Evaluation of a mechanistic mathematical model to predict nutrient uptake by field-grown paddy rice and selection of a chemical extractant to assess phosphorous availability in rice soils

Three rice (Oryza sativa L.) cultivars were grown on a Crowley silt loam at the Rice Research and Extension Center to evaluate the ability of a mechanistic model to predict NH{dollar}sb4sp+,{dollar} P and K uptake. Total nutrient uptake and root and soil parameters were measured on soil samples representing 5 cm depth increments down to 40 cm during vegetative and reproductive growth. The soil and plant parameters were input into the Barber-Cushman model to predict nutrient uptake. The model accurately predicted nutrient uptake during vegetative growth but underpredicted uptake during reproductive growth. It is postulated that rapid root senescence during reproductive growth prevented accurate estimation of root length developed during this stage of growth and resulted in the underprediction. Approximately 90% of the total NH{dollar}sb4sp+,{dollar} P and K uptake by rice occurred within the surface 20 cm of soil. Ammonium and P uptake in the 0- to 20-cm of the soil depth were most sensitive to half-distance between root axes, root radius, nutrient solution concentration and buffer power. Potassium uptake was most sensitive to maximum influx rate, root radius, Michaelis constant and root growth rate throughout the soil profile.; The Mehlich-3 extractant, used by soil testing, is not consistent in predicting rice response to P fertilization on three soils (Crowley silt loam from Arkansas county and a Hillemann silt loam from Poinsett and Cross counties). A greenhouse study was conducted to evaluate the effect of P fertilizer on plant growth parameters and to identify a suitable chemical extractant to assess P availability in flooded rice soils. Rice was grown on these soils with fertilizer rates of 0, 5, 10, 20 and 40 mg P kg{dollar}sp{lcub}-1{rcub}.{dollar} Shoot and root measurements were taken at three sampling dates (37, 44 and 137 days after emergence (DAE)). Phosphorus fractionation and P extracted by Mehlich-3, Bray-1, Olsen and anion exchange resin (resin) were conducted on these sampling dates and also at 0 days after flooding (DAF). At 137 DAE, kernel weight and total P uptake by rice grown on the three soils were maximized at 10 mg P kg{dollar}sp{lcub}-1{rcub}.{dollar} Phosphorus fertilizer applied to the three soils stimulated organic P mineralization and increased the nonoccluded and solution P fractions. Nonoccluded P and solution P fractions were significantly correlated to most plant growth parameters measured. Extractable P measured by resin at 0 DAF was consistently correlated to total P uptake measured at 24, 31 and 124 DAF. The Barber-Cushman model indicated that the resin method is suitable to assess available P in these soils. The resin, with its high exchange capacity compared to soil, removes the most available P in soil.