Correlation of soil test nitrate level and wheat yields
1. Analysis of yields taken from plots that received no N fertilizer shows a strong positive relationship with fall soil profile nitrate-N (Figure 1). Wheat yields increased rapidly as soil N levels increased to about 80 pounds soil N per acre, and then leveled off.
2. We then converted check plot yields to a relative yield, or percentage of the maximum fertilized yield obtained at each location (Figure 2). The results reveal not only the yield of the check plot, but also the N responsiveness of the site. This shows that at low soil nitrate levels, sites respond well to applied fertilizer. When fall soil profile nitrate-N levels are greater than 80 to 100 lb/acre, relative yield is approaching 100%, and it is unlikely the site will respond to additional fertilizer N applied in the spring.
click image to zoomFigure 3. Increase in yield due to N fertilization, Delta Yield, as a function of soil N level. 3. A third way to show this relationship between fall soil nitrate and N response is to calculate the Delta Yield, or the increase in yield obtained from the addition of fertilizer at each site. This is a good measure of N responsiveness of an individual research site. The relationship between fall profile N level and Delta yield is shown in Figure 3. It is clear from this graph that at low soil nitrate levels in the profile, sites respond well to applied nitrogen fertilizer. However, as the profile N level increases beyond 75 to 80 pounds N per acre, little or no N fertilizer response was found.
4. A commonly used way to measure the efficiency of N use is to determine the amount of N fertilizer required to produce one additional bushel of yield. This relationship is shown in Figure 4.
click image to zoomFigure 4. Pounds of N fertilizer required per bushel of yield increase at different levels of N responsiveness, or Delta Yield. On highly N responsive sites, those with a large Delta Yield, the amount of N required to increase yield by one bushel is relatively low, less than the 2.4 pounds N per bushel used in the K-State fertilizer recommendations. However, as the yield response decreases, the amount of N required to obtain that response increases dramatically. This relationship provides a good explanation of why fertilizer recommendations are generally made not to obtain the maximum yield, but rather the economic optimum yield. The efficiency of squeezing out those last one or two bushels is just too low. A number of additional conditions such as drought, disease, and poor root growth can influence this relationship. Many of the new technologies being developed to enhance N management and NUE, should help reduce the pounds of N fertilizer required to obtain a bushel of N response.