The C:N ratio of organic fertilizers is critical to the amount of nitrogen that will be available to a crop. This is referred to as Plant Available Nitrogen (PAN).
Almost all organic fertilizers are composed of natural organic sources of nitrogen and require microbial breakdown. This is also referred to as mineralization, which is the conversion of organic nitrogen to ammonia and nitrate.
Microbes require both nitrogen and carbon with carbon being the source of energy. The higher the carbon content relative to nitrogen the greater amount of nitrogen will be consumed by microbes and not available to a crop. The balance point where the nitrogen is fully consumed by the microbes with none left for the crop is generally agreed upon to be about 24:1.
An input, such as poultry manure, that has over 12:1 C:N ratio will release approximately 50% of the nitrogen for crop uptake whereas a high protein product, such as feather meal, will have 4-5:1 C:N ratio with a PAN level of over 80%.
Average soil C:N ratio is 13-15:1
(This is the stability point. Any organic material added will eventually be mineralized to reach this point. Changes to this ratio are very slow due to high mass of the soil relative to the inputs.)
Compost varies from 30:1 to 10:1
(Note that if the compost has a C:N ratio greater than the soil then the compost will consume nitrogen as it mineralizes resulting a deficit of nitrogen regardless of the compost nitrogen content)
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Plant Available Nitrogen (PAN) is the quantity of nitrogen made available during the growing season after fertilizing materials are applied. A certain amount of the nitrogen is immobilized, and the remaining nitrogen is available to the plant.
A Soil test is the best way to determine what nutrients your soil needs.
To calculate the pounds of nitrogen in a bag of fertilizer, multiply the weight of the bag by the percent nitrogen (this is the first number in the N-P-K designation on the label).
For example, an 8-4-4 product contains 8% nitrogen. A fifty pound bag of 8-4-4 would therefore contain 4 lbs. of nitrogen (50 lbs. x 8%).