Organic matter is decomposed by organisms—mostly bacteria and fungi—in the soil. These microorganisms use the carbohydrates in the organic matter as fuel in the same way that we turn carbohydrates into energy in our bodies. Carbohydrates spur them into increased growth and reproduction.
If there is not enough nitrogen present, the microorganisms will scavenge all they can from the soil to make the protein they need to reproduce. Because they are omnipresent in the soil, they get to the available nitrogen before plant roots can, causing a nitrogen shortage in the plants that share their soil. This shortage is called a nitrogen draft.
A nitrogen draft occurs when organic matter that is high in carbohydrates but low in nitrogen is mixed into the soil. It might also occur from a mulch being added to the surface, but the effect is much less because only the organic matter in contact with the soil is available to the microorganisms. The nitrogen draft is only severe if it is mixed into the soil.
As the carbohydrate is consumed, the microorganisms decline, releasing the nitrogen in their bodies to the soil again. Depending on conditions, this might take several months to two years.
The ratio of carbohydrate to nitrogen in organic matter is called the C:N ratio. It is measured as the ratio of carbon to nitrogen. A C:N ratio of about 30:1 (30 parts carbon to 1 part nitrogen) is ideal for organic matter decomposition. The higher the ratio of carbon to nitrogen, the worse the nitrogen draft will be. The table below shows the C:N ratios of several common materials used as soil amendments.
Materials with a C:N ratio of less than 30:1 add nitrogen to the soil and are a source of nutrients for the plants.
Materials that have been thoroughly composted do not cause nitrogen draft; their available carbohydrate has already been digested by microorganisms.
Common inducers of nitrogen draft include ground bark, straw, and sawdust, or manure containing large amounts of straw or sawdust.
You can compensate for nitrogen draft by adding about 1 pound of actual nitrogen for each cubic yard of soil amendment. Ammonium sulfate is a good source of nitrogen for this purpose. Add 5 pounds of ammonium sulfate, which contains 20 percent nitrogen, for each cubic yard of organic matter.
Carbon/Nitrogen Ratio of Soil Amendments
|Horse manure with litter||60:1|
|Sawdust weathered for 2 months||325:1|