PLSOIL 120
ORGANIC FARMING AND GARDENING

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CHAPTER 7

FERTILIZERS

If the results of testing of soil or diagnosis of plants reveal that nutrients are deficient in a soil, fertilization is necessary for optimum plant growth. Fertilizers are chemical substances that are used to carry plant nutrients to a soil. Fertilizers may supply one or more nutrients. Fertilizers may be manufactured or naturally occurring. Naturally occurring fertilizers sometimes, but not always, are classified as organic fertilizers. Designation of fertilizers as organic is a matter of definition and of acceptance and is not necessarily based on origin of the materials. Manufactured fertilizers commonly are called chemical fertilizers. Manufactured fertilizers often are mixed to form what is called a complete fertilizer, that is, a fertilizer with nitrogen, phosphorus, and potassium. The concentrations of plant nutrients in a fertilizer specify its grade or analysis.

Fertilizer Grades

Grade or analysis of a fertilizer is designated by three numbers that indicate the guaranteed minimum concentrations of available plant nutrients. Commercial fertilizers, organic or manufactured, must have the guaranteed mininum analysis printed on the package. A grade of 10-10-10 has a guaranteed analysis 10% N, 10% P2O5, and 10% K2O, which is specified on the container as guaranteed available nitrogen, phosphoric acid, and potash. Fertilizers that have more than 30% total available nutrients are called high analysis fertilizers, whereas those with less than 30% total available nutrients are called low analysis fertilizers. A 15-15-15 is a high analysis fertilizer; a 5-10-10 is a low analysis fertilizer, and a 10-10-10 is right on the borderline. The division between high analysis and low analysis is arbitrary. Sometimes, recommendations imply limitations on the amounts of high analysis fertilizers that must be applied. Growers should not feel that they can apply low analysis fertilizers at substantially higher amounts than high analysis fertilizers, when in fact only minor differences exist between the two materials. Growers should consider most modern, mixed, chemical, multinutrient fertilizers as high analysis fertilizers.

A matter that needs explanation is the method of presentation of grades of fertilizers as amounts of N (nitrogen), P2O5 (called phosphoric acid on the bag but chemically phosphorus pentoxide), and K2O (potash or potassium oxide). These expressions are related to the practice of presentation of constituents of inorganic substances as oxides. This procedure has been accepted by the fertilizer industry and is used in the expression of grades of all fertilizers, organic or chemical, for which the manufacturer guarantees an availability of nutrients. A similar kind or reporting of constituents is used in the expession of calcium and magnesium contents of liming materials (Chapter 8). Reports in scientific literature require that analyses be reported as actual elemental concentrations, that is, as N, P, and K. If a grower wants to convert fertilizer grades into amounts of actual elements delivered by the fertilizer, the following conversions should be used (Table 18). Recommendations for application of fertilizers are based on N, P2O5, and K2O. Changes in the bases of reporting of analysis or in making recommendations are unlikely because of the amount of confusion that would exist and the great effort in education of the public that would have to be made to effect the changes. On the other hand, when nutrient removal by crops is presented, the amounts always are given in amounts of actual element removed. Grower and advisers of growers, who base recommendations of fertilization on amounts of nutrients removed, must be certain to coordinate recommendations of nutrients needed with the amounts delivered by fertilizers.

 

Table 18. Conversions of expressions of analyses as oxides into concentrations of actual plant nutrients present.

Element

Term in trade

Usage

Conversion factor for actual element

Nitrogen

Available N

Fertilizer analyses

1.00 X N

Phosphorus

Available phosphoric acid, P205

Fertilizer analyses

0.44 X P205

Potassium

Available potash, K20

Fertilizer analysis

0.83 X K20

Calcium

Calcium carbonate equivalent, CaC03

Limestone analysis

0.4 X CaC03 (calcite) or 0.23. X CaC03 (dolomite)

Nutrients other than the three primary macronutrients (nitrogen, phosphorus, and potassium) may be supplied by a fertilizer, but with a few exceptions, their concentrations are specified only if a guaranteed analysis for these other nutrients is claimed. Fertilizers with nutritients in addition to nitrogen, phosphorus, and potassium are referred to as specialty fertilizers.

Nitrogen Fertilizers

Effects of nitrogen fertilizers on plant growth

Nitrogen is the essential element most frequently deficient in soils around the world. Most of the nitrogen in plants is in proteins, genetic material, and chlorophyll. The amount of nitrogen accumulated by plants varies with species, cultivar, plant part, and age of the part as well as with the nutritional status. Typical ranges of concentration are from 1.5 to 5% total nitrogen on a dry-weight basis. Nitrogen in mature grasses falls in the lower end of the range and in young plants and legumes at the middle and higher end of the range. Although the threshhold for nitrogen deficiency varies with kind, position, and age of tissues, leaves that have less than 1.5% total nitrogen probably are nitrogen deficient. Nitrogen-deficient plants (Table 17) are identified by poor growth and poor color which are due basically to the lack of production of proteins. Adequate nitrogen nutrition is associated with vigorous vegetative (roots, stems, leaves) and reproductive (flowers, fruits, seeds) growth. Excesses of nitrogen generally lead to supraoptimal vegetative growth and suppressions of reproductive growth. A proper total supply and balance of nitrogen with other elements is very important in plant nutrition. Optimum and balanced fertilization with nitrogen promotes the growth of all plant organs. Generally, nitrogen fertilization promotes shoot growth more than root growth and, in the shoots, promotes vegetative growth more than reproductive growth. Nitrogen-fertilized plants may be soft, succulent plants with a high content of water. This characteristic is the result of alterations in the relative contents of proteinaceous and carbohydrate-like materials.

Nitrogen fertilization enhances protein synthesis. Photosynthetic products that might go to formation of sugars, starches, and cell walls are directed toward protein synthesis. The thin cell walls that are associated with high water and protein contents make leafy vegetables crisp and succulent. On the other hand, thin-walled cells of stems make plants weak-stemmed, and these plants may fall over or lodge. Nitrogen fertilization normally delays the rate of maturation of plants, that is, the plants are maintained in a vigorous vegetative state, and flowering is delayed. Sometimes delays in maturity are detrimental in that crops do not have sufficient time to produce fruits or seeds before the growing season is ended by frost. For some crops that flower and fruit over a prolonged period, such as tomatoes and cucumbers, the period of productivity may be enhanced by a delay in maturity.

The relative responses of roots and shoots to nitrogen must be considered. Nitrogen fertilization promotes shoot growth more than root growth, and higher than optimum amounts of nitrogen will inhibit root growth while more markedly enhancing shoot growth. With beets, carrots, radishes, sweet potatoes, and other root crops, overfertilization with nitrogen gives lower yields and lower quality of produce than optimum fertilization. Irish potatoes are stems (tubers), and heavy applications of nitrogen give favorable yield responses with this crop.

Nitrogen fertilizers are potent materials. It is important to be well acquainted with the kinds of fertilizers that are available and with their composition and release of available nutrients. Common nitrogen fertilizers (Table 19) are discussed in the following section.

 

Table 19. List and characteristics of common nitrogenous fertilizers.

________________________________________________________________________

 

Kind of fertilizer

Type

-----------Availability of nitrogen ----------

Total %N | ----------------Release of Nz----------------

Urea

Chemical

46

Rapid (water-soluble)

Ammonium nitrate

Chemical

34

Rapid (water-soluble)

Ammonium sulfate

Chemical

20

Rapid (water-soluble)

Diammonium phosphate

Chemical

18

Rapid (water-soluble)

Sodium nitrate

Chemical

16

Rapid (water-soluble)

Calcium nitrate

Chemical

15

Rapid (water-soluble)

Potassium nitrate

Chemical

13

Rapid (water-soluble)

Dried blood

Organic

12

Rapid (90%) y

Feather meal

Organic

14

Rapid (70%)

Seed meal

Organic

6

Rapid (80%)

Sewage

Organic

4

Slow (10 to 50%)

Farm manures

Organic

2

Slow (10 to 50%)

Composts

Organic

1 to 3

Slow (10 to 50%)

________________________________________________________________________

 zExpected release in first growing season.

 yExpected mineralization of nitrogen in first growing season.

 

The chemical or manufactured fertilizers have much higher total concentrations and availability of nitrogen than the organic fertilizers. Release (Table 19) is defined as the amount of nitrogen that is expected to be available in soluble form from the fertilizer in one growing season. Chemical fertilizers, being water-soluble, will release all of their nitrogen. Organic fertilizers must be mineralized before they will release their nitrogen. Mineralization is a slow, microbiological process, the rate of which is dependent highly on the concentration of nitrogen present in the organic material. Performance of nitrogen fertilizers is increased if they are incorporated into the soil rather than left on the soil surface after application. Incorporation is virtually a mandatory practice with ammoniacal or organic fertilizers, such as urea or farm manures, respectively, because of the potential of losses of nitrogen by ammonia volatilization. Nitrate-based fertilizers are less vulnerable to losses from the surface. Washing soluble nitrogenous fertilizers into the soil usually is equivalent to mechanical incorporation.

