Predicting Corn N Response Using Alkaline Mineralizable-Nitrogen and Haney Soil Health Tool-Nitrogen in TN

Soil properties

Soil organic matter (SOM), estimated nitrogen release, and Mehlich III extractable phosphorus (P), potassium (K), boron (B), manganese (Mn) iron (Fe), copper (Cu) and Zinc (Zn) were higher at the 0-6 inch compared to the 6-12-inch depth. The decline in SOM, K, and P from 0-6 to 6-12-inch depth supports the prediction that soil organic matter and the most limiting nutrients for plants tend to have shallow distributions. Mehlich III extractable calcium (Ca), magnesium (Mg), sulfur (S) and sodium (Na) increased with depth. This soil property trend was observed at both trial locations (Table 1). Apart from soil organic matter, Ca, Mg, and B, the measured soil properties at the Jackson location were higher than those at the Milan location.

Table 1. Mean preplant chemical properties of soil at 0-6 and 6-12-inch depths collected from the Jackson and Milan locations. Nutrient concentrations are based on Mehlich III extraction (n =24).

TEC, Total Exchangeable Cation; OM, Organic Matter; ENR, Estimated Nitrogen Release

Mehlich III and H3A-4 extractable nutrients

The relationship between Mehlich-III and H3A-4 extractable nutrients at depths (0-6 and 6-12-inch) locations (Jackson and Milan) is presented in Figure 1. Mehlich III and H3A-4 extractable nutrients were significantly correlated and showed a positive linear relationship with the combined data across soil depths and locations except Ca. On average, Mehlich-III extracted approximately 75% more P, 36% more K, 78% more Ca, 73% more Mg, 61% more S, 17% more Na, 43% more B, 86% more Fe, 44% more Zn, 93% more Mn, and 83% more Cu than H3A-4 extractable nutrients (Figure 2). The H3A-4 extracting solution consists of a dilute mixture of four weak acids with a weakly buffered pH of approximately 3.75, that may account for the lower concentration of H3A-4 extractable nutrients. Phosphorus, sulfur, boron, and manganese correlated well (R² = 0.75 - 0.95) while potassium, zinc, copper, iron, and sodium correlated moderately (R² = 0.39 – 0.61). Although the relationships between some nutrient appear highly correlated, existing calibration and correlation data relating conventional soil test to relative yield may not be appropriate for current mineral fertilizer recommendations.

Correlation of alkaline mineralizable N and HSHT’s plant available N with N response

The mineralized N  component of the Haney Soil Health Test (HSHT), often reported as estimated biological N (EB-N) was estimated using water extractable organic carbon, water extractable total nitrogen, and soil respiration). Data on the parameters used to compute EB-N are presented in Tables S1, S2, & S3. Estimated biological N was assessed at preplant for both locations; however, at sidedress (V6 growth stage), EB-N was evaluated at only the Jackson location. At both locations, preplant EB-N decreased significantly with depth, with EB-N at the 0-6-inch depth being at least 50% higher than the other depths. There was no significant difference between EB-N at the 6-12- and 12-18-inch depths, regardless of location (Figure 2 A & B). Estimated biological N at sidedress was significantly lower when compared to the preplant N (Figure 3A; Table S4). In contrast to preplant EB-N, sidedress EB-N increase with soil depth. 

Supplementary Table S1. Preplant Haney soil health test and alkaline-hydrolyzable nitrogen at 0-6, 6-12, 12-18, 0-18, 0-24-inch depths collected from the Jackson location.

SR, Soil Respiration; WEOC, Water Extractable Organic Carbon; WEON, Water Extractable Organic Nitrogen; SHS, Soil Health Score; NO₃^-, H3A-4 extractable available nitrate; NH₄⁺, H3A-4 extractable available ammonium; HAN, H3A-4 Extractable Available Nitrogen; EB-N, Estimated Biological N; PAN, Plant Available N; AH-N, Alkaline-Hydrolyzable Nitrogen

Supplementary Table S2. Preplant Haney soil health test and alkaline-hydrolyzable nitrogen at 0-6, 6-12, 12-18, 0-18, 0-24-inch depths collected from the Milan location.

