Determine the right rate for manure or compost application

Factors to consider when determining how much manure or compost should be applied to get the desired crop productivity.

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“With the high cost of fertilizer inputs, now more than ever, it is important to make the best use of available on-farm nutrients such as manure and compost,” says Trevor Wallace, nutrient management specialist with the Alberta government. “Ensuring that nutrients are used effectively minimizes nutrient loss. Adding too much manure or compost can negatively impact air, soil, groundwater and surface water quality, not to mention be very costly.”

Key information needed to calculate or determine an appropriate manure or compost application rate includes:

• nutrient requirements for proposed crop or rotation based on target yield

• potential soil nutrient availability

• potential environmental nutrient additions, for example, sulphur from irrigation water

• nutrients from past fertilizer, manure or compost applications

• an estimate of the available nutrient content of the source, and

• proposed method and timing of application

This information is assessed and used to determine an appropriate application rate. Wallace points out some assumptions must be made, such as:

• proportion of nitrogen or phosphorus in the manure or compost in organic form

• rate of nutrient mineralization from manure or compost and soil each year, and

• amount of nutrients lost after manure or compost application, that never becomes part of the soil-plant system Start with a realistic yield goal in mind to determine the crop nutrient requirements. The nutrient recommendation can be estimated using a variety of tools, such as the Alberta Farm Fertilizer Information and Recommendation Manager (AFFIRM), soil test laboratory recommendations, past yields or fertility applications.

Testing and analyzing the soil in fields provides an understanding of what nutrients are available for crop growth, as well as the physical, chemical, and even biological characteristics of the soil (see Consider the right source of manure or compost for field application). This information is invaluable when selecting the right rate for manure or compost application.

“The ideal time to assess your soil is just before a crop’s active growth stage when soil nutrient uptake is high. In most areas of Alberta, it is generally safe to begin fall soil sampling by mid-October. Fall sampling in forage fields can begin any time after September 1.”

Use a reputable soil testing laboratory that uses Alberta data to determine fertilizer recommendations. Producers can contact their local agricultural fieldmen, applied research or forage association, industry or private agronomist, or 310-FARM (3276) for more information on soil and manure sampling and labs available to complete the analysis.

Assess and account for other nutrient additions such as previous crop and past applications of fertilizer, manure or compost application, or irrigation water. Use book values found in the Manure Characteristics and Land Base Code (https://open. alberta.ca/publications/6921902) or test the manure or compost to estimate the nutrient content to calculate manure application rates.

“The best time to collect a manure or compost sample is during application,” says Wallace. “The initial manure or compost application rate is determined based on book values or previous analyzed samples. Once the latest samples are analyzed, and bulk density and manure analysis information is available, the calculations can be updated to determine the correct application rates and determine whether additional fertilizer is required.”

Manure and compost application rates should be adjusted based on method and time of application to reduce potential nutrient losses. Method of application influences the potential for nutrient loss through runoff and volatilization, which can be reduced by incorporation or injection. Applying the nutrient source as close to crop need as possible also minimizes the risk of nutrient loss. The greater the lag between application and crop need, the more opportunity for microbial or environmental activities to tie up or transform nutrients resulting in transport and loss.

Finally, weather conditions at the time of application can significantly influence loss. The least optimal time for application is on frozen or snow-covered ground, as infiltration is limited, and plants are not growing to utilize applied nutrients. Adjust application rates based on application method and timing as well as anticipated environmental conditions.

“Once the application rate has been determined, you can determine if additional fertilizer is required to meet shortfalls for any one nutrient. Supplemental fertilizer application may be needed in response to weather-induced losses after manure or compost application. Monitor conditions through the season to determine if any additional nutrient applications are warranted based on changing economic or weather conditions.”

Based on the application rate used, producers can also estimate the amount of nutrients that will become available in that year plus subsequent years, from that application. If a whole farm nutrient management plan is being developed, determine whether additional area is required to apply all the manure available at agronomic rates (see Field selection for fall manure or compost application at https://www.alberta.ca/ field-selection-for-fall-manure-or-compost-application).

Chapter 6.1 of the Nutrient Management Planning Guide (https://open.alberta.ca/publications/7086752) provides practical examples of calculating manure application rates. The Alberta Farm Fertilizer Information and Recommendation Manager (AFFIRM) application is another useful tool for planning manure applications and monitoring throughout the season.

“Before you apply to a field be sure to check if you are following the soil protection limits set by the Agricultural Operation Practices Act (AOPA). If your operation produces more than 500 tonnes of manure or compost, application to that field should only occur if the soil has been tested within the previous 3 years, and not exceed the soil nitrate-nitrogen and salinity limits set in AOPA,” says Wallace.