Chemical Fertilizers for Nitrogen

Urea (CON2H4) . Urea is the most concentrated and most commonly used dry, granular or pelleted nitrogen fertilizer. Urea, however, is not the most widely used nitrogen fertilizer. Anhydrous ammonia (gaseous ammonia compressed into a liquid) is used in the largest total amounts and is the most concentrated of the nitrogenous fertilizers (80% N). Anhydrous ammonia is used widely in corn production. Special equipment is needed to apply anhydrous ammonia; consequently, it is used infrequently in small-scale agriculture or in gardening.

Urea is sold as such for fertilization of any crop. Urea is a common product in mixed dry or liquid fertilizers used in large-scale commercial agriculture. It is ofter a major constituent in concentrated, high-analysis, soluble, mixed fertilizers used on greenhouse crops, gardens, and house plants. Frequently, urea is sold as a sulfur-coated or plastic-coated material for fertilization of lawns or container-grown plants. The coatings give the urea a property of slow-release, mimicking organic fertilizers, so that it can be applied directly to turf or mixed in containers without causing damage to plants. The coatings can be manufactured with varying stabilities so that the availabitities of nitrogen from the slow-release materials can be governed. Selection of the appropriate material varies with the crop being fertilized and with the season. With container-grown crops, enough plastic-coated fertilizer may be added at planting to carry the crop through its growth cycle.

Commercial urea is considered to be a chemical fertilizer because it is manufactured. Commercial urea is chemically identical to urea in urine. If equal amounts of nitrogen from commercial urea or from urine were applied to a crop, crop responses would be expected to be the same. Urea is an ammoniacal fertilizer, for the first nitrogenous product of its breakdown in the soil is gaseous ammonia (or ionic ammonium). Phytotoxicity may be a problem if the ammonia (or ammonium) is not oxidized rapidly to nitrate. Slow oxidation may occur in cold or wet soils. High applications of urea may lead to ammonia (or ammonium) concentrations that are phytotoxic.

Although distinctions are made between ammonia and ammonium above, farmers and gardeners need not be concerned with this distinction. Gaseous ammonia dissolves rapidly in water. A fertilizer of ammonia dissolved in water is called aqua ammonia. In the field, if ammonia dissolves in water and reacts with other dissolved substances, it forms the ammonium ion. An equilibrium exists between ammonia and ammonium, and whichever form that dominates is dependent on the pH of the solution. In alkaline solutions, ammonia dominates, and in acidic solutions, ammonium dominates.

Applications of urea must be incorporated into the soil. Surface-applied urea is hydrolyzed rapidly to ammonia and carbon dioxide which react to form ammonium carbonate. Ammonium carbonate is an unstable, alkaline substance that decomposes quickly back to ammonia and carbon dioxide. On the surface of the ground, the resulting ammonia will be lost to the atmosphere. If pelleted urea is left on the soil surface, as much as one-third of the urea nitrogen can be lost in a day or withinin a few days. If urea is incorporated into the soil immediately after application, losses are nil, for soil water and colloids hold the ammonia and ammonium. If it rains or if irrigation water is applied, the urea will be carried into the soil, and losses will be nil. Losses of urea from surface-applied liquid fertilizers are much lower than those from surface-applied dry fertilizer.

Ammonium nitrate (NH4NO3). This fertilizer was the first solid one to be produced on a commercial scale. It became common after World War II, when industrial plants that had been producing ammonium nitrate for blasting or for munitions were converted into fertilizer plants. With respect to supplying nitrogen, ammonium nitrate is an excellent fertilizer. It is a concentrated material (34% N) Half of its nitrogen comes from nitrate and half from ammonium. This combination makes ammonium toxicity unlikely, even if the ammonium is not oxidized rapidly by nitrification. On the other hand, ammonium nitrate is a strong oxidizing material. If it becomes contaminated with carbonaceous materials, it may be somewhat unsafe to handle. This fertilizer should be used up and not stored around the house or farm. Ammonium nitrate also has a tendency to absorb water and to cake even if prilled (pelleted). Production of dry ammonium nitrate as fertilizer is declining, so that the availability of ammonium nitrate for direct application is limited. One usually has to purchase ammonium nitrate early in the season, before supplies stocked for sale are exhausted. Ammonium nitrate along with urea is a constituent of liquid fertilizers commonly called UAN (urea ammonium nitrate) solutions.

Ammonium sulfate [(NH4)2SO4]. Ammonium sulfate is also a fertilizer with a long history of use, as it is a by-product of many industrial processes. It also is produced directly for use as a fertilizer. Today its use is limited by its relative low analysis (21% N). Ammonium sulfate acidifies the soil, and its use in the United States is mainly on alkaline soils. It is recommended sometimes for use on crops that are said to be acid-loving, such as rhodendrons, azalea, and blueberries. It is used frequently in rice production. On acid soils, ammonium toxicity can be a problem with use of ammonium sulfate, even with acid-loving crops. The grower must be careful to avoid overapplication of ammonium sulfate because of the potential for ammonium toxicity. Ammonium sulfate has an advantage over other nitrogen fertilizers in that it supplies sulfur as well as nitrogen.

Diammonium phosphate [(NH4)2HPO4]. This fertilizer is used primarily for its phosphorus (46% P2O5), although it has a relative high nitrogen concentration (18% N). It is water-soluble fertilizer and is used frequently in mixed liquid fertilizers and concentrated mixed dry fertilizers. It can be applied at any time but has good use as a starter fertilizer.

Sodium nitrate (NaNO3). Sodium nitrate was the first commercial nitrogen fertilizer in the United States. Its origin was ore deposits in Chile and was sold as Chilean nitrate. Today, although some is manufactured, most of the sodium nitrate used in the United States is imported from Chile. Chilean nitrate a naturally occurring fertilizer. It is not considered as organic because it is purified in processing and has a relatively high analysis (16% N) and solubility in water. Sodium nitrate has a only a minor role as a nitrogen fertilizer today in the United States and is not readily available in the marketplace. Sodium is

not a plant nutrient, and its application to soils, particularly acid, clayey soils, should be avoided. Sodium in exchange sites of clays causes colloidal dispersion and leads to poor soil structure.

Calcium nitrate [Ca(NO3)2]. Calcium nitrate was among the first manufactured nitrogen fertilizers. Almost all of the calcium nitrate available in the United States comes from Norway, where it was first manufactured with use of electricity from hydroelectric power. Calcium nitrate is a high quality nitrogen fertilizer but is expensive per unit of nitrogen. Transportation costs make this material uneconomical for use on an agronomic scale. It is not a commonly available fertilizer in the United States but is an important source of nitrogen in Europe. Calcium nitrate absorbs water, and this tendency limits its use as a bagged, dry fertilizer. This fertilizer supplies calcium, which is a plant nutrient and which has favorable effects on soil structure. Clays that have a predominance of calcium on their exchange sites can have a good, aggregated structure. Calcium nitrate is recommended as a calcium fertilizer to control blossom-end rot on tomato.

Potassium nitrate (KNO3). Potassium nitrate provides two nutrients, nitrogen (13%) and potassium (46% K2O). Although this fertilizer is an excellent material for use on any crop, it is expensive relative to other commercial sources of nitrogen and potassium and is used mainly on high-value crops. It is excellent for supplemental fertilization as side-dressings or top-dressings after plants are established and growing. The fertilizer is water-soluble and can be applied in solutions.

Organic Fertilizers for Nitrogen

Dried blood. This organic fertilizer is mostly protein, has a high nitrogen content (12% N), and mineralizes rapidly. Its action in the soil is much like that of a chemical fertilizer. Dried blood can be used at any time in the growing season. Since its nitrogen becomes available rapidly, it would be a good organic source of nitrogen for supplemental fertilization of an established crop. Since dried blood mineralizes rapidly, ammonium build up in the soil may be rapid. Because of the possibility of ammonium injury, dried blood should be applied away from germinating seeds and roots of young plants. Dried blood is an expensive source of nitrogen and, even if purchased in bulk, is affordable only for use on small plots. Because of its rapid mineralization, very little of the nitrogen in the blood would be expected to be carried over in the organic form for the next cropping season. Dried blood has little or no value as a source of phosphorus or potassium. Dried blood should be mixed in the soil. Nitrogen will be lost rapidly from surface applications.

Feather meal. Feathers are proteins and are a good source of nitrogen. Feather meal should be used in the same way as dried blood. The proteins in feather meal may be slightly more resistant to mineralization than those in dried blood, but this difference is not likely to be evident in practice. Other nutrients in feather meal are in very low concentrations, and it should be considered only as a nitrogen fertilizer.

Seed meals. Seed meals are the residues left after oils are extracted from seeds. Cottonseed oil meal and castor pomace are fairly well known seed meals, which are used as organic fertilizers. Soybean oil meal is used less frequently as a fertilizer. Cottonseed oil meal and soybean oil meal are valuable as protein supplements in animals feeds; consequently, they are expensive as fertilizers. Castor pomace is poisonous, for the castor bean from which it is derived has a very toxic alkaloid (ricinine). Castor pomace is not used as feed and is cheaper than the other oil meals. Castor pomace is poisonous to humans and should be handled with care to ensure that none of its powder is inhaled.

