Laboratory Manual

Experiment # 5 Use of phosphorus and potassium fertilizers

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Phosphorus. With the possible exception of nitrogen, no other plant nutrient is more critical for crop production than phosphorus. It is needed for cell division, flowering, fruiting, and seed formation. It enhances crop maturity, counteracting the adverse effects of over applications of nitrogen. Straw or stem strength is increased by phosphorus fertilization of small grains, and lodging is reduced. The development of branch or fibrous roots is dependent upon an adequate supply of phosphorus. Crop quality and resistance to some diseases is increased by phosphorus fertilization.

The phosphorus contents of plant tissues are seldom high, ranging from 0.1 to 0.4% P on a dry weight basis. Plant tissues may be 1 to 5% N and 1 to 6% K on a dry weight basis. Legumes tend to be somewhat higher in phosphorus than nonlegumes and are normally more sensitive to phosphorus shortages in the soil.

Phosphorus can be supplied to soils as commercial fertilizers, natural rock materials or animal residues. Manure and plant residues are not good phosphorus sources. Several good commercial sources are of natural origin and hence can be used by the organic gardener. Several of the commercial man-made fertilizers are derived from natural materials treated with acids. Untreated natural phosphate materials and bonemeal are not very soluble and are usually most effective when applied to acidic soils.

Plants which are deficient in phosphorus are usually stunted and show a reddish or purple discoloration of the foliage, particularly visible on the undersides of leaves.

Potassium. The essentiality of potassium in plant nutrition has been known for a long time. Only relatively recently, however, have potassium fertilizers been used to correct potassium deficiencies in plants. Most soils were originally high in potassium, and many years of cropping were needed to deplete them of this nutrient. Increased crop yields from the application of nitrogen and phosphorus fertilizers have enhanced the drain of potassium from the soil. Cropping of land may have also increased erosion and leaching losses of potassium. Widespread potassium deficiencies have been known for only the last 25 to 30 years.

The precise function of potassium in plants is virtually unknown. All of the potassium in the plant is inorganic and soluble. Potassium appears to be essential for protein synthesis and stability. Potassium is important in carbohydrate metabolism. It strengthens stems and increases plant resistance to diseases and lodging. Applications of potassium tend to delay maturity, thus balancing the effect of phosphorus applications which accelerate maturity of a crop. Potassium is essential for chlorophyll formation. Starch formation appears to require potassium, and potassium-deficient crops tend to have shriveled grain or tubers. All root crops need liberal applications of potassium.

Potassium may be added to soil by returning residues high in potassium. Tobacco stems are used as an "organic" source of potassium. Natural sources are materials such as granite dusts or greensand (glauconite). Commercial chemical sources are usually readily soluble potassium salts, many of which are of natural origin but purified or refined. Many potassium-rich ores are used directly, providing a soluble potassium fertilizer of natural origin.

These ores are called manure salts or mine-run potash salts and are 20 to 30% K2O. Potassium deficiency in a crop is characterized by drying and scorching at the edges of leaves. In many plants, these symptoms are preceded by the formation of small dots arranged somewhat regularly near and around the edge of the leaves.

One of the symptoms of potassium deficiency in tomato is the appearance of lesions on the stems when the plants are given ammonium nitrogen fertilizers. In order for a potassium fertilizer to be adequate, it must prevent ammonium toxicity which is manifested by these lesions.

Experiment

1. PHOSPHORUS. The objective of this experiment is to evaluate the effects of phosphorus source, soil pH, and organic matter on the availability of phosphorus to plants.

You are provided with three phosphorus carriers:

You are also given a soil which is deficient in phosphorus and which has a pH of 4.8.

Peat moss will serve as your organic matter source.

Agricultural limestone will be used as a liming material.

A solution of KNO3 (potassium nitrate) will be used as a source of potassium and nitrogen; this material will not add any additional organic matter to the system or change the pH of the soil appreciably upon addition or with growth of the plants.

a. Using the materials above, create the following conditions:

(1). Colloidal rock phosphate in acid soil (pH 4.8).

Add 2 g of colloidal rock phosphate per pot (1500 g) of soil.

Fill pot to within one inch of top.

(2). Colloidal rock phosphate in limed soil (pH 6 to 6.5).

Add 2 g of colloidal rock phosphate, per pot (1500 g) of soil and mix in 10 g of agricultural limestone.

Fill pot to within one inch of top.

(3). Colloidal rock phosphate in acid soil plus organic matter.

Add 2 g of colloidal rock phosphate and 100 g of compost or peat moss per pot of soil. Note: To keep the volume of soil and organic matter within one pot, measure one pot of soil, mix in the peat moss, then refill pot to within one inch of the top, and discard excess soil mixture. Dump the soil on the bench and mix in the rock phosphate.

(4). Colloidal rock phosphate in limed soils plus organic matter.

Add 2 g of colloidal rock phosphate, 10 g of limestone, and 100 g of peat moss to a pot of soil. Proceed as in #3 above.

Continue with part a of the experiment to set up the treatments with bonemeal, rock phosphate, and superphosphate.

(5). Bonemeal as in 1, 2, 3, and 4 above.

(6). Superphosphate as in 1, 2, 3, and 4 above.

(7). No phosphate fertilizer as in 1, 2, 3, and 4 above.

b. Transplant one tomato seedling into each pot of soil. Be certain to select uniform plants.

c. Water the plants with the KNO3 solution. Repeat this treatment weekly until the experiment is completed.

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d. Make the following measurements on the plants at harvest. Results

------Material added------

--------Height, cm ---------

-------Shoot weight, g------

---Deficiency symptoms*--


Acid Soil

No phosphate

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Rock phosphate

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Bone meal

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Superphosphate

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Limed Soil

No phosphate

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Rock phosphate

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Bone meal

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Superphosphate

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Acid soil plus organic matter

No phosphate

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Rock phosphate

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Bone meal

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Superphosphate

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Limed soil plus organic matter

No phosphate

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Rock phosphate

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Bone meal

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Superphosphate

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*Use the following numerical ratings for deficiency symptoms; 0, no symptoms; 1, slight; 2, moderate; 3, severe symptoms.

Other comments________________________________________________________________________

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Phosphorus-deficient tomato, Rating =3, severe

Close-up view of underside of leaf

POTASSIUM. The objective of this experiment is to evaluate the effects of three potassium fertilizers for supplying potassium to plants. Conditions of potassium deficiency will be created in the soil and plants through the use of heavy applications of ammonium sulfate.

You are provided with four potassium fertilizers:

You will use the greenhouse potting soil (7 parts soil: 3 parts peat moss: 2 parts sand) for this experiment.

a. Mix into the potting soil the K fertilizers as follows:

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-------------------------------------------Amount of fertilizer added, g/pot-------------------------------------------------------

--------Greensand-----------

----------Kelp---------------

---------Wood Ashes--------

-----------KCl---------------

0

0

0

0

15

5

2

0.3

30

10

4

0.6

60

20

8

1.25

120

40

16

2.50

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b. Transplant one tomato seedling into each pot of soil. Be certain to select uniform plants.

c. Water plants with tap water. Allow the plants one week to overcome transplanting shock and to become established in the new medium. At this point begin treating the plants with an ammonium sulfate solution. This solution will be given to the plants daily.

d. After 21 days of ammonium treatment, observe whether or not stem lesions appear on the plants. Rate lesion development as follows: 0 --no lesions; 1 -- a few lesions on the stems; 2 -- several lesions on the stems but lesions remain distinctly separate; 3 -- lesions run together and completely cover some areas of the stem (0, no ammonium toxicity; 1, slight toxicity, 2, moderate toxicity, 3, severe toxicity).

Ammonium Toxicity Ratings. Results

-------------------------------------------Amount of fertilizer added, g/pot-----------------------------------------------------

--------Greensand-----------

----------Kelp---------------

---------Wood Ashes--------

-----------KCl---------------

15 __________________

5 __________________

2 __________________

0.3 __________________

30 __________________

10__________________

4 __________________

0.6 __________________

60 __________________

20 __________________

8 __________________

1.25 _________________

120 __________________

40 __________________

16 __________________

2.50 __________________

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Plant Weights at Harvest, g/plant. Results

-------------------------------------------Amount of fertilizer added, g/pot-----------------------------------------------------

--------Greensand-----------

----------Kelp---------------

---------Wood Ashes--------

-----------KCl---------------

15 __________________

5 __________________

2 __________________

0.3 __________________

30 __________________

10__________________

4 __________________

0.6 __________________

60 __________________

20__________________

8 __________________

1.25 __________________

120 __________________

40__________________

16 __________________

2.50 __________________

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Answer the following questions.

1. Describe the deficiency symptoms which developed on the plants grown on the various phosphatic fertilizers or with no phosphorus fertilization.

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2. Account for the effects or lack of effects of the following factors on alleviating phosphorus deficiency in plants.

a. Soil pH ____________________________________________________________________________

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b. Organic Matter _____________________________________________________________________

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c. Rock phosphate ____________________________________________________________________

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d. Bone meal _________________________________________________________________________

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e. Superphosphate_____________________________________________________________________

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f. Interactions (Identify and describe any factors that acted together to affect the results) ___________________

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3. Were the results of the experiment with phosphorus fertilizers consistent with the results which one would expect to obtain?

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If not, speculate why not.

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4. Describe how you would use each of the following fertilizers in your own garden or farm. If you do not have a plot of land, use the following conditions: Area, 1 acre; soil, a fine sandy loam, 1% organic matter (humus), pH 4.8, well-drained and level, phosphorus test: 120 lb P2O5 required per acre.

a. Rock phosphate _____________________________________________________________________

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b. Bonemeal___________________________________________________________________________

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c. Superphosphate_____________________________________________________________________

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5. What is the relationship between the potassium-supplying power of a soil or fertilizer and the appearance of ammonium injury symptoms?

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6. Comment on the value of the following potassic fertilizers. What special precautions should be taken when using these fertilizers?

a. Granite dust_________________________________________________________________________

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b. Wood ashes________________________________________________________________________

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c. Potassium chloride ______________________________________________________________

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 d. Tobacco Stems or Kelp_________________________________________________________________

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Produced and maintained by Allen Barker
University of Massachusetts, Amherst.