Nitrogen Fertilizer by Two Rice Cultivars Grown on an Arkansas Clay Soil

Rice (Oryza sativa L.) grown on a clay soil in a direct-seeded, delayed-flood system as used in the southern U.S. requires 30 to 60 lb/acre more nitrogen (N) fertilizer to achieve maximal grain yields compared to rice grown on silt loam soils. Seasonal N fertilizer accumulation studies are almost non-existent for rice grown on clay soils. Therefore, a two-year field study was conducted to evaluate uptake of fertilizer 15N applied preflood to ‘Wells’ and ‘Cocodrie’ rice grown on a Perry clay (very-fine, smectitic, thermic, Typic Epiaquert). Aboveground plant samples were taken throughout the season for total dry matter, total N, and fertilizer 15N accumulation. Grain yield was higher for Cocodrie during 2000 and Wells during 2001. Fertilizer 15N accumulation patterns were dissimilar between study years. Fertilizer 15N uptake in both cultivars was maximized at 50% heading during 2000. However, during 2001, peak 15N uptake occurred at 21 or 28 days after flooding, which is comparable to previous fertilizer 15N uptake studies of rice grown on silt loam soils. In 2001, the preflood-N fertilizer application had to be delayed for a week or two because of wet soil conditions from frequent rains. The resulting delay in preflood-N fertilizer application to larger rice plants with probably a large root biomass led to a rapid uptake of preflood-N fertilizer with an efficiency of around 70%. Consequently, delaying the preflood-N fertilizer and flood on clay soils a week or two past the 4to 5-lf growth stage might result in increased root biomass and allow rice grown on clay soils to utilize fertilizer-N more efficiently. AAES Research Series 550