PLSOIL 120
ORGANIC FARMING AND GARDENING

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CHAPTER 3

MANAGEMENT OF FARM MANURES

 

Benefits of Application of Farm Manures to Land

The principal benefit obtained from application of farm manures to land is the delivery of plant nutrients that are in the manures. Applications for fertilization of crops are based on the nitrogen contents of manures. Generally if the nitrogen requirement of crops is met by manuring, the potassium, calcium, magnesium, and sulfur requirements will be met also. Increased organic matter content of soil occurs with applications of farm manures. To increase organic matter in a few years, applications of manures should exceed those that are required for fertilization of crops. Smaller applications of manures will increase organic matter in the long term. Well-rotted manures make good mulches for moisture conservation and possibly weed control in crops. Farmland is often a site for disposal of farm manures from livestock and poultry production. Care must be taken in disposal of manures on land to ensure that the land is not overloaded with nitrogen.

Composition of Farm Manures

The value of farm manures with respect to contents of nutrients varies with source and handling of the manures. A ton of average fresh manure without bedding from large farm animals (cattle, hogs, horses) will have about the same amounts of nitrogen, phosphorus, and potassium as 100 lb of 10-5-10 commercial fertilizer. Bedding in the manure will dilute its nutrient content. Some manures may have so much bedding from straw, hay, or sawdust that the manures should be composted before they are applied to the land or additional nitrogen must be added to overcome the potential for nitrogen immobilization if the manures are incorporated in the soil. Fresh manures from sheep may have twice the nitrogen and potassium as manures from other large animals. Poultry manures may have three times as much nitrogen and twice as much phosphorus as manures from other farm animals.

Manures often are marketed as dehydrated or composted products. The dehydrated manures are ones from which water has been removed. Manures from large animals will have a dry composition of 2-1-2 (%N, %P2O5, %K2O). Dried chicken manures are about 3-1-2. Composted manures have been rotted before bagging and will have a composition of 1-1-1. Composted commercial manures also have a considerable water content, as much as 25%. In general, the dehydrated manures are the superior product.

Farm manures contain some of all of the elements that are essential for plant growth, but the nitrogen content of a farm manure governs the amount of manure that is applied to land. In general, if enough manure is applied to meet the nitrogen needs of a crop, the needs for most of the other nutrients will be met (Table 11). The phosphorus concentration in farm manures is low, and they are not considered to be adequate materials to provide phosphorus in sufficient total or available amounts to a crop in the short term. The same reasoning applies for minor elements. Generally, another fertilizer should be used to supplement the phosphorus from manures. Often, the supplemental fertilizer is incorporated directly in the manure before application to the land. Minor element deficiencies are rare enough that amendment of manures with minor elements is not a practice. Long-term applications of farm manures will lead to phosphorus accumulation in soils.

Handling of farm manures

Farm manures should be incorporated into the soil soon after they are applied to land. If manures lie on the surface for even a few hours after application, nitrogen is lost by volatilization, and the value of manures to increase crop yields is reduced greatly (Table 12). Generally, after periods of two weeks or more on top of the ground, the value of manures is cut in half. If the manures are never incorporated, their value is nil unless the application is very large, meaning amounts of one or more inches in layers or mulches over the soil surface.

Manure that is stored in piles loses much of its nitrogen and potassium. Nitrogen is lost by leaching, volatilization of ammonia, and denitrification of nitrates. Potassium is leached. Losses of nitrogen are rapid. Half of the nitrogen may be lost in two weeks. Piling is preferable to application to fields without plowing the manures under. In the latter case, losses of nitrogen are higher from volatilization and runoff than from piles. Manure from piles should be handled as fresh manure and turned under as soon as it is spread to avoid further losses (Table 12).

Application of manures

The best time to apply manures is about two weeks ahead of the time that the crop is to be planted. This lead time allows for some decomposition of the manure. Decomposition will narrow the C:N ratio and will lessen the likelihood of creating nitrogen deficiency through immobilization. The lead time also allows for any ammonium that is released from the manures to be oxidized to nitrates. Ammonium is first available form of nitrogen that is released from mineralization of organic matter. Manures are relative high in nitrogen, and the chance of substantial ammonium accumulation is high soon after application of manures, particularly if soils are cold, as they might be at planting time in the spring. The toxicity, often referred to as burning, that is encountered on plants that are fertilized with chicken manure is ammonium toxicity. Chicken manures are high in ammonium, and if sufficient time is not allowed between application of manures and planting, substantial damage from ammonium toxicity may occur to seeds or seedlings. Also, when manures are rotting rapidly in the soil, the microorganisms that are carrying out the rotting will attack other materials as well. These other materials may be seeds or roots of seedlings. In about two weeks time, the easily decomposable materials in the manure will be broken down, and the likelihood of damage to seeds or young plants is alleviated. The usual function of applying manures to land is to supply plant nutrients. Manures are applied also to increase organic matter in soil or as mulches. The amount of manure that should be applied depends on the purpose of manuring, the quality of the manure, and the crop that is to be supported.

 

Table 11. Portions of total annual nutrient requirements that will be met if enough farm manure is applied to satisfy the nitrogen requirement of a crop.

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Nutrient

Portion supplied, % of total need

Nitrogen

100

Phosphorus

25

Potassium

100

Calcium

100

Magnesium

100

Sulfur

100

Minor elements

25

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Table 12. Effect of time that manures lie on surface until they are incorporated on the relative value of manures for increasing yields of crops

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Time between application and incorporation (hours)
Relative value of manure

0

100

6

85

24

75

48

70

96

55

336 (2wks)

50

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Table 13. Recommended amounts of fresh manures according to purpose for manuring land.

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Purpose

Amount to apply

lb/100 sq ft

tons/acre

Fertilization

100

20

Increasing organic matter

150

30

Mulching (1.5 inches)

500

100

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About half of the nitrogen, about a quarter of the phosphorus, and most of potassium in manures will become available to a crop in one season. Often, the amounts of available nitrogen and phosphorus are lower than those mentioned in the preceding sentence. As little as 10 to 25% of the nitrogen may become available depending on the quality of the manure. Crop yields will be limited by applications of manures with low availabilities of nutrients. Applications should be increased or other fertilizers should be applied to compensate for this low release of nutrients. If the manure is high in bedding, elements in the soil may be immobilized creating deficiencies in plant nutrition. Manures that have been leached from rain may be depleted of much of their available nitrogen and will have low value as fertilizers.

 

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Produced and maintained by Your Name Allen V. Barker
University of Massachusetts, Amherst.
last updated - April 6, 1999