Most of the soil nitrogen (N) is present in organic form, especially in the top 6 inches (75 – 95%), which is not readily available for plant uptake. Only about 0.1 – 0.3% is in inorganic or bioavailable forms such as nitrate and ammonium that plants can utilize. So, organic soil N is converted to inorganic forms for plants use, a process referred to as mineralization. The fraction of organic soil N that can be converted to inorganic form is referred to as potentially mineralizable N. Several factors including soil, previous crop, weather, and nitrogen management determine the extent of N mineralized. Nitrogen rate trials (>30) conducted over the past five years in West and Central TN suggested that mineralized soil N can contribute more than 20% of the total crop N requirements. Current N fertilizer recommendation for corn in Tennessee is based on realistic yield goals, which does not account for potentially mineralizable N that is available for the plant.
Adjustments to fertilizer N can only be made for corn grown on field receiving manure by using the Pre-Sidedress Nitrate Test (PSNT). A number of tests have been proposed to estimate potentially mineralizable N in row crops including the Illinois soil N test (ISNT) and the direct steam distillation (DSD) that measure the alkaline-hydrolyzable N. Several studies have been conducted to predict N fertilizer needs using ISNT in North central and Northeastern regions of the United States. However, there are discrepancies in the results. The DSD was shown to predict N crop response, and it is used for fertilizer N recommendation for rice in Arkansas. A crop response to mineralizable N may help develop a site-specific N fertilizer recommendation tool for corn, thus improving the efficiency of fertilizer N application. Below are the findings of the work done in TN to examine the suitability of alkaline mineralizable-N to predict corn N needs.
Research in Tennessee
Soil samples were collected from the 0 lb N ac-1 plots at different depths (0-6, 0-12, 0-18, and 0-24-inch depth) at sidedress and analyzed for alkaline mineralizable-N using University of Arkansas’ DSD (Roberts et al., 2011). The nitrogen fertilizer rates, which varied across trial locations ranged from 0 to 300 lb N ac-1. A linear model was used to correlate the yield of the No N (check) plants to mineralizable N at 4 soil depths.
The alkaline-hydrolyzable N for 0-6, 0-12, 0-18, and 0-24-inch depth ranged from 248 – 348, 188 – 254, 140 – 224, and 124 – 208 lb/A, respectively. These values are within the ranges reported by Scientists in the Southern US. Significant differences in average mineralizable N were observed among soil depth, with the highest amount (289 lb/A) at the 0-6-inch and decreased with depth (Figure below).
All the N trials showed a positive linear response to N fertilizer application. Grain yield of the check plots ranged from 50 to 131 bu per acre, with the relative grain yield values ranging from 23 to 59% of the maximum yield. A significant and positive linear relationship was observed between the check plot yield and alkaline-hydrolyzable N for all depths, with the best predictive relationship occurring at the 0-18-inch depth (R2 = 0.682) (Figure below). This suggests that mineralizable-N at soil 0-18 inch has substantial influence on the predictability of crop response to N fertilizer, which aligns with previous studies in other states.
In conclusion, current findings suggest that alkaline-hydrolyzable N using the DSD method moderately correlates with corn N response in Tennessee, so there is a potential to predicting N response, especially at the 0-18-inch soil depth. On-going research funded by the Tennessee Corn Promotion Board will provide more information to ensure a robust prediction of DSD and the probability of success.