Seeds meals are slower to mineralize in the soil than the nitrogen-rich animal products. Seed meals may be incorporated into the soil at planting but application before planting is preferable. Their rate of mineralization is moderate but not rapid enough to be considered as a fertilizer for side-dressing or other supplemental applications. Initial decomposition of seed meals is rapid. If the seed meals are placed close to germinating seeds or seedlings, seeds will rot, and damping-off disease of seedlings will occur. If possible, seed meals should be applied two weeks ahead of planting to allow for their rotting and to alleviate possibilities of seeds rotting or damping-off. Seed meals should be considered only as nitrogen fertililzers, for their contents of phosphorus and potassium are too low to be considered practical.

Sewages. The sludge or biosolids from municipal wastewater treatment plants is a source of nitrogen. This material has some phosphorus but is essentially void of potassium, which has been washed out in the effluent. Sewage biosolids are commercially available. Sewage bioslids from Milwaukee have been marketed for decades. Sewage biosolids occasionally may be available from local wastewater treatment plants. Toxic heavy metals, such as, cadmium, nickel, copper, zinc, and lead, are suspected hazards in virtually all sewage biosolids. Although with control of point sources of pollution, the concentrations of heavy metals in sewage biosolids is being reduced to levels that are safe for application to farmland.

If present in biosolids, metals are adsorbed by the soil and will remain there for long periods of times. Contamination of soils with heavy metals is a drawback to the use of sewage biosolids as nitrogen fertilizer. Because of this hazard, sewage biosolids should be avoided or used with caution in the production of food crops. Sewage biosolids from local treatment plants may be high in paper if the sludge is raw or undigested. This kind of material should be composted before it is applied to the land. Some treatment plants may have facilities in which biosolids and wood chips are composted into a product that is made available for sale to the public. Sewage biosolids is sometimes surface-applied to turf; otherwise, it should be mixed into the soil. Ammonia loss from surface applications will be high; particularly, since biosolids are usually made alkaline by incorporation of calcium carbonate at the treatment plant.

Farm manures. Applications of farm manures to land are discussed more extensively in Chapter 3. In summary, farm manures are nitrogen fertilizers ranging from 1% to 3% N on a dry weight basis and from 10 lb N/ton to 30 lb N/ton on a wet (fresh) weight basis. Manures applied as fertilizers should be incorporated into the soil about two weeks ahead of planting. This mode of application will conserve nitrogen against losses from ammonia volatilization and will permit ammonium formed in the soil to be oxidized to nitrate before the crops are in the soil. The amount of application should be about 1 lb of fresh material per square foot of land (20 tons/acre).

Composts. Use of composts is discussed in Chapter 4. In general, composts are applied in the same manner as manures except that the lead time of two weeks ahead of planting is not necessary. Chances of damaging seeds or seedlings from compost are much lower than from manures. Chapter 4 deals with how growers can make their own composts. Today many composts are manufactured commercially. Some of these composts are made from farm products, such as manures, products of food processing plants, and wood chips. Others are made from municipal solid wastes, such as paper, yard wastes, leaves, and sewage sludge. All users of commercial composts should know the composition of the composts with respect to plant nutrients and other elements.

Miscellaneous and specialty nitrogen fertilizers.

Many of these materials are imported or are only of local origin. Scarcity and cost limit their general use in many cases.

Urea reacts with formaldehyde to produce a slow release fertilized (urea-form, 38% N) which is used is used in formulation of fertilizers for turf. Isobutylidenediurea (IBDU, 32% N) also is a slow-release material commonly used on turf. Peanut hulls (1.5% N), cotton bolls (1% N), cocao shells (2.5% N), alfalfa hay or meal (2.5% N) are plant-derived nitrogen fertilizers with slow to moderate rates of release of nitrogen. Wool wastes and felt have about 8% N each. These materials treated with sulfuric acid to increase their release of nitrogen are called rough ammoniates. Fish meal and fish scrap have 5% to 10% N of which half or more will be available in a season. Guano (excrement of birds and bats), about 10% N, is a scarce material rarely used today.

Most farmers and gardeners will use mixed or multinutrient fertilizers in all or in a portion of their programs in soil fertility. Multinutrient fertilizers are guaranteed to have more than one of the primary macronutrients, nitrogen, phosphorus, and potassium. Examples of such fertilizers are 5-10-10, 10-10-10, and 20-20-20. The nitrogen in these fertilizers will be derived from at least one of the above-mentioned chemical fertilizers.

Phosphorus Fertilizers

Effects of Phosphorus Fertilizers on Plant Growth

The role of phosphorus is most prominent as a constituent of genetic material, the nucleic acids, DNA and RNA. Phosphorus has another major function in the metabolic processes of energy transfer. Energy from respiration and photosynthesis is stored in phosphate bonds of energy-rich compounds (e.g., ATP, adenosine triphosphate) in plants and in animals. The phosphorus requirements of plants is about one-tenth to one-fourth the amounts of nitrogen required.

Plants absorb phosphorus as phosphate ions (principally H2PO4-). Plants respond with increased growth or yield in about half of the cases in which phosphorus fertilizers are applied. New land or land that has not been cropped or fertilized recently is more likely to be phosphorus-deficient than land that has been cultivated and fertilized. The high frequency of response of crops to fertilization with phosphorus is the reason that phosphorus is considered as a primary macronutrient, for its accumulation in plants does not exceed that of calcium, magnesium, and sulfur, which are considered as secondary macronutrients. The elemental phosphorus concentration of phosphorus-sufficient plants typically ranges from 0.2 to 0.4% of the dry weight of foliage. Leaves that have less than 0.15 to 0.2% actual P probably are phosphorus deficient. Plants grown in soil normally will have less than 0.5% actual P. Concentrations approaching or exceeding 0.8% occur with plants grown hydroponically or in well-fertilized media in containers.

Phosphorus-deficient plants are stunted (Table 17). The stunting may at first be difficult to recognize, for the plants may appear dark green and otherwise normal. A well-nourished plant would have to be available as a standard of comparison to indicate optimum growth. Phosphorus is a mobile element in plants. It will move from old leaves to young leaves and growing points and into fruits and seeds. If phosphorus becomes deficient through exhaustion of available soil reserves, deficiencies will appear as reddening or purpling of the lower leaves, particularly on the undersides of the leaves. These leaves may later become yellow or dead and drop off. A plant that is stunted or otherwise exhibiting deficiencies of phosphorus deficiency is very difficult to nourish back to a state of full nutrition. Losses of yields or quality are certain to result any time that phosphorus deficiency occurs.

The concentration of phosphorus in the soil solution is only a few tenths parts per million in unfertilized soils. Most phosphorus salts in the soil are present as sparingly soluble phosphates of iron, aluminum, calcium, and magnesium. Plants receive only a fraction, perhaps 20% or less, of the phosphorus that is applied in fertilizers. Soils have strong capacities to fix phosphorus. Phosphorus fixation by soils refers refers to the precipitation of phosphate by iron and aluminum in acid soils and by calcium and magnesium in alkaline soils. Fixation in acid soils is the more common process in humid temperate regions.

Generally, phosphorus in soil or from fertilizers is most available to plants in the pH range of 6 to 7. An exception, might be in the case of rock phosphate for which availability of phosphorus is favored by acidic conditions, which help to dissolve the rock. An abundance of organic matter in the soil helps to limit fixation and to keep phosphorus in solution. Organic matter forms complexes (chelates) with iron and aluminum ions and lessens their reaction with phosphates, thereby lessening precipition. The acids from the decay of organic matter also help to dissolve rock phosphate or other difficultly soluble phosphorus fertilizers.

Because of fixation, phosphorus builds up in soils, and after many years of fertilization, use of phosphorus fertilizers may not be necessary. Soil tests are accurate in determining if this condition of enrichment exists. Also, because of fixation and the relatively low solubility of most phosphorus fertilizers, overfertilization with phosphorus may occur but is unlikely. The problems that occur with overfertilization with phosphorus fertilizers may be imbalances that render minor elements unavailable to plants.

Chemical (manufactured) and organic phosphorus fertilizers are marketed (Table 20). As a rule, the chemical fertilizers are the superior products and are the easiest to obtain. Much more attention needs to be given to procedures for application of organic fertilizers and to the conditions of the soil than for use of the chemical fertilizers. Proper use of each fertilizer will be discussed below.

Chemical Fertilizers for Phosphorus

Superphosphates. Superphosphates are manufactured by treatment of rock phosphate with sulfuric acid or with phosphoric acid. The purpose of this process is to increase the effectiveness of rock phosphate by its acidulation into more soluble compounds. This process dates back to about 1840, when Justus von Liebig, a physical chemist in Germany, demonstrated that the value of bones as fertilizer could be increased by treating them with sulfuric acid. Liebig is credited with the development of the first chemical fertilizer.

Table 20. List and characteristics of some common phosphorus fertilizers.