SR, Soil Respiration; WEOC, Water Extractable Organic Carbon; WEON, Water Extractable Organic Nitrogen; SHS, Soil Health Score; NO₃^-, H3A-4 extractable available nitrate; NH₄⁺, H3A-4 extractable available ammonium; HAN, H3A-4 Extractable Available Nitrogen; EB-N, Estimated Biological N; PN, Plant Available N; AH-N, alkaline-hydrolyzable Nitrogen

Supplementary Table S3. Sidedress Haney soil health test and alkaline-hydrolyzable nitrogen at 0-6, 6-12, 12-18, 0-18, 0-24-inch depths collected from the Jackson location.

SR, Soil Respiration; WEOC, Water Extractable Organic Carbon; WEON, Water Extractable Organic Nitrogen; SHS, Soil Health Score; NO₃^-, H3A-4 extractable available nitrate; NH₄+, H3A-4 extractable available ammonium; HAN, H3A-4 Extractable Available Nitrogen; EB-N, Estimated Biological N; PN, Plant Available N; AH-N, alkaline-hydrolyzable Nitrogen

Alkaline hydrolyzable N (AH-N) was estimated using the direct distillation procedure outlined by Roberts et al., (2009), which involves a 7-minute distillation using 10 mL of NaOH. Alkaline hydrolyzable N was estimated at preplant for both locations; however, at sidedress (V6 growth stage), AH-N was evaluated at only the Jackson location. At both locations, AH-N decreased significantly with depth, where the AH-N at the 0-6-inch depth was significantly higher than the other depths, but there was no significant difference beyond the 0-6-inch depth for both locations (Figure 2 C & D). AH-N at sidedress was significantly lower when compared to the preplant N (Figure 3B). However, sidedress AK-N, exhibited similar trend as preplant N with respect to the relationship between AH-N and soil depth. A significant positive relationship (R² > 0.8) was observed between organic matter and AH-N, meaning the lower AH-N observed with depth and sampling time may partly be attributed to lower soil organic matter content. 

Relationship between AH-N and EB-N at preplant and sidedress across soil depths and locations is presented in Figure4. Alkaline-hydrolyzable exhibited a negative significant linear relationship with EB-N over soil depth and location, regardless of the time of soil sampling. At preplant and sidedress, the highest R² value for the AH-N and EB-N was 0.15 and 0.65 at the 0-6-inch and 12-18 inch-depth, respectively. 

Application of N significantly increased corn yields at both locations (Figure 5). The Jackson and Milan locations exhibited a significant and a positive response to fertilizer N. Corn response to N at the Jackson location did not plateau. In contrast, grain yield increased with N rate until a plateau was reached where yield stabilized at the Milan location. The predicted fertilizer N rate required to maximize yield at the Milan location was 162 lb N a^-1. Correlation of check plots with AH-N resulted in a significant linear regression model for 0-6- and 12-18-inch depths at preplant (Figure 6). At preplant, the best predictive relationship between the check plots and AH-N was found at the 12-18-inch depth (R² = 0.46). In contrast, the correlation between the check plots and EB-N was not significant at all depths (Figure 6).

Results for Objectives (2) develop N fertilizer rate calibration using alkaline-hydrolyzable N, and (3) compare current UT fertilizer recommendation, HSHT N fertilizer recommendation, and alkaline-hydrolyzable fertilizer recommendation will be provided after the second-year research.

2022

Soil properties

Soil organic matter (SOM), estimated nitrogen release, and Mehlich III extractable phosphorus (P), boron (B), manganese (Mn) iron (Fe), copper (Cu) and Zinc (Zn) were higher at the 0-6 inch compared to the 6-12-inch depth at both locations. The decline in SOM, and P from 0-6 to 6-12-inch depth supports the prediction that soil organic matter and the most limiting nutrients for plants tend to have shallow distributions. Mehlich III extractable calcium (Ca), magnesium (Mg), sulfur (S) and sodium (Na) increased with depth. This soil property trend was observed at both trial locations (Table 1). 

Table 1. Mean preplant chemical properties of soil at 0-6 and 6-12-inch depths collected from the Jackson and Milan locations. Nutrient concentrations are based on Mehlich III extraction (n =24).

The soil samples collected for the alkaline mineralizable-N and Haney Soil Health Test were not analyzed because of in-season drought. The concern was the possibility of crop failure, which would render any soil information less useful. The drought severely affected corn yields in most counties in West TN. The fields that we set up our experiments were no exception. We were unable to harvest one location and for the harvested field, yields were very poor. As a result, we requested for a no-cost extension to repeat the trial for another year.