Page 173 - Nutrient Management Planning Guide

167

Chapter 4.3

If C:N ratio is provided for the manure, it can provide a sense of how rapidly and to what extent nutrients will become available from the manure. In general, the lower the C:N ratio the more rapidly organic nutrients will be released in crop available forms.

Net mineralization of organic N occurs when C:N is less than 20:1. When C:N exceeds 30:1, N becomes a limiting nutrient for decomposer organisms, and this can reduce the rate of decomposition and results in N immobilization (i.e., N tie-up). This can be an issue in manure with large amounts of bedding mixed in, such as poultry litter or certain types of beef manure. In these situations, requesting a C:N ratio from the lab may be valuable for identifying potential issues with N availability.

Similar to N, organic forms of P are mineralized by soil microorganisms to inorganic forms, but can also be immobilized depending on the ratio of carbon to P (C:P ratio). The C:P ratio can be estimated:

C:N ratio × Total N ÷ Total P = C:P ratio

When the C:P ratio in residues is between 200:1 and 300:1, mineralization and immobilization balance each other to result in no net release of P from the decomposing manure. When C:P is below this range, P is released, while above this range P will be tied up and not released for crop use.

Nitrate-N levels are typically higher in finished compost than fresh manure. Most inorganic N in fresh manure is in the NH

4

-N form but nitrification during the composting process converts some of this to NO

3

-N. The ammonium-N/nitrate N ratio is often used as an index of compost maturity with lower ratios denoting more mature or stable material. Ratios fall from 1000:1 for fresh manure to <10:1 for compost.