Changes in Soil Test Phosphorus over Time As Affected By Phosphorus Rate and AVAIL.

Soil test phosphorus (P) continues to be monitored along with cotton yield from a 4-year field study initiated in 2010 to evaluate cotton response to P fertilization in a continuous rain-fed production system.  The fertilizer P was applied with and without AVAIL® Phosphorus Fertilizer Enhancer (Specialty Fertilizer Products, Inc.).  The original study consisted of a factorial arrangement of P rates (0, 20, 40, and 60 lb P/acre) and AVAIL (0 or 0.5 gal/ton) with no additional P added after the 4^th year.  All treatments were included in a randomized complete block design with six replications.  Pre-weighed P fertilizer (CSP, 0-46-0) was applied to the surface in the fall of each year following cotton harvest.  Tillage was then used to incorporate the fertilizer material.  Cotton was grown with uniform cultural practices and harvested following defoliation with a commercial cotton picker adapted for plot harvest.  Grab-samples were collected at the time of harvest and then ginned through a 10-saw micro-gin to determine lint percentage and calculated lint yields.  Soil Samples were collected immediately following harvest and prior to the next P addition.  Seedcotton and lint yields have shown no significant response to increasing P rates.  Soil test P levels increased with fertilizer P additions.  Adding AVAIL Phosphorus Fertilizer Enhancer was not effective in increasing cotton yields in this study where soil test P levels were high.  Fertilizer additions under high test situations should result in buildup of soil test levels and with time should decline as crops are removed.  In this study, AVAIL has not affected the soil test P levels in the top six inches of soil.  Soil test P was significantly increased with the higher P applications in excess of crop removal.