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Nitrogen is not static in soil. There is a constant interchange between forms of nitrogen that plants can use and those they cannot, and nitrogen in the air. That is why soil nitrogen is not normally measured except for situations where plant needs are very high and the fertilizer can be added immediately according to the measurement. This occurs, for example in greenhouse production and with field corn in early growth stages at the point farmers are about to sidedress fertilizer. Otherwise a nitrate test is meaningless.
Here is a diagram showing the complex processes that affect the amount of nitrogen in the soil at any moment. Ideally, Nitrogen is added to the soil near the time plants will take it up and use it rapidly.
This site was last updated 11/22/06