Kind of fertilizer

Type

-------Availability of phosphorus----------

Total % P2O5 |---------Release of P-----------

Ammoniated phosphates

Chemical

46+

Rapid (water -soluble)

Triple superphosphate

Chemical

46

Rapid (moderately water-soluble)

Ordinary superphosphate

Chemical

20

Rapid (moderately water-soluble)

Rock phosphate

Organic

30

Very slow

Colloidal rock phosphate

Organic

20

Very slow

Bonemeal

Organic

24

Slow

________________________________________________________________________

 

The product of the reaction of sulfuric acid and rock phosphate is ordinary superphospate. This material is a mixture of phosphates, mostly monocalcium phosphate, a fairly water-soluble material, and gypsum. Ordinary superphosphate has the chemical formula, Ca(H2PO4)2.CaSO4. Triple superphosphate is produced by treatment of rock phosphate with phosphoric acid. The end product is mostly monocalcium phosphate, Ca(H2PO4)2; hence, triple superphosphate, missing the gypsum of ordinary superphosphate, is about 46% P2O5 compared to 20% P2O5 for ordinary superphosphate. The availability of phosphorus does not differ between the two superphosphates. About 85% of the phosphorus is water-soluble in these fertilizers. The name triple superphosphate dates back to the time when ordinary superphosphate was 16% P2O5 and triple superphosphate was 48% P2O5. Triple superphosphate sometimes is called concentrated superphosphate. Strictly speaking concentrated superphosphate is another product with about 54% P2O5 and is used mainly in mixed nutrient fertilizers.

Because of its lower analysis, ordinary superphosphate is declining in importance. More than twice as much ordinary superphosphate would have to be applied to supply the same amount of total phosphorus carried in triple superphosphate. Costs of transportation make it more economical to ship the more concentrated material. About one-third of the fertilizer phosphorus used in the United States is supplied by triple superphosphate. Triple superphosphate or concentrated superphosphate is common in dry, bulk blends of mixed fertilizers.

Superphosphates may be applied directly to the soil. Modern superphosphates are pelleted or prilled materials. These materials can be incorporated by broadcasting and mixing in the soil or by placing in a band along side of rows of seeds or transplants. All of the phosphorus fertilizer should be applied at one time at planting. Most recommendations for applications of superphosphates specify that they be broadcasted and disked in or drilled in about 2 inches below and 2 inches to the side of the seed row. Banding usually allows for more efficient use of phosphorus but may require more labor and care or special implements than broadcasting.

Because of precipitation by fixation, phosphorus is not very mobile in soils and will not move more than a centimeter or so (about a half inch) from a pellet. With banding, the concentrated placement, which is not reacted with the soil, helps to provide a constant supply of phosphorus. A one-time large application helps to ensure that young, sparsely rooted seedlings in cold soils of the spring season are able to obtain adequate phosphorus.

Superphosphates are 17% to 22% calcium. Supplying the phosphorus requirement of plants with either of these fertilizers generally ensures adequate nutrition with calcium. Ordinary superphophate is richer in calcium than concentrated superphosphate and also has 10% to 12% sulfur of which concentrated superphosphate is essentially void. The absence of sulfur in concentrated fertilizers may be of concern in areas in which sulfur contents of soils are marginal for crop production.

Ammoniated phosphates. Ammoniated phosphates are produced by reactions of ammonia and phosphoric acids. Depending on the manufacturing process one or more chemical salts or their mixtures are produced. Ammoniated phosphates are marketed in liquid and dry formulations. These phosphates are more soluble in water than the superphosphates and are used frequently to make mixes of soluble fertilizers for use in greenhouses and gardens and for houseplants. They are available also for direct application in production of agronomic and horticultural crops. The principal ammoniated phosphates are monoammonium phosphate (NH4H2PO4), diammonium phosphate [(NH4)2HPO4], and ammonium polyphosphate (pyrophosphate). They have concentrations of P2O5 from 46% to 62% and are equally effective in suppling phosphorus to plants.

Liquid fertilizer applied in bands or broadcasted and worked in gives the same response as dry, water-soluble fertilizer applied in the same way. As with dry fertilizers, band application of liquid gives superior results to broadcasted applications. Banding lessens contact between soil and fertilizer and restricts phosphorus fixation. Monoammonium phosphate and diammonium phosphate are the most commonly used dry materials. Diammonium phosphate is the most widely used phosphorus fertilizer in the United States.

Some care must be taken in the use of diammonium phosphate as a starter fertilizer. Diammonium phosphate, which has loosely bound ammonia, gives an alkaline solution (pH 8 in saturated solutions), whereas monoammonium phosphate makes an acid solution (pH 4). Application of diammonium phosphate in direct or close contact with seeds can result in ammonium toxicity to the seeds. Although any soluble salt can damage seeds or roots by plasmolysis, the problems of ammonium toxicity are less likely with monoammonium phosphate or ammonium polyphosphate than with diammonium phosphate.

Liquid or dry fertilizers sold as starter fertilizers or for other uses have their constituents printed on labels of the cartons, and users should take care to read these labels before application of the fertilizers.

Organic Fertilizers for Phosphorus

Plant and animal tissues. Phosphorus concentrations in plant materials, farm manures, and composts are too low for these materials to be practical sources of phosphorus for the short term. The amounts of materials that need to be applied limit the feasibility of use of these materials as phosphorus fertilizers for single applications in one season. Long-term use of these materials may lead to build up of phosphorus in the soil, and the organic matter in these materials makes them good amendments for application with phosphorus fertilizers. Organic matter additions to the soils generally improve the nutritional availability of phosphorus from any source.

Phosphatic materials derived from animal bodies have much higher phosphorus concentrations than those from plant materials and can be used directly as phosphorus fertilizers as well as being processed into the chemical fertilizers mentioned above.

Rock phosphate [Ca10(PO4)6F2]. Rock phosphate occurs as sedimentary deposits of marine organisms. Principal deposits in the United States are in Florida, North Carolina, Tennessee, Wyoming, Idaho, and Montana. About 80% of all rock phosphate mined in the United States comes from Florida and North Carolina. The products that are sold for direct application to land come from Florida and Tennessee. Regardless of source, rock phosphate is an often very hard, nearly inert material. It has a chemical composition similar to enamel of teeth. Since rock phosphate is of biological origin, the phosphorus content of ores varies. At the fertilizer plant, the ore is washed to remove impurities such as sand and clay. The washed materials go into waste ponds. Methods of pumping the washings into the waste ponds give separation of sand and clay deposits. The clay deposits are marketed as colloidal rock phosphate (from Florida). Material marketed as regular rock phosphate is from Tennessee.

Because of the fineness of the waste-pond colloidal phosphates, claims are made that they are superior to regular rock phosphate. These claims should be discounted, for colloidal rock phosphate has a lower analysis (20% P2O5) as it is regular rock phosphate (30% P2O5) diluted with clay or silt. On the other hand, regular rock phosphate should be pulverized to at least silt-sized particles before it has much value as a fertilizer. If colloidal rock or regular rock phosphate are not used properly, crop responses may not be any different than if no phosphorus fertilizers were applied at all.

Rock phosphate or colloidal rock phosphate should be applied at double or quadruple the recommended rate for superphosphates or ammoniated phosphates. If the user is close to the site of production of rock phosphates, shipping costs may be low enough so that these amounts of application are economically reasonable.

The rock phosphates must be pulverized finely and mixed well into the soil. The soil should be about pH 5.5. Mixing the finely divided material into an acid soil brings the rock into close contact with the soil acids so that the rock may be solubilized. In essence, the acids of the soil acidulate the rock into a form of superphosphate. More strongly acid soils should be limed to raise them to pH 5.5. If the soils are much below pH 5.5, so much iron and aluminum are in solution that the dissolved phosphate is unable to move in the soil because of fixation. Organic matter should be added in generous amounts, at least 20 tons/acre (1,000 lb/1,000 sq. ft.). Organic matter increases the availability of phosphorus in rock in that organic acids produced during decomposition of the organic matter help to dissolve the rock and in that the organic matter complexes (sequesters or chelates) iron and aluminum allowing the dissolved phosphate to remain in solution. Irrigation may help in increasing the availability of phosphorus from rock sources.

Rock phosphates can be mixed in direct contact or in close proximity to seeds, roots, or bulbs, for the low solubility of the rock prohibits the possibility of the creation of any stresses from salinity.

Bonemeal. The product that should be used is steamed bone meal. The principal sources of bones are from slaughter houses. Raw bones are boiled and steamed to remove fats and proteins. The fats and proteins may go to the manufacture of gelatins and glues. The resulting bones are nearly nitrogen-free, maybe 1% N, so that steamed bonemeal is not a nitrogen fertilizer. Raw bonemeal may have 4% N but should not be used as it has not been steamed and sterilized. The steamed bones are ground into meal. The resulting product has 22% to 30% P2O5, averaging about 24%. Bone meal is much superior to rock phosphate and is approaches the value of ordinary superphosphate. The phosphorus in bones is a little less available than that in the superphosphate, for bones are about chemically equivalent to tricalcium phosphate [Ca3(PO4)2], which is sparingly soluble compared to the monocalcium phosphate of the superphosphate.

Bonemeal should be mixed well into soils of pH 6. Organic matter should be used generously as with the rock phosphates. Bonemeal can be placed in direct contact with seeds, roots, or bulbs without damage to the plant tissues. The sparingly soluble nature of the phosphorus compounds in the bonemeal prohibit any damage from salinity.

