Quantifying Clay-Fixed Ammonium in Paddy Soils and Determining Its Availability Using Alkaline Distillation

The Nitrogen Soil Test for Rice (N-STaR, Oryza saliva L) has been calibrated and is being used to predict rice nitrogen (N) fertilizer rates. Research has identified the organic forms of N quantified via N-STaR (NH4 released by distillation with 10 mol L-1 NaOH), but no research has looked at the ability of N-STaR to quantify clay-fixed NH4 (CF-NH4). This laboratory trial was initiated to determine the amount of CF-NH4 quantified via N-STaR as well as the influence of soil drying conditions (oven dry vs. field moist) and alkali distillation method (NaOH vs. KOH) on the CF-NH4 recovered. Seven soils representing five clayey and two loamy soil textures were collected from various rice-producing regions across Arkansas from the 0- to 15-cm and 15- to 30-cm soil depths. Total CF-NH4 and CF-NH4 recovered by N-STaR varied greatly across soils and soil depths with a range of 35 to 299 mg N kg soil(-1) and 6 to 21 mg N kg soil(-1), respectively. The N-STaR recovered as much as 15% of total CF-NH4 and recovery was greatest for silt loam soils. Alkaline hydrolyzable-N (AH-N) concentrations were strongly influenced by soil drying conditions and the alkali used with a higher level (22%) of AH-N recovered from oven-dry than field-moist soils and a higher (7%) recovery of AH-N with NaOH than KOH. These results indicate that the N-STaR method recovers a small, but significant percentage of total CF-NH4 and may capture the portion of CF-NH4 that may be plant available during the growing season.