Composting In Schools

Use of Fertilizer Nitrogen to balance C/N Ratios

Tom Richard

When composting high carbon materials such as leaves, additional nitrogen may be required to reduce the C/N ratio to the optimal range. If considering using fertilizer as an N source, one needs to proceed cautiously. While the theoretical number should be the same as calculated using the C/N ratio formulas, the nitrogen in fertilizers is released much more rapidly than that in organic nitrogen (from which the rules of thumb are derived).

Organic nitrogen sources provide a natural "time release" that makes them available at a rate comparable to the growth rate of microorganisms in the compost, so they are utilized efficiently. The rapid availability of nitrogen fertilizers is especially a concern in the fall and winter, when low temperatures slow down the growth rate of microorganisms, and nitrogen uptake will be correspondingly slow. To mimic a natural time release with synthetic fertilizers, they should be applied sparingly and in a series of applications. While there is not a research base on which to estimate what the right rate would be, sniffing for ammonia volatilization may indicate if too much was applied too soon. In addition, because none of the fertilizer nitrogen is locked into compounds that are difficult to degrade (as is the case with organic sources), the total applied should be significantly less than the calculations indicate - perhaps half to two thirds.


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