Bonemeal is an expensive material compared to all other commercial sources of phosphorus. Its high price limits its use to garden-size or smaller applications. Its rate of application to soils should be about the same as that of ordinary superphosphate.

Miscellaneous and specialty phosphorus fertilizers. Basic slag is a by-product of refining of pig iron (crude cast iron) to steel. In the Bessemer process, pig iron and lime are heated to a molten state. The lime reacts with phosphoric acid and other impurities and floats to the surface, where it is poured off as slag. The cooled slag is ground to a fine powder. Slag is 8% to 12% P2O5. It is a liming material, being 70% as effective as agricultural limestone. Slag is best suited for long-season crops in acid soils. It is not used much in the United States, with most of its use being in Europe.

Practically all crops that are fertilized with nitrogen and phosphorus are fertilized with multinutrient materials. These materials may be supplemented with potassium fertilizers, usually potassium chloride, to make a complete commercial fertilizer. Some nitrogen and phosphorus compounds are manufactured specifically for mixed fertilizers. These materials include some of the fertilizers described above but also others such as nitric phosphates and ammoniated superphosphates. The latter materials are mixtures of nitrogen-containing compounds and phosphorus-containing compounds. These multinutrient fertilizers generally are as effective as the individual fertilizers, especially for nitrogen. Some of the phosphorus-containing compounds may not be as water-soluble as some of the individual fertilizers discussed above. It is doubtful that differences in crop response could be noted between crops fertilized with multinutrient fertilizers and mixtures or individual applications of the phosphorus fertilizers described above.

Potassium Fertilizers

Effects of Potassium Fertilizers on Plant Growth

Potassium is usually the most abundant cation in plants. Nitrogen is the only nutrient accumulated consistently in quantities that exceed those of potassium. Some plants accumulate more calcium than potassium. The sufficiency zone for potassium accumulation typically is from 1.5% to 3% actual K. With well-fertilized plants, potassium may accumulate to levels well above 5% into what is known as the concentration range of luxury consumption. Accumulation in the luxury range has no ill effects on plants. Accumulation above the luxury range may have ill effects in that calcium and magnesium accumulation may be suppressed to the point of deficiency.

In spite of the requirement for large amounts of potassium, the specific function of potassium eluded scientists for many years, mainly because all of the potassium is in plants is in ionic form and no organic compound has potassium in any permanent combination. Recently it has been ascertained that the roles of potassium in plants are catalytic or osmotic functions. Potassium has a role in activation of enzymes, which are proteins that catalyze metabolic reactions. Potassium appears to impart a specific structure that makes the enzymes functional. Potassium has a known catalytic role in protein synthesis and apparent roles in photosynthesis. The opening and closing of stomates in leaves is regulated by turgor pressure brought about by the movement of potassium into and out of the cells surrounding the stomates. The turgor pressure in leaves may be regulated by potassium. Growth of cells by expansion may be due in part to the turgor pressure brought about by high concentrations of potassium in cells. Potassium is a mobile element in plants. It will move from sites where it is needed less to sites where the demand is greater. If potassium is depleted in the soil, the potassium that is in the plant will move from the old leaves to young leaves. The old leaves will become deficient in potassium and will die at their edges and tip. This condition is called firing or scorching.

Stems of potassium-deficient plants may become weak due to internal breakdown or failure of supporting cells to develop. Although grain is not high in potassium, seeds of cereals will be unfilled or chaffy, and ears or corn will fail to fill at the tips, producing what are called nubbins. Often fruits of potassium-deficient apple or pepper are misshapen. Blotchy ripening of tomato has been associated with potassium deficiency. Although potassium appears to have a role in the growth and development of fruits and seeds, they are not rich in potassium, as about two-thirds of the potassium remains in the vegetative portions of the shoots.

About half of the soil in the United States are deficient in potassium for optimum plant growth. These soils may be sandy or organic soils from which potassium has leached. Potassium-deficient soils also may be ones from which the readily available potassium has been exhausted leaving behind the the difficultly available potassium, which supplies potassium too slowly to meet the needs of crop growth. Potassium exists in the soil in several forms (Figure 15). Plants absorb potassium (K+) from only the ionic form in soil solution. Exchangeable potassium from the soil colloids (clays and humus) is readily available, for this form enters easily into the soil solution. Nonexchangeable potassium is fixed in the lattice structure of clays. It is trapped in the structure and is not released unless some mechanism opens the lattice to permit the potassium to diffuse into the soil solution. The nonexchangeable fraction is from 2% to 10% of the total soil potassium and represents a reservoir of slowly available potassium from which a plant may draw during the growing season. The release of potassium from the nonexchangeable sites depends on the types of clay, moisture, pH, and presence of other cations in the soil. Almost all of the potassium in the soil is in the primary minerals or slowly available fraction. These primary minerals are feldspars and micas, which are derived from the weathering of rocks from the parent material. They are resistant to weathering further and are very slowly soluble; hence, the amount of potassium released from this fraction is very small although the total amount present is large.

Figure 15. Equilibrium relationships between forms of potassium in soils.

Previously, it was believed that soils did not have to be fertilized with potassium, for tests for total soil potassium, which includes the primary minerals, revealed an abundance of potassium. After World War II, when the production and consumption of commercial nitrogen and phosphorus fertilizers increased, the potassium supplying power of many soils was exhausted. High yields brought about by fertilization with nitrogen and phosphorus placed high demands on the soil to supply potassium. The readily available soluble and exchangeable potassium and the slowly available nonexchangeable potassium were removed by the crops. The virtually unavailable potassium in the primary minerals remained but was unable to supply potassium rapidly enough to meet the needs of crops. A similar event occurred in the early agricultural history of the colonial United States. The practice of recommending that a fish be placed in a hill of corn ultimately failed because the fish provided nitrogen and phosphorus but little potassium. The enhanced corn production brought about by fertilization by fish soon depleted the soils of their available potassium, and yields were diminished afterwards. Today, several potassium fertilizers are marketed individually or in mixed carriers (Table 21).

 

Table 21. List and characteristics of some common potassium fertilizers

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----------Potassium fertilizer----------------

-----------------Characteristics---------------------------

Availability of potassium

Kind

Type

Total%K20

Release of K

Potassium chloride

Chemical

60

Rapid (water-soluble)

Potassium sulfate

Chemical

52

Rapid (water-soluble)

Potassium magnesium sulfate

Chemical

21

Rapid (water-soluble)

Potassium nitrate

Chemical

44

Rapid (water-soluble)

Wood ashes

Organic

5-10

Rapid (water-soluble)

Seaweed

Organic

6

Rapid (water-soluble)

Greensand

Organic

6

Unavailable

Granite dust

Organic

5

Unavailable

Farm manures

Organic

2

Rapid (water-soluble)

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The highest quality materials are chemical fertilizers. The organic farmer and gardener has difficulty in preventing depletion of soil of its potassium, for they do not have good fertilizers in an abundant supply to replenish the potassium removed by crops. The organic farmer and gardener depend on plants to mine the soil for available potassium.

Chemical Fertilizers for Potassium

Potassium chloride (KCl). In the trade, this fertilizer is known as muriate of potash, with its name being derived from muriatic acid (hydrochloric acid). It is the most widely used potassium fertilizer in the world, constituting as much as 95% of the total consumption of potassic fertilizers. Most of the potassium chloride comes from refining of mined products of deep salt deposits on the earth. The world's largest deposit of high-grade ore is in Saskatchewan, Canada. The United States is a relatively minor producer, along with Germany, France, Spain, Ukraine, Israel, and Zaire. Potassium chloride is the most inexpensive carrier of potassium on the market.

Potassium chloride is a water-soluble, concentrated fertilizer. Although it is naturally occurring, its high solubility and high analysis prevent its acceptance as organic. It is sold for direct application to soil and is the most common potassium carrier for mixed fertilizers.

Potassium sulfate (K2SO4). This fertilizer is manufactured by refining of ores by washing or by treatment of potassium chloride with sulfuric acid. Potassium sulfate has a lower analysis than potassium chloride and is more expensive. On a potassium-equivalent basis, potassium sulfate is as good as or better than potassium chloride for production of crops. Irish potatoes, tobacco, and maybe corn are sensitive to high amounts of chloride, and potassium sulfate is preferred by some growers of these crops. Most of the potassium sulfate used in the United States is used by tobacco growers.

Potassium magnesium sulfate (K2SO4.2MgSO4). This material, known in the trade as sulfate of potash magnesia, is produced by refining langbeinite ores by washing, which removes the more soluble KCl. It is a useful fertilizer in that it supplies potassium (21% K2O), magnesium (11%), and sulfur (21%). It is sold under several registered trade names for direct application to the soil. It is listed in some mail order catalogs as being an organic fertilizer. Its origin at the mine is no different from that of potassium chloride or sulfate, and it is also a refined product cleaned by washing. Perhaps, its lower analysis of potassium and somewhat slower dissolution than potassium chloride and its contents of magnesium and sulfur permit its acceptance as organic.

Potassium nitrate (KNO3). Potassium nitrate is known also as saltpeter or nitre and is viewed as being primarily a nitrogen fertilizer. It is worth considering as a potassium fertilizer, because of its relatively high analyses. High costs of production limit its widespread use. Its limited use is in some water-soluble concentrates and lawn fertilizers.

