ABSTRACT

Soil fertility is an important controllable factor that affects cotton growth and subsequently affects water use efficiency. If water is available through irrigation or timely precipitation, the availability of plant nutrients becomes a growth limiting factor. In previous years, Southern High Plains producers have continued nitrogen (N) fertilizer applications while reducing phosphorus (P) inputs. A cooperative soil test survey of over 250 locations indicated that 60% of the surface samples tested were low to very low in P, and over 80% were low to very low in the second foot. The objectives of this study were to demonstrate the effects of P application and placement on lint yield, using area producers fields.

In 1987, six sites, all low to very low in soil test P, were selected. Plots ran the length of the field, and were randomized but not replicated at each location. All plots were machine harvested. Treatments were: 1) an untreated check, 2) 60 lb N/a, 3) 60 lb N/a + soil test recommended P (ST-P) in a conventional band application approximately 10 inches on either side of the bed and six inches deep, 4) 60 lb N/a + ST-P 1/2 applied conventional and 1/2 applied in a band on either side of the row greater than the one foot depth, 5) 60 lb N/a + ST-P all applied deep, and 6) 60 lb N/a + ST-P conventional with chisels pulled deep. Fertilizer P applications ranged from 20 to 40 lb P₂O₅/a. Yields were subjected to appropriate statistical analysis with locations treated as blocks.

Yields in 1987 ranged from an average of 377 lb lint/a at the Crosby County site to 1113 lb lint/a at the Parmer County location. Treatment means separated at p=0.05 based on P fertilization. The untreated check and the N only treatments averaged 543 lb lint/a compared to an average of 622 lb lint/a for all P treatments. Placement, conventional or deep P, had no effect on lint yield.

In 1988, six new sites were selected. Plot design was the same as in 1987, but treatments were: 1) untreated, 2) 60 lb N/a, 3) 100 lb N/a, 4)60 lb N/a + ST-P conventional, 5) 100 lb N/a + ST-P conventional, 6)60 lb N/a + ST-P conventional + 150 lb P₂O₅/a applied in a single band 3 inches offset from the planting row and greater than 14 inches deep, 7) 100 lb N/a + ST-P conventional + 1 50 lb P₂O₅/a deep, and 8) 60 lb N/a + ST-P conventional + deep chisel.

Lint yields averaged 140 lb/a greater in 1988 compared to 1987. Results, however, were similar to those in 1987. The two N only treatments averaged over 100 lb lint/a more than the untreated check, but were not statistically different. All treatments receiving P fertilizer yielded significantly higher than the check plots, but fertilizer P placement effects could not be separated out. These results indicate that P fertility is important in Texas Southern High Plains soils. While deeper placement of P may offer some advantages, it can not be justified simply on the basis of soil fertilizer P application.