Changes in Cotton Yield and Soil Test Phosphorus As Influenced By Applied P Fertilizer in Continuous Rain-Fed Cotton.

Soil test phosphorus (P) continues to be monitored along with cotton yield from a long term field study initiated in 2010. Cotton yield response to P fertilization was the primary objective of the original research with treatments maintained for four years. Fertilizer P was initially applied with and without AVAIL® Phosphorus Fertilizer Enhancer (Verdesian Life Sciences). The study had 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 4th year. All treatments were included in a randomized complete block design with six replications. Pre-weighed P fertilizer (CSP, 0-46-0) was fall-applied to the surface following cotton harvest and spoil sampling. 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 initially but have shown a downward movement. Adding AVAIL Phosphorus Fertilizer Enhancer was not effective in increasing cotton yields in this study where soil test P levels were high and has had no influence on soil test P. 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 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.