Project Details:

Adequate P nutrition of rice is critical for production of optimum grain yields. Unfortunately, our research data suggests that soil test methods commonly used for upland crops are poor predictors of rice response to P fertilization. We have improved our P fertilizer recommendations for rice by considering soil pH with Mehlich 3 extractable soil P. Rice grown on high pH silt loam soils tends to be more responsive to P fertilizer than when grown on acid soils. In fact, significant rice yield reductions from P fertilizer application to rice grown on acid silt loam soils has been documented several times by Arkansas researchers over the past 30 years.

Over the past four years our research focus has shifted to gaining a better understanding of P nutrition of rice and soybean and Mehlich 3 extractable soil P with respect to P fertilization practices and previous crops grown in the rotation. We continue to investigate these aspects of crop nutrition but are also increasing our efforts to better understand the seasonal P uptake of soil P and fertilizer P.

We have previously evaluated the effect of P application time (in relation to rice growth stage), rate, and fertilizer source on rice yield and total P uptake. The previous tests did not evaluate the effect of P application timing on P uptake at different intervals after different P application times to estimate the uptake efficiency of P fertilizer, via whole plant P content. Additionally, the latest time of P fertilizer application studied was ½ inch internode elongation. Observations of P deficient rice in fields where preplant P was applied suggest that P fertilizer may be unavailable when applied several weeks before flooding when the rapid growth of the rice plant requires P availability. Our previous time of P application research suggested that P applied at emergence, preflood, or within 7 days after flooding was sufficient to maximize dry matter and grain yield where significant yield increases were measured. However, P application at ½ inch IE resulted in numerically higher yields than the control but were also lower than P applied at emergence, preflood, or within 7 days after flooding. Because much of our P research must be performed in grower fields where P response is most likely to occur we have not evaluated P fertilizer applied in the fall or several weeks before seeding. One way of evaluating the effect of application time on P fertilizer uptake efficiency is to collect sequential whole plant samples and track tissue P concentration and content over time. The attached protocol will be researched during 2001 in two or three high pH silt loam rice fields located in Cross County, AR. This data should help us refine our recommendations and better answer grower questions as to the best time to apply P fertilizer.