Impact of Annual Phosphorus Fertilization On Mehlich-III Extractable Phosphorus of an Alluvial Soil Under Corn and Cotton Rotation.

Corn (Zea mays L.) and cotton (Gossypium spp.) are commonly rotated on the alluvial plain along the Mississippi River in Northeast Louisiana. Phosphorus calibration research for corn and cotton under rotation is understudied in this area. This study was established in 2008 in Northeast Louisiana to evaluate the effect of annual application of P fertilizer on Mehlich-3 extractable P (M3-P) and yield of corn and cotton under rotation. Two fields (A and B) both having Commerce silt loam soils were selected. Prior to planting, two sets of P application rates (0, 34, 67,101, and 134 kg P₂O₅ ha^-1) were established as treatments where one set was applied one time before planting of the first cropping year, and the second set was annually applied before planting from 2008 to 2011. During the first cropping year, Field A was planted to corn while Field B to cotton; the following year, corn in Field A was rotated with cotton and cotton in Filed B was rotated with corn. The rotation of corn and cotton in the two fields was continued in the subsequent years. The treatments were arranged in a randomized complete block design with four replications. Soils (0-15 cm depth) in Field A and B were initially tested low of M3-P with only 15 and 17 mg kg^-1, respectively. On average, the highest corn and cotton yield were both obtained in Field A in 2008 and 2011, respectively. Across fields and years, corn unlike cotton responded to P fertilization. The annual application of P fertilizer from 2008 to 2011 did not result in large differences in M3-P when compared with P application done only in 2008. In 2011, an average of only 10 mg kg^-1 increase in soil test P was recorded across plots which annually received P fertilizer for both Fields A and B. Soils samples collected from 15 to 30 cm depth did not show  observable changes on M3-P as well. It appears that M3-P did not entirely reflect the P nutrition status of these two soils which eventually influenced corn and cotton yield. Our findings suggest that applied P fertilizer may have resulted in build-up of different P functional pools that could not be accounted by simply measuring P extracted by Mehlich-3 solution.