Alkaline Hydrolyzable-Nitrogen, Seeding Date, and Clay-Fixed Ammonium as Potential Indicators of Rice Response to Nitrogen Fertilization in Arkansas

Several potential indicators of rice (Oryza sativa L.) response to nitrogen (N) fertilization include the soil's alkaline hydrolyzable-N (AH-N) concentration, seeding date, and the soil's clay-fixed NH4-N content. Three studies were conducted to: (1) correlate AH-N, determined using Illinois Soil Nitrogen Test (ISNT) or Nitrogen Soil Test for Rice (N-STaR), to plant parameters of interest and develop a fertilizer N rate calibration curve capable of predicting the 95% relative grain yield (RGY) fertilizer N rate for rice grown on clayey soils; (2) evaluate the influence of seeding date and N fertilization on rice plant growth and grain yield; and (3) evaluate the plant-available portion of clay-fixed NH4-N using N-STaR and quantify total clay-fixed NH4-N using dry combustion. Alkaline hydrolyzable-N, quantified using ISNT or N-STaR in the first study, explained 83% of the variability in calibration based on a 0 to 30-cm sampling depth. The second study revealed that grain yield exhibited a tendency to be greater for rice seeded in April or May compared to June seeding dates in Arkansas. Results of the third study indicated a low percentage (4-6%) of total clay fixed NH4-N was predicted to be plant-available NH 4-N based on N-STaR. Fertilizer N rate recommendations based on the ISNT or N-STaR soil test methods evaluated in the first study revealed the potential of soil-based tests to improve economic, agronomic, and environmental aspects of rice produced on clayey soils. Seeding date selection can influence rice grain yield which may in turn influence fertilizer N management. However, the fertilizer N rate required to maximize grain yield when seeding date was delayed remained unclear following completion of the second study and continued research is required to identify the influence of seeding date on the yield-maximizing, fertilizer N rate for rice grown in Arkansas. The recovery of clay-fixed NH 4-N by N-STaR indicated a portion of total clay-fixed NH4-N was expected to be available for plant uptake. However, additional research is required to verify the low recovery of clay-fixed NH4-N and the influence of clay-fixed NH4-N on the quantity of AH-N determined by N-STaR.