Organic Fertilizers for Potassium.

Wood ashes. Wood ashes are an excellent source of potassium. Burning of wood drives away all of the carbon and leaves an ash that is usually more than 5% K2O. Ashes from softwoods may be lower, and ashes from hardwoods may be higher than this analysis. After burning the potassium may be present mostly as potassium oxide, but with time it will react with the air, and potassium carbonate will be formed. In any case, the potassium in wood ashes is water-soluble and on a potassium-equivalent basis would be as good as any chemical fertilizer. Wood ashes are also high in calcium oxide or carbonate and are valuable as a liming material. Wood ashes have about half the liming equivalency of agricultural limestone. Care needs to be taken in application of wood ashes to avoid creating alkaline soils and what is known as the over-liming effect. Dumping or prolonged disposal of wood ashes on the same site can elevate soil alkalinity to levels that will inhibit plant growth. Reports of pH 11 in soils on which wood ashes are dumped are common. The limiting factor in the use of wood ashes in agriculture is their limited supply.

Seaweed and other plant materials. A truly remarkable phenomenon is that algae (seaweed) grow in the ocean and accumulate potassium and do not accumulate sodium. The concentration of sodium in sea water is nearly a hundred times the concentration of potassium; yet potassium is accumulated, and sodium is excluded by the seaweed. One does not have to worry about the transmission of sodium by application of seaweed to land. The kelp of seaweed can be washed free of any sodium that adheres to their external surface. The potassium that is held in the kelp will wash out only after the kelp is dead. The limiting factor in use of kelp is its limited supply and distribution and its high costs. Commercially available liquid seaweed, after it is diluted for application, has little value as a source of potassium because of its low analysis.

Essentially all vegetative plant materials are rich in potassium unless they have been grown under potassium-deficient conditions. Plant materials may range from 1% to 9% K2O. All of their potassium would be available, for it can be extracted from dead plants by water. Seeds and the flesh of fruits are low in potassium, but peelings, rinds, and hulls are comparable in potassium to that of vegetative organs. On an potassium-equivalent basis, plant materials are as good as chemical fertilizers. Low analysis and bulk limit their use as potassic fertilizers. Plant diseases, such as mosaic virus from tobacco, may present problems in the application of plant materials on crops that are susceptible to the diseases.

Manures. Farm manures, being of plant origin, are good sources of potassium, and if applied in quantities to meet the nitrogen requirements of a crop will also meet its potassium requirement. Dehydrated, unleached manures have about 2% K2O, and composted manures have about 1% K2O. Fresh manures with some bedding deliver about 10 lb of K2O per ton. Manures or composts that are subjected to leaching from rain water will have low concentrations of potassium, maybe no more than half that of the unleached materials.

Greensand and granite dust. Greensand is potassium glauconite , a naturally occurring mineral. It occurs in deposits of sand-sized materials in coastal New Jersey and vicinity. Its total K2O is about 6%, but none is water-soluble. Thus, none of the potassium is considered available. Granite contains minerals of feldspars and micas, which are potassium-containing minerals. Finely ground granite dust from Georgia or Massachusetts is sold as a source of potassium. These dusts are about 5% K2O, but potassium availability is nil. These materials are expensive and would have to be used in very large quantities to improve soil fertility. Some recommendations are to apply as much as 5 to 10 tons of granite dust or greensand per acre per year. To apply potassium in one application in these quantities could exceed the value of the land. In most soils, weathered granite dust already exists in the primary minerals. Further additions of granite dust may contribute little to the total potassium reserves in these soils and give negligible increases in crop yields.

Calcium Fertilizers

Effects of Calcium on Plant Growth

Calcium is necessary for cell division and growth by expansion. The layer of pectin that is between cells is a calcium-containing material. Calcium is needed for maintenance of membranes in cells. If calcium is deficient, nutrient absorption by plants may be perturbed. The growing points (vegetative buds and surrounding leaves) of calcium-deficient plants die. Shoots will die back to the first fully expanded or mature leaf. Dieback occurs only in conditions of severe calcium deficiency; however, a number of disorders occur in fruits and vegetables (Table 22). Produce that has these disorders is unmarketable or inedible.

Table 22. Disorders resulting from calcium deficiency of fruits and vegetables.

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Disorder

Fruits

Vegetables

Bitter pit (apple)

Blossom-end rot (tomato, pepper, watermelon)

Scald (apple)z

Blackheart (celery)

Internal breakdown (apple)

Internal tipburn or browning (lettuce, cabbage, brussels sprouts)

Water core (apple)

Brownheart (escarole)

Cracking (cherry)

Cavity spot (carrot)

Soft nose (mango, avocodo)

Unfilled kernels (peanut)

Pod split (bean, pea)

______________________________________________________________________________________________

z Possibly calcium related disorder

Fertilization with Calcium

Fertilization of plants with calcium normally occurs with the application of the primary macronutrients or with liming of soil. Many of the carriers of nitrogen and phosphorus are calcium-containing compounds (Table 23). All liming materials contain calcium. Gypsum (calcium sulfate), which is used as an amendment to improve structure of clayey soils, supplies calcium. Calcium sulfate is applied specifically to increase available calcium to peanuts. Calcium chloride (CaCl2) is one of the rare materials that are applied specifically to crops to improved their status with respect to calcium nutrition. It is used occasionally to increase the calcium supply to apple fruits.

 

Table 23. Fertilizers or soil amendments that contain calcium.

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Material

Use

% Calcium

Calcium nitrate

Nitrogen fertilizer

24

Superphosphate

Phosphorus fertilizer

21

Triple superphosphate

Phosphorous fertilizer

17

Bonemeal

Phosphorus fertilizer

35

Wood ashes

Potassium fertilizer

20

Gypsum

Soil amendment

29

Calcite

Lime

40

Dolomite

Lime

21

Calcium chloride

Calcium fertilizer

36

________________________________________________________________________

If farm manures or composts are suppied in quantities to meet the nitrogen requirement of plants, the calcium requirement will be satisfied also. If rock phosphate is supplied in sufficient amounts and under conditions that supply adequate phosphorus nutrition, sufficient calcium will be available.

Plants are unable to mobilize calcium from one portion of the plant to another portion. Calcium which enters a leaf or another organ cannot be transported to young, growing areas such as growing points or fruits. For example, old leaves of cabbage may have an abundance of calcium, whereas young leaves at the center of the head will be showing internal tipburn. Plants must have a constant supply of calcium from the soil to avoid deficiencies.

Calcium deficiency occurs frequently in dry weather. Blossom-end rot of tomato, pepper, and watermelon often can be corrected by irrigation without addition of calcium to the soil. In dry weather, restricted water uptake limits calcium absorption, for research shows that calcium enters plants passively and is distributed in plants with the flow of water. Also in dry soils, the concentrations of soluble potassium salts may be sufficiently high relative to calcium salts so that potassium absorption competively suppresses calcium absorption. Irrigation helps to overcome these difficulties in calcium absorption and transport.

Generally, calcium cannot be supplied to plants by foliar sprays, for calcium that enters through the foliage will remain in the foliage that absorbs the calcium. Foliar sprays of calcium chloride on apple has provided some success in preventing disorders of the fruits.

Magnesium Fertilizers

Functions and Effects of Magnesium on Plant Growth

Magnesium is a constituent of chlorophyll, and magnesium activates more enzymatic reactions than any other cation. It is a cofactor in reactions participating in photosynthesis, respiration, and protein synthesis. Magnesium in leaves ranges from 0.15% to 1% of their dry weight. The threshhold value for sufficiency is about 0.2 to 0.3%.

Magnesium-deficient plants have mottled leaves, which are yellow, bleached, or necrotic between the veins and green along the veins. This mottling is due to a combination of the effects that magnesium deficiency has on protein synthesis and chlorophyll synthesis.

Fertilization with Magnesium

Soils are enriched with magnesium when they are limed (Table 24). Limestones are grouped chemically into calcite and dolomite. Calcite is calcium carbonate, and dolomite is a 1:1 molecular mixture of calcium carbonate and magnesium carbonate. In nature, deposits of limestone are rarely, if ever, pure deposits of calcite or dolomite; therefore, agricultural limestones and liming materials derived from limestones are mixtures of calcite and dolomite and have varying magnesium contents. The magnesium concentration of limestones is provided on the package in which they are sold or on other labels for bulk sales. The concentrations of actual magnesium in limestones commonly vary from 0.6% to 13%, averaging about 5%.

Epsom salts (magnesium sulfate, MgSO4.7H2O, 20% Mg) is water soluble. Magnesium deficiency on foliage can be corrected by sprays of 10 g epsom salts dissolved in 20 liters (0.33 oz in 5 gal) of water. Magnesium is mobile in plants, so it will move from one area of the plant to another. Although a complete spraying of the plants with the salt solution is desirable, complete coverage is not mandatory, for magnesium will be transported to sites where it is needed. Since foliar symptoms of magnesium indicate depletion of available magnesium in the soil, repeated sprayings will be needed. Spraying should begin as soon as the deficiencies are diagnosed. Recovery of severely stressed tissues will not occur.

Potassium magnesium sulfate (11% Mg) is used primarily as a potassium fertilizer. Fertilization of a crop with potassium magnesium sulfate to meet the potassium requirement will satisfy the magnesium needs. Manures and composts contain sufficient magnesium so that crops supplied with an ample supply of nitrogen from these materials will have adequate magnesium nutrition. Talc, the mineral in soapstone, is used occasionally as an organic source of magnesium.

 

Table 24. Fertilizers or soil amendments that contain magnesium.

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Material

Use

Percent magnesium

Dolomite

Lime

13

Magnesium sulfate

Magnesium fertilizer

20

Potassium magnesium sulfate

Potassium fertilizer

11

Wood ashes

Potassium fertilizer

5

Talc (soapstone)

Magnesium fertilizer

19

________________________________________________________________________

Sulfur Fertilizers

Functions and Effects of Sulfur on Plant Growth

Sulfur is needed for protein synthesis in that it is a component of the amino acids, cysteine and methionine. Sulfur is a constituent of several vitamins that have important roles in plant metabolism. The characteristic odor and taste of plants in the mustard family (radish, turnips, cabbage, caulflower, broccoli, mustard, and others) and of onions and garlic are due to sulfur-containing compounds. Since sulfur is needed for protein synthesis, symptoms of its deficiency resemble those of nitrogen deficiency with the exception that the symptoms of sulfur deficiency appear first on the young leaves rather than on the old leaves. Sulfur is not as mobile in the plant as nitrogen, and sulfur in the old leaves cannot be transported rapidly enough to the young leaves to prevent deficiency after the available sulfur in the soil has been depleted. Normal leaf tissue has from 0.15% to 0.5% actual sulfur, about the same concentration as for phosphorus. Deficiencies may occur if the concentration in leaves falls below 0.15%.

Fertilization with Sulfur

Most of the sulfur in soils of humid regions is in the organic matter. In dry areas, sulfur may be precipitated in the soil as gypsum (calcium sulfate). The annual raindown of sulfur (5 to 15 lb S/acre) meets part of the sulfur requirements of crops. Sulfur additions to the soil may be made in the process of fertilizing the soil with the other macronutrients (Table 25). Applications of sulfur to land with fertilizers is diminishing, for many of the modern, high-analysis fertilizers are free of sulfur. Urea, ammonium nitrate, ammonium phosphates, triple superphosphate, and potassium chloride which are used in formulation of concentrated fertilizers contain no sulfur. Ammonium sulfate, ordinary superphosphate, and potassium sulfate are used infrequently for direct application to the land or in the manufacture of mixed fertilizers. Limestones add virtually no sulfur. Sometimes elemental sulfur or aluminum sulfate are used to acidify soil.

Possibly, in heavily cropped soils in the future, sulfur deficiency may become common because of the lack of return of sulfur back to the land.

Table 25. Fertilizers or soil amendments that contain sulfur.

________________________________________________________________________

Material

Use

Percent sulfur

Ammonium sulfate

Nitrogen fertilizer

24

Potassium sulfate

Potassium fertilizer

18

Potassium magnesium sulfate

Potassium fertilizer

21

Ordinary superphosphate

Phosphorus fertilizer

8

Gypsum

Soil amendment

23

________________________________________________________________________

 

Rock phosphate provides essentially no sulfur other than that which might be present as a contaminate. Organic farmers and gardeners supply sulfur through applications of farm manures and composts. Fertilization to satisfy the nitrogen needs of a crop with farm manures or composts also will meet its sulfur requirement.

Micronutrients

Because plants accumulate small amounts of iron, copper, zinc, manganese, molybdenum, boron, and chlorine, these elements are called micronutrients. Sometimes these nutrients are called minor elements or trace elements. These names refer to the the small amounts that are required and to the infrequent occurrence of deficiencies of these elements. Most growers normally do not apply specialty fertilizers for micronutrients and rely on the soil to provide ample supplies of these nutrients. In most cases, this practice is acceptable. Chlorine deficiency has never been observed in nature, but deficiencies of the other micronutrients occur sporadically and warrant some attention. Deficiencies of minor elements are not easy to diagnose, expert advice should be sought before applying of fertilizers to supply minor elements.

Iron, zinc, copper, and manganese

Deficiencies of the metallic micronutrients (iron, copper, zinc, manganese) except molybdenum may occur in alkaline soils, in heavily leached soils, and in organic soils. Carbonates, hydroxides, and phosphates of these elements are sufficiently insoluble in alkaline soils at pH 7.5 or above that availability of these nutrients is reduced to the level of deficiency. In acid, sandy soils, these elements may be leached to the level of deficiency. In organic soils (peats, mucks), they may be held so tightly in organic complexes that they are unavailable or, in some cases, may be leached to deficient levels.

Iron deficiency develops first on the youngest leaves and may advance to older leaves but seldom appears on fully mature leaves. Symptoms are characterized by interveinal (early stages) or uniform light-green or yellow color or even bleaching to white leaves. Leaves may die so that the plant dies back, or even the whole plant may die. Iron deficiency is a world-wide problem in alkaline soils. The deficiency under this condition is referred to as lime-induced chlorosis. Acidifying the soil with organic matter, sulfur, or ammonium sulfate may premit sufficient native soil iron to dissolve to provide adequate nutrition (Table 26).

Table 26. Approximate amount of soil amendment to apply to lower pH one unit.

____________________________________________________________________

Application l lb/100 sq ft

Material

Sands

Loams

Clays

Sulfur

2

3

4

Aluminum sulfate (alum)

12

18

24

Organic matter

50

75

100

_____________________________________________________________________

Applying soluble iron salts to alkaline soils gives no relief or only temporary relief from iron deficiency. Chelated or sequestered iron compounds (Table 27) may be applied to the soil or as a foliar spray. The best success is achieved by application to the soil (Table 28). Iron chelates protect the iron from fixation as hydroxides, carbonates, or phosphates and help to keep the iron in solution.

 

Table 27. List of iron chelates.

________________________________________________________________________

 

Name of compound

Condition of use

Common

Scientific

FeEDTA

Iron ethylenediamine tetraacetate

Acid soils

FeHEEDTA

Iron hydroxyethyl-

ethylenediamine triacetate

Neutral soils

FeDTPA

Iron diethylene triamine

pentaacetate

Neutral soils

FeEDDHA

Iron ethylenediamine di-(o-hydroxyphenylacetic acid)

Alkaline soils

________________________________________________________________________

 

Table 28. Application of iron chelates to correct deficiencies in soils.

________________________________________________________________________

 

Kind of crop

--------------------------Application of chelate--------------------------------

Amount

Method

Trees

0.25 lb to 0.5 lb/tree

Apply under crown with enough water to carry to roots

Shrubs

1 oz/shrub

Same as for trees

Vegetables

1 oz/100 sq ft

Dissolve in water and apply to base of plants

Flowers

1 oz/100 sq ft

Same as for vegetables

Lawns

1 oz/100 sq ft

Spray across surface

Potted plants

0.1 oz/pot

Apply with water

___________________________________________________________________________________

Foliar feeding of iron is of questionable value for most plants. Iron does not enter into the leaves readily, and high concentrations of iron salts or chelates applied foliarly in solution may be toxic. If a grower wants to employ foliar spraying, the concentration of iron in the sprays should be in the range of 10 ppm to 50 ppm (mg/kg). The spray should be applied over the plant to the point of runoff.

Plants with sufficient nutrition have from 100 to 200 ppm iron in their foliage. Deficient concentrations occur in the range of 25 to 50 sometimes up to 100 ppm with some vegetables.

Zinc deficiency in most plants affects the termninal growth first. Early stages appears as interveinal chlorosis which produces mottling of leaves of trees. Acute deficiency appears as little leaf, rosetting, or dieback. Corn shows white striping or banding on the lower half of leaves. Plants with sufficient zinc nutrition have from 30 to 200 ppm (mg/kg) in their foliage. Deficient levels of zinc vary with species but are 5 to 25 ppm, with the lower end of this range being woody plants such as fruit trees and the upper limit being herbaceous plants.

For trees, shrubs, fruit-bearing trees, and field or garden crops, zinc can be applied as foliar sprays of zinc sulfate (0.1 to 0.3% solution of compound by weight) or zinc chelate (ZnEDTA, ZnHEEDA). These chelates are comparable to those of iron (Table 27), except they have zinc as the complexed metal. Zinc sulfate can be applied to the soil of garden and field crops at amounts of 10 to 20 lb/acre (0.25 to 0.5 lb/1,000 sq ft). Most of the soil-applied zinc remains about where it is applied as its mobility in soil is low, but the mobility is sufficient to provide zinc to a crop. Zinc chelates (about 1 lb/tree) may be applied to the soil around trees to provide adequate zinc nutrition for 2 or 3 years.

Excesses of zinc may occur with repeated applications. Symptoms of excess zinc resemble iron deficiency. Applications of lime or phosphates help lower the solubility of zinc and its absorption by plants. A problem of excessive phosphate nutrition of plants is suspected to be zinc deficiency.

