Nitrogen is, of course, an essential plant nutrient – but it also can negatively affect both growth and quality of a crop and so must be carefully managed.
Crops take up nitrogen that is released to the soil as a direct result of several catalysts, including atmospheric deposition … soil organic matter mineralization … crop residue decomposition … and animal manure and/or inorganic fertilizer applications.
A deficiency in nitrogen causes severe damage to crop yields – and can even cause a catastrophic, total crop failure. However, an excess of nitrogen may lead to excessive vegetative growth, lodging, delayed maturity, increased disease susceptibility and low crop quality. Excesses also may contribute to acid rain, destruction of the ozone layer in the stratosphere, the greenhouse effect, an increase in chemical nutrients in surface waters, contamination of ground water, and fish and other marine-life kills, among other negative side effects.
It’s clear, then, that it’s important to growers – both from an economic and an environmental standpoint – to carefully control the nitrogen content of field soil. The ideal situation is to maintain adequate inorganic nitrogen during the growing season and to minimize the occurrence of inorganic nitrogen during the off-seasons, when nitrogen may be introduced into field soil via surface and groundwater.
The PSNT is different from routine soil tests in that nitrogen testing shows specifically when nitrogen fertilizer applications need to be adjusted to suit crop- and field-specific conditions.
The PSNT generally is most useful for confirming legume and manure nitrogen content and for determining the amount of nitrogen in a specific field. It’s especially important to do a PSNT when not enough hard data is available to determine nitrogen content using more standard techniques.
For example, a PSNT can answer a lot of questions when a grower doesn’t know the previous manure application rate or nutrient content of a particular field. It’s also useful when the stand density of a previous crop is unknown. (Stand density is an absolute measurement based on basal area, number of trees per acre or volume per acre. It reflects the degree of crowding of stems within a stand.) Another instance when PSNT is of particular value is when unusually cool weather conditions may have impacted nitrogen mineralization rates, or when excessive rainfall causes a dramatic loss of inorganic nitrogen – both of which conditions would be missed without the PSNT.
Soil samples for the PSNT generally are taken after planting, when the crop has begun its initial growth and is several inches above the ground. By this stage of the growing cycle most of the conversion of organic nitrogen sources to forms of nitrogen able to be utilized by plants has occurred.
PSNT core soil samples are collected to a specific depth, determined by the type of crop, and are collected randomly over the full field. Then, the cores are mixed to obtain a composite sub sample and are submitted to a soil-testing laboratory.
The United States Department of Agriculture (USDA), as well as local colleges and universities, will have charts and graphs for growers in all areas of the country to determine the ideal soil nitrogen content for their specific crops. The wise grower will have his or her soil undergo a PSNT to assure the best possible crop.