Manure Nutrient Availability
Table: "Average daily production and total content of manure" (or better yet, your manure analysis) will tell you the total amounts of these major nutrients on either a per-gallon or a per-ton basis. With N, however, the total amount is not the amount available to the crop in the year of application. The proportion of the manure N available to the crop in the year of application depends on how the manure is managed.
Surface-applied manure potentially can lose large amounts of N if it is not incorporated mechanically or by rain. The fraction of the total manure N available to the crop (the availability factor), in relation to when and how the manure is applied, is given in Table: "Manure nitrogen availability factors for use in determining manure application rates based on planning condition", part A. To determine manure N availability you need to answer several questions:
- When will the manure be applied?
- What is the target crop for manure nutrient utilization?
- How will the manure be applied?
- What type of manure is being applied?
Note that even under the best of conditions only about one-half to three-quarters of the nitrogen in the manure is available to the crop in the year it is spread. The remaining nitrogen will become available slowly over a period of years. The availability of this residual nitrogen can be estimated from the manure history of a field. Adjustments for residual nitrogen are given in Table: "Manure nitrogen availability factors for use in determining manure application rates based on planning condition", part B. The example given later in this section illustrates how to use these factors in application rate calculations.
If, in addition to total N analysis, the manure also is analyzed for NH4-N, an even better estimate of available N can be made using the factors in Table: "Factors for calculating manure nitrogen availability based on time of application, incorporation, field history, and manure analysis with ammonium and organic N fractions. Recommended for all manures, but required for atypical or treated manures". This approach also is illustrated in the example and should be used any time the manure has been treated in any way. Examples of treatment that might influence N availability include composting; digestion for methane production; separation, either mechanically or by settling; and aerobic or anaerobic treatment lagoons. Similar information is required to determine the manure N availability as with the previous table.
The phosphorus (P) in manure is available only slowly to a crop; however, manure P is not fixed readily into unavailable forms in soil like highly soluble P fertilizers are. The net result is that the P in manure is very similar to the P in fertilizer for building soil P levels and supplying crop needs. Thus, in most cases, manure P can be substituted for P in fertilizer on a one-to-one basis.
There is one exception to this rule: manure P cannot be substituted for starter fertilizer P. Thus, even if manure P is applied in amounts adequate to meet the needs of a crop, there still may be some benefit in using a starter fertilizer, especially if soil test levels were marginal or low. Manure P will accumulate in the soil. Therefore, P in manure, applied in excess of one crop’s needs, can be used by a later crop in the rotation.
The potassium (K) in manure occurs mostly in a soluble form similar to the K in fertilizer. Thus the K in manure can be substituted one-to-one for fertilizer K. This high solubility of manure K can result in excessive levels of soluble salts if large applications of manure are made, particularly on lighter-textured soils. This is a problem especially when poultry manure is applied at high rates immediately before the crop is planted. Like P, K accumulates in soil and thus can be built up in one part of the rotation so that it may be used later in another.
The nutrients in manure have no immediate value if the crop does not need them, since there already is more than enough in the soil. As with fertilizer, you get the greatest return from manure by applying it to crops that require N and that are grown in fields testing the lowest in P and K. Your first step in developing a manure management plan should be to prioritize farm fields on this basis, with an additional consideration of the fields’ relative environmental sensitivity. Then, allocate the manure to the high-priority fields at a rate that meets crop nutrient requirements, but at a rate no greater than that which meets the need for manure’s most vulnerable nutrient, N. If there is a potential for P transport to water, the manure application rate should be limited to P crop removal. In this case, significant supplemental fertilizer N will usually be needed to meet the crop N requirement. The Phosphorus Index can be used to determine the risk of P loss to water and to guide appropriate management to minimize this risk.
Available N is less likely to be present the following year if not removed by the crop. As mentioned above, P and K applied in excess of crop uptake will build soil levels and generally are available to succeeding crops. In fact, with advance planning, excess manure P and K applied during the corn years of a rotation will supply the high P and K requirements of a following hay crop. A good approach is to balance the nitrogen in the manure with the needs of the current crop and to balance the P and K in the manure with the needs of the crop rotation (see "Changes in Soil Test Levels Over Time" [right] Reference: Figure 1.2-7, the Agronomy Guide).
Applying manure to legumes generally is not recommended. If properly inoculated, legumes do not need supplemental nitrogen for best growth. Thus, unlike a nonlegume such as corn, for which potential returns from the nitrogen in the manure are great, there will be no economic return from the nitrogen in manure applied to a legume. Legumes, however, will take up nitrogen applied in manure even though they don't need it; thus, legumes may serve as a place to dispose of excess nitrogen. The uptake of nitrogen by legumes is indicated in Table: "Nitrogen removed by legumes".
Several other considerations exist when applying manure to legumes. If manure is applied to a legume to use excess nitrogen on the farm, because of the relative proportions of nitrogen and phosphorus in manure, phosphorus likely will be applied in excess of crop needs; thus, the goal of balancing the P throughout the crop rotation as discussed earlier cannot be met.
Applying manure to legume stands can encourage competition from weeds and grasses. Manure also may introduce additional weed seed to the fields. Both effects will result in lower-quality forage and in reduced stand life for the legume. Manure can be used effectively, however, on an old legume stand that is to be rotated, not only to increase productivity for one more year but also to build up soil nutrient levels to prepare for the following crop.