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Fluid Journal : Fluid Journal 1999-2001
2 Fluid Journal Spring 2000 measured in heat unit accumulation. The long-term average for the Lubbock area is 2,300 heat units (°F), which is sufficient to produce 1,000 to 1,200 pounds per acre yield if water supply is adequate. However, disturbance in the spring (such as hail storms) can cause replanting, shortening the season length by 15 to 20 percent. Over 85 percent of the yield variability in cotton is due to the boll number component. Boll size (lint and seed weight per boll) has a strong genetic influence but contributes only 10-12 percent to total lint yield variation across environments. Boll number is a function of plant density, fruiting sites per plant, and fruit retention. Plant density is largely controlled by the producer. Previous research (also funded by the FFF) has clearly defined the optimum plant density for cotton production on the Texas High Plains. Optimum density varies based upon the water supply and ranges from 2 plants per foot of row under limited water conditions to 4 plants per foot of row under optimum water supplies. Water supply has the major influence on production of fruiting sites on a cotton plant. Fruit retention is strongly dependent on the supply of reduced C and N to the developing fruit. Fruit size is largely controlled by the number of developing embryos in the fruit, which is dependent upon the food supply to the developing embryo during the first 10 days after pollination. North Carolina research has indicated that P fertilization influences boll size but not fruit numbers. Studies in Mississippi have suggested that cotton responds to P only when adequate N is available. The purpose of this research was to first determine whether P fertilizer can be effectively applied through irrigation water in combination with N and S and, secondly, to determine the optimum N:P205 ratio for cotton production using fertigation. Effect of Application Method Although soil test P levels were in the 25-30 ppm range for the Olsen bicarbonate test, which is considered medium to medium-high, lint yield responses were observed every year for the fertigation treatment. The preplant banded P provided a response over no P in two of three years. Sidedress application resulted in sufficient root pruning to negate the effect of multiple P applications. Sidedressing at first flower pruned sufficient roots to interfere with water and nutrient uptake, which decreased boll numbers. A 50-lb/ A yield increase was observed by simply using fertigation rather than preplant P applications. Lint yield increases were due to increased boll numbers and increased boll size. Boll numbers were related to an increase in bolls found on the 6th to 10th fruiting branches, whereas boll size was a function of increased micronaire (more mature fibers). Water is critical N:P205 ratios showed that cotton response to P is largely determined by water supply. At 2 gallons per minute per acre (GPM/A), plants did not respond to added P as water was the most limiting factor. Figure 3. Effect of N:P2O5 ratio on boil size, Reiter, et al. , