If the plant dies or is harvested, the accumulated nitrate stays in the plant material. It does not disappear with time. Nitrate levels may be reported in three different ways depending on the analytical procedure used. Be sure you know which method was used before trying to interpret the results. Refer to the following table.
Nitrates accumulate with time in an injured or damaged crop.
Injury to a plant can be from hail, frost, or other forms of physical damage that reduces photosynthesis. It is best to cut or harvest the crop as soon as possible after damage. Nitrate will accumulate in a plant as long as it is taking up more nitrate than it can convert to protein. Soil nitrogen levels greatly influence plant nitrate content. Large applications of nitrogen fertilizer or manure increase soil nitrate and thus the nitrate available to the plant.