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Nutrient Management
Manure is applied to crops, pastures, and hayland as a way to utilize plant-available nutrients in the manure. These nutrients have significant value if managed properly. Furthermore, proper management reduces environmental risks associated with manure spills or over-application.
All manure contains the macronutrients nitrogen (N), phosphorus (P), and potassium (K), as well as organic matter that can benefit agricultural crops. Assessing the amount of N, P, and K in manure is critical to determine application rate. Additionally, the amount of manure nutrients available for crop uptake, crop nutrient needs, timing of nutrient applications, and potential nutrient loss mechanisms are also important.
Factors Affecting Nutrient Availability
The following information must be known to determine proper manure application rates:
- What is the crop to be grown?
- What is the soil type in the field? (Not needed in all states)
- What is the expected yield for this crop?
- What level of nutrients already exists in the soil?
- How is the manure collected, stored, and treated?
- How is the manure land-applied?
- What is the level of nutrients in the manure?
- What fraction of manure nutrients are available to the crops?
It is impossible for plants to use applied nutrients with 100% efficiency, regardless if source is animal manures or commercial fertilizers. Studies have shown that nutrient use efficiency for nitrogen ranges from 30% to 75%, and is dependent on the crop, the specific nutrient, weather, and many other environmental factors. The goal of a nutrient management planner and waste applicator is to obtain the best use of the manure nutrients. This requires intensive management.
Determining Nutrient Availability of Manure
The availability of manure nutrients is not cannot be calculated with 100% confidence. Good estimates can be obtained using the correct, local values and coefficients when making manure applications, and then closely monitoring the system. In some cases, it may be necessary to add supplemental nutrients through the growing season based on soil tests and plant tissue tests. In other cases (i.e. where crop yield was much lower than expected), a manager should assess nutrient buildup in the soil and adjust subsequent manure application rates. Nutrient management plans are based on average sets of conditions, and constant management which includes frequent manure testing, soil testing, and application equipment calibration are crucial to insure efficient nutrient utilization. A webcast on nitrogen availability from organic sources is available.
Nutrient Management Planning
Nutrient management guidance is typically done at the state level. General guidance may be available at a regional or national level from USDA Natural Resources Conservation Service (NRCS). You can also seek the advice of a local expert who is with the Cooperative Extension Service, land grant institution, state department of agriculture, or the state regulatory agency to obtain manure nutrient generation values relevant to the area, specieds, and system you are working with.
Tables that offer production volumes for manure as well as manure nutrient concentration are available for planning purposes, but should not be used to determine application rates on a daily basis. Frequent manure sampling is the only way to make a good assessment of manure nutrient value. Then, with data that relates to loss potential as it relates to manure application method and timing, one can make good recommendations as to appropriate application rates that assure maximum crop use efficiency and minimize losses.
Author: Karl Shafer, North Carolina State University