Copper deficiency is not diagnosable until symptoms become advanced. The terminal growth is the first to be affected, but often symptoms are nonspecific. Small or abnormally large leaves, dark green or chlorotic leaves, necrotic spotting, rosetting, witches' brooms, or dieback may develop depending on species. Bark of trees may become rough. Heads of lettuce or cabbage may fail to form. Plants typically have 5 to 25 ppm (mg/kg) copper in their foliage. Deficiencies appear in foliage with less than 4 ppm copper.

Copper is applied as copper sulfate (CuSO4.5H2O) at 5 to 25 lb/acre to mineral soils and at 100 lb/acre to organic soils. In orchards, 1 or 2 lb copper sulfate applied to the soil under each tree is sufficient. Foliar sprays used to control diseases are valuable emergency treatments. For example, plants respond quickly to the copper in Bordeaux mixture (5 to 10 lb copper sulfate and an equal amount of calcium hydroxide in 100 gallons of water).

Single applications of copper produce residual effects that last for many years, for copper is held tightly to the organic matter in all soils and is not subject to leaching. Also, the amount of copper removed with a harvested crop is small, being of the order of 1 lb/acre per year. Repeated applications of copper to meet the nutritional requirements of plants are not needed. Excesses of copper may produce stunting of plants or symptoms resembling the chlorosis of iron deficiency. Virtually all of the copper applied to soils remains in the top few inches of the ground where it may build up to toxic concentrations. Excesses of copper in soil are associated usually with copper-containing compounds applied to control diseases.

Manganese deficiency appears as chlorosis on new flushes of growth and closely resembles iron deficiency. In severe cases with fruit trees and broadleafed vegetables, light green areas become grey, white, or necrotic; entire leaves become dull yellow green, and twigs may dieback. Corn develops yellow and green striping over the full length of the leaves. Manganese concentrations in plants vary with species, but generally the range of sufficiency is from 25 to 500 ppm (mg/kg) manganese in the foliage with the deficiency threshhold being 10 to 25 ppm.

Foliar sprays of manganese sulfate often are more successful than soil fertilization. A Bordeaux-like mixture of a 5% by weight of manganese sulfate and an equal amount of calcium hydroxide in water makes a satisfactory spray for foliar application to fruit trees. Ground applications of 2 to 4 lb manganese sulfate per tree are effective in the second season after application.

Molybdenum. Plants need molybdenum for the metabolism of nitrogen. Nitrate assimilation requires molybdenum. Fixation of nitrogen by free-living microorganisms or by those living in symbiosis with plants such as legumes requires molybdenum. Molybdenum-deficient legumes may appear, and may in fact be, nitrogen deficient because they are unable to fix atmospheric nitrogen. Legumes or nonlegumes may appear nitrogen deficient because they are unable to assimilate nitrate. Deficiency symptoms appear when concentrations in foliage are under 0.1 ppm molybdenum (0.1 mg/kg). Marginal scorching and rolling and cupping of leaves occurs. Whiptail of cauliflower and yellow-spot of citrus are easily recognizable disorders. Symptoms rarely appear on grasses. Although plants require minute amounts of molybdenum, toxicity from molybdenum is rare, and plants may accumulate molybdenum to several 100 ppm with no toxic effects.

Unlike the other metallic elements, the availability of molybdenum increases with rise in pH. Below about pH 5.5, the solubility of molybdenum is so low that it is deficient for growth of some crops. Except for the rare soils that are naturally low in molybdenum, liming of the soil to above pH 5.5 will allow for release of soil-borne molybdenum to sufficient levels. Notable historical cases of molybdenum deficiency occurred with cauliflower grown in unlimed fields previously cropped to potatoes with soil at pH 5.3 to control scab disease.

Fertilization with molybdenum is rare. Seeds from plants grown in normal areas often contain enough molybdenum to meet the needs of a crop. Ammonium or sodium molybdate salts may be applied at 2 oz to 1 lb per acre to the soil or foliage of crops. Because of the small amounts applied, molybdenum fertilizer is diluted with sawdust, sand, or another fertilizer. Since molybdenum deficiency frequently occurs in association with phosphorus and sulfate deficiency, molybdenized superphosphate is a frequently used carrier that provides molybdenum, phosphorus, and sulfur. Except in acid soils below pH 5.5 in which molybdenum is rendered unavailable quickly, a single application of molybdenum will last for several years. Economics and convenience dictate whether liming of soil or foliar fertilization will be employed. In a gardening situation, liming is probably the better practice.

Boron. Deficiencies of boron occur more frequently than deficiencies of any other micronutrient with the possible exception of iron. Celery, cauliflower and other cole crops, apple, pear, alfalfa, clovers, and beets commonly express deficiencies of boron and are considered to be good indicator crops for identifying boron-deficient soils. Boron-deficient soils include acid leached sandy soils, acid leached organic soils, mineral soils low in organic matter, and soils with high pH. Most of the boron in soils of humid regions is in the soil organic matter and that which is not in organic matter is water soluble and subject to leaching. In organic soils, much of the boron is leached and depleted after the decay of the fresh organic matter into the more stable peat or peat-like matter. The solubility of boron is low in alkaline soils with free calcium carbonate. Soils that are derived from igneous rock or from rocks of marine deposits are naturally low in boron.

In early stages of development, boron deficiency is not identifiable by visual diagnosis. Tissue analysis is necessary to detect incipient deficiencies. Acute deficiency has characteristic symptoms, and generally by the time of their appearance, the crop is lost. Many disorders of fruits and vegetables result from boron deficiency and result in a totally unmarketable crop or undesirable product. Boron deficiency affects zones of cell division. The terminal buds and young leaves die. Rosetting and witches' brooms may occur. Plants may appear bushy because of all of the lateral branches that develop. These lateral branches later die back. Leaves and stems become thickened and brittle. Fruits, stems, tubers, and roots may fleck, crack, dry rot, or be blackened and water-soaked.

 Table 29. Disorders caused by boron deficiency in plants.

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Crop

Disorder

Apple

Cracking; corking; brown lesions with bitter taste

Beets (table & sugar)

Crown rot; heart rot and dry rot of roots

Cauliflower, cabbage, broccoli

Hollow stem; brown curd of cauliflower

Celery

Cracked stem

Tomato

Open locule; internal browning, and darkened, dried covering of fruits

Turnip and rutabaga

Brown heart

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 Some of these disorders are identified readily from external symptoms, but some of the diseases such as brown heart of turnip may not be detected until the roots are cut.

Plants with sufficient boron nutrition have between 25 and 100 ppm (mg/kg) in their leaves. Concentrations below 20 ppm are in the deficient zone. Concentrations above 200 ppm are associated with symptoms of excessive boron nutrition. Plants are sensitive to toxicity from boron, as is evident from the narrow range of concentrations between deficient and excessive nutrition. Moderate care needs to be taken in rotation of crops that have a high requirement for boron with crops that have are sensitive to boron toxicity. Fertilization of a crop with a high boron requirement may lead to excesses of boron in the soil for the succeeding crop. Generally, however, boron fertilizers are soluble and will leach from the root zone so that the possibility of toxic carry over of boron from one season to another is unlikely.

If boron deficiency is not noted, boron-containing fertilizers do not need to be applied. If deficiency symptoms are apparent or were detected in a previous crop, the following amounts of boron are recommended (Table 30).

 

Table 30. Suggestions for application of boron to selected crops.

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Crop

Application,

lb boron/acre

Alfalfa

1 to 4

Apples

1 to 3

Beets

1 to 2

Broccoli

2 to 4

Cabbage

1 to 3

Cauliflower

2 to 4

Citrus

1 to 2

Clovers

1 to 2

Corn

1 to 2

Strawberries

1 to 2

Tomato

1 to 2

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Boron may be sprayed on foliage or applied to the soil. Soil-applied boron remains effective longer that foliarly applied boron. Some fertilizers and methods of application are listed in Table 31.

The spray materials are applied directly to the foliage of the crop. Boric acid is water soluble. Solubor (U. S. Borax and Chemical Corporation) is partially dehydrated pentaborate specially formulated for application in solution. The materials of lower solubility are used for direct application to soil. The common practice, because of the low amounts of materials needed, is to mix borax or coarse granules of borates with sand or with fertilizer for soil application. Boron frits are fragments of borosilicate sintered glass. The fragments have a large surface area and are relatively insoluble for slow, steady release of boron to the soil and plant roots. Finely granulated borates are blended with fertilizers which are use provide macronutrients. For example, a 0-10-40 WB could be used to provide potassium. A typical blend might have 0.45% boron. If this fertilizer were used to provide, 100 lb of K2O per acre, 250 lb of the fertilizer would be applied and would deliver 1.13 lb of boron per acre, as well as 25 lb of P2O5.

 

Table 31. Fertilizers for application of boron to crops.

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Fertilizer

Boron (%)

Application

Borax

11

Soil

Boric acid

17

Spray or soil

Boron frits

10 to 17

Soil

Sodium tetraborate

hydrate

14

Soil

anhydrous

20

Soil

Sodium pentaborate

18

Soil

Solubor

20

Spray

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Maroon Divider
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Maroon Divider

Produced and maintained by Your Name Allen V. Barker
University of Massachusetts, Amherst.
last updated - April 25, 1999