Get a Better Handle on Crop Nutrient Needs with Pre-Sidedress Nitrate Testing

Get a Better Handle on Crop Nutrient Needs with Pre-Sidedress Nitrate Testing

For growing plants, nitrogen provides essential nourishment. The use of nitrogen fertilizers has revolutionized global food production, dramatically increasing the efficiency and productivity of the crops we rely on for an endless list of uses — from food to fuel and thousands of products in between.

Identifying better ways to make more efficient use of nitrogen and other fertilizers is important to farmers and consumers alike. Management practices such as pre-sidedress soil nitrate testing (PSNT) help growers ensure that they are using the right rate of nitrogen — enough to optimize crop production but not an excessive amount that could potentially contribute to environmental concerns.  

The goal of PSNT is to improve the efficient use of nitrogen fertilizer applications and reduce the potential for water quality issues that can be associated with nitrogen use. Developed primarily for field corn production systems, the PSNT has been adapted for use in vegetable crops such as tomatoes, peppers, squash and sweet corn. 

Getting a Better Handle on Crop Nutrient Needs

The nitrogen requirements of plants can be met in several ways. The soil itself supplies a certain amount of N through the decay of organic matter and, in some cropping systems, legumes such as alfalfa also contribute nitrogen to the soil. Applications of inorganic or organic fertilizer — commercially available fertilizers or livestock manure — fill the gap between the amount of nutrients available in the soil and the amount needed for optimum crop production.

In situations where legumes and/or manure are significant nitrogen sources, it can be difficult to arrive at economically and environmentally sound nitrogen fertilizer recommendations. Nitrogen poses nutrient management challenges because it goes through complex changes that are impacted by dynamic environmental conditions, such as temperature and moisture, affecting mineralization rate and N loss. Its availability to plants is affected by a number of factors. The amount and timing of nitrogen availability from the organic matter of previous crops, the nitrogen fixation of legumes, and the transformation of proteins in manure can be difficult to predict.

Although laboratory testing can effectively measure the amount of nitrogen in the soil, quantifying available nitrogen within a specific time frame — when the crop most needs it, for instance — requires a different approach.

Measuring Nutrient Content at the Right Time

The concept behind PSNT is that a soil test near the time of the crop’s greatest need for nitrogen should most accurately reflect existing nitrogen availability for the crop. If growers know the amount of nitrate available to the plants, they can use more efficient fertilizer rates. The term “sidedress” refers to an in-season application of fertilizer beside the rows of actively growing plants.

The PSNT is a prediction of the near-term availability of nitrogen mineralized from decaying plants, animal manure, compost or other organic matter. Only the nitrate form of nitrogen is measured since that is what is available to the growing crop.

If a grower has a more accurate estimate of what’s available, he or she can then make a better decision regarding the right rate of fertilizer to apply in order to meet production goals.

Measuring Current Nitrate Concentration

When a grower intends to use the PSNT, a minimal amount of nitrogen is applied in the spring, such as that supplied by a low rate starter fertilizer at planting. Samples for the Pre-Sidedress Soil Nitrate Test are taken during the growing season, after the typically wet spring period but prior to the crop’s major demand for nitrogen.  The corn crop is approximately 6 to 12 inches tall at this time and 15 to 25 core samples to a depth of about 12 inches are taken. A fairly large number of core samples are important because nitrate levels can vary from one location to another within a field. Timeliness of sampling is important and growers should avoid unique field areas where known fertility differences exist. Core samples are combined, and sent to a soil-testing laboratory for analysis.

Nitrate level is measured in parts per million (ppm) and that measurement, combined with a yield goal for the crop, are utilized to arrive at a recommended sidedress nitrogen rate, or possibly no additional nitrogen if adequate supplies are available in the soil. In some areas, for example, a PSNT level of 25 ppm would indicate to the corn grower that no extra nitrogen is required to attain a level at which a lack of nitrogen limits yields. At the lower PSNT levels, the amount of nitrogen required to achieve optimum yields depends on the level of the PSNT and other factors, including the previous crop and potentially attainable yield.

In either case, the goal is for the grower to use the right rate of nitrogen, neither shorting the crop nor applying an excessive amount. Using the PSNT for a number of years, and recording the results, can help growers fine-tune recommendations by providing a more complete picture of how environmental conditions impact soil nitrate concentrations.

Another important benefit from using the PSNT and applying the majority of fertilizer requirements during the growing season is that the grower has the opportunity to assess the crop’s yield potential. With the plants up and growing, he or she can make a better decision whether or not the crop can utilize a given nitrogen rate.

On Target With the Right Rate at the Right Time

Fine-tuning nitrogen rates and targeting the high-demand stage of crop development can help growers optimize their fertilizer investment, protect water quality and grow better crops. Agronomic professionals can help growers tailor the PSNT and other practices to their specific production systems.