Managing Livestock Manures 4: Managing Manure on Organic Farms

The nutrient values of manures


Factors affecting composition

Animal manures are an important source of organic matter and plant nutrients. Knowledge of manure composition is an important part of good management, either when importing manure onto the farm or transferring nutrients around the farm. Many factors affect the nutrient content of animal manure:

the livestock diet affects the amount of nutrients excreted
feed quality influences the partitioning of excreted nutrients between faeces and urine
bedding material type and quantity
losses during livestock housing
manure storage both length and management.

Animal manures can therefore vary greatly in composition. It is important to know the nutrient content of manure before land application.

You should measure or estimate manure nutrients, not rely on experience or guess work. You can:

Refer to standard values these are useful for general planning purposes. These are based on the analysis of large numbers of samples. Values for conventional manure are given in Table 1. Values for manure produced on organic systems are given in Table 2, though the latter are based on much less information.

Sample manures for analysis as the nutrient content of manures can depend on the individual circumstances of production, it is good practice to periodically sample manure. Careful sampling and analysis can establish typical nutrient values on a farm.

Manure sampling

Sampling must be carried out carefully so that the sample is a true average of the whole store. Particular care must be taken when sampling slurry stores due to the risk from lethal gases and the need to obtain a safe access point in order to take the samples. Further guidance on sampling can be found in Managing Livestock Manures: Booklet 3, Spreading systems for slurries and solid manures, (Appendix II).

Laboratory analysis should include: dry matter (DM), total N, P, K, S, Mg and ammonium-N. For well-composted FYM, nitrate-N should also be measured and for poultry manures, uric-acid N.

For slurries, laboratory results can be supplemented by on-farm rapid N meter measurements of ammonium-N. A slurry hydrometer can also be used to estimate DM, total N and P contents.

Table 2 indicates that cattle slurry from an organic holding, on average, has a slightly lower nutrient concentration than conventionally produced slurry. On average, there is little difference in FYM from the different systems. However, many factors affect manure nutrient contents on individual holdings. Two key issues to establish are the dry matter content of slurry and the length of storage for solid manures.

The more dilute the slurry, the lower its nutrient concentration (e.g. Figure 1 below). Dry matter is not as good a guide for K2O concentration because this is excreted mainly in urine.

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Solid manures lose carbon (as CO2 and methane) during storage,which can lead to a higher concentration of nutrients over time. Losses will be more rapid if the manure is actively composted. However, concentration may not occur if nutrients are also lost during storage/composting. Figure 2 shows changes based on manure samples collected from organic farms in England.

There is little information on the nutrient content of manures from organic poultry production. What there is, suggests there may be little difference to conventionally produced poultry, but it would be advisable to confirm this with analysis. The largest difference may be in the dry matter content of the manure if it is lower than the standards quoted in Table 1, then the nutrient content will be proportionally lower when expressed on a fresh weight basis.

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