Page 171 - Nutrient Management Planning Guide

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Chapter 4.3

Calculating Available N in Compost

Nitrate concentration is usually very low in raw manure, but can be present in higher concentrations in compost. For composted material, a nitrate analysis should be requested. If compost contains detectable amounts of NO

3

-N, this should be subtracted along with any NH

4

-N from total N to estimate organic N content. Any NO

3

present should be included in the estimate of crop available N, together with retained NH

4

-N and available organic N.

To calculate organic N content, subtract the NO

3

-N and NH

4

-N from total N: Organic N = Total N – NH

4

-N – NO

3

-N

Because properly composted manure is a more stable source of organic N, less N will be mineralized. It is safe to assume that 13 percent of organic N will be mineralized to crop-available forms in the year following application: Available Organic N (year of application) = Organic N × 0.13

Additional organic N will be mineralized in subsequent years and can be estimated at 7 % in year 2 and 4 % in year 3 when planning future manure applications. Available Organic N (year 2) = Organic N × 0.07 Available Organic N (year 3) = Organic N × 0.04

To calculate crop available N, add NO

3

-N to retained NH

4

-N and percentage of organic N that will become available.

Estimated Crop Available N = Available Organic N (year 1) + Retained NH

4

-N + NO

3

-N

Crop Available P

Similar to N, P in manure is present in organic and inorganic forms, but most labs only report the amount of total P in a sample. Based on experience and research, about 70% of total P in manure will be crop available in the year it is applied:

Estimated Crop Available P (year 1) = Total P × 0.7

Similar to N, some of the residual applied P will be mineralized and become crop available in subsequent years:

Estimated Crop Available P (year 2) = Total P × 0.2

Estimated Crop Available P (year 3) = Total P × 0.06