Correlation of soil test nitrate level and wheat yields
Wheat yield with no N fertilizer applied was compared with fall nitrate-N levels and a strong relationship was established. Although new practices have been developed to improve N management in winter wheat, soil sampling in the fall for nitrate-N remains an important practice to manage N efficiently and can result in considerable savings for producers.
When soil sampling for N is not done, the K-State fertilizer recommendation formula defaults to a standard value of 30 lb/acre available N. In this particular dataset, the average profile N level was 39 lb N/acre. However the N level at individual sites ranged from 11 to 197 lbs N/acre. Most recommendation systems default to a standardized set of N recommendations based on yield goal and/or the cost of N. Without sampling for N or using some alternative method of measuring the soil’s ability to supply N to a crop, such as crop sensing, the recommendations made for N will be inaccurate, resulting in a reduction in yield or profit per acre and increased environmental impact.
Due to the drought of the past two years, there have been many situations where large amounts of N have been present in the soil at planting of wheat or summer crops such as corn or grain sorghum. Early samples requesting soil N tests from western Kansas coming to the lab are showing consistently high soil N levels. Failure to account for that valuable resource can result in excess vegetation, increased plant disease, inefficient use of soil water, and reduced yield.
Soil sampling in fall for nitrate-N can have a significant impact on N recommendations for winter wheat, thus improving N management, and is still strongly recommended.