Cotton response to phosphorus fertigation using subsurface drip irrigation

Subsurface drip irrigation (SDI) has expanded dramatically in cotton producing areas of the southwest United States, especially where furrow irrigation was practiced. There is ancillary evidence that cotton may experience shortages of phosphorus (P) and other nutrients more readily under SDI compared with furrow irrigation. This may be related to the smaller wetted volume of soil and hence smaller root volume under SDI. Some cotton producers using SDI in the Trans Pecos region of Texas have observed large increases in cotton lint yield by the addition of small amounts of P; however, cotton response to different P rates under SDI has not been documented in this region. This article presents results of a field experiment repeated during the 2003, 2004, and 2005 growing seasons where cotton lint yield, quality characteristics, and lint gross return were evaluated for different P application rates. The experiment was completely randomized with four treatments and three replications. Treatments were: 1) no phosphorus applications; 2) injection of 4.6 kg ha(-1) y(-1) P contained in Miller Solugro (R) (12-48-8); 3) injection of 32.9 kg ha(-1) y(-1) P contained in phosphoric acid; 4) injection of 65.7 kg ha(-1) y(-1) P contained in phosphoric acid. The cost of P treatments ranged from $21.10 to $42.20 ha(-1) y(-1), but marginal gross returns for lint were $520 to $1,252 ha(-1) y(-1) more from phosphoric acid and $233 to $872 ha(-1) y(-1) more from Miller Solugro (R). In 2003 and 2004, lint yields and gross returns were greatest for the 65.7-kg ha(-1) y(-1) P rate, which was similar to recommended rates based on a 2004 soil fertility analysis. In 2005, lint yields and gross returns were greatest for the 32.9-kg ha(-1) y(-1) P rate, which was slightly below recommended rates based on pre-plant 2005 soil fertility analysis. These results support injecting P at rates based on soil fertility analyses. Both phosphoric acid and Miller Solugro (R) were suitable for injecting P into SDI. systems. The P2O5 application rates increased the productivity per unit of water applied during the three years of the study. Plant petiole analysis in 2004 correlated with P application rates, suggesting a qualitative method to detect P deficiencies during the growing season.