Excess Nitrogen in Winter Wheat
Taking 24-inch soil profile-Nitrogen samples in the fall has been a recommended practice for making a nitrogen recommendation for winter wheat for many years. However, due to the mobility of nitrate-N in the soil, soil test values observed in the fall may be completely different from values observed in the spring, particularly on soils prone to leaching. Because many producers wait until spring green up to make their nitrogen application, does soil sampling in the fall for nitrate-N really provide useful information for nitrogen management in wheat? That is a legitimate question.
Analysis of yields taken from K-State research plots that received no nitrogen fertilizer shows a strong positive relationship with fall soil profile nitrate-N. Wheat yields increased rapidly as soil nitrogen levels increased to about 80 pounds soil nitrogen per acre, and then leveled off. We found that at low soil nitrate levels, wheat yields responded well to applied fertilizer. We also found that when fall soil profile nitrate-N levels are greater than 80 to 100 lb/acre, it is unlikely the site will respond to additional fertilizer nitrogen applied in the spring.
In short, a strong relationship was found between wheat yield and fall nitrate-N levels from 24-inch profile soil test analyses when no nitrogen fertilizer was applied. Although new practices have been developed to improve nitrogen management in winter wheat, soil sampling in the fall for nitrate-N remains an important practice to manage nitrogen efficiently and can result in considerable savings for producers. When soil sampling for nitrogen is not done, the K-State fertilizer recommendation formula defaults to a standard value of 30 lb/acre available nitrogen. In this particular dataset, the average profile nitrogen level was 39 lb N/acre. However, the nitrogen level at individual sites ranged from 11 to 197 lbs N/acre. Most recommendation systems default to a standardized set of nitrogen recommendations based on yield goal and/or the cost of nitrogen. Without sampling for nitrogen or using some alternative method of measuring the soil’s ability to supply nitrogen to a crop, such as crop sensing, the recommendations made for nitrogen will be inaccurate, resulting in a reduction in yield or profit per acre and increased environmental impact.
Failure to account for the nitrogen present in the soil wastes a valuable resource and can result in excess foliage, increased plant disease, inefficient use of soil water, and reduced yield. Soil sampling in fall for nitrate-N can have a significant impact on nitrogen recommendations for winter wheat, thus improving nitrogen management, and is strongly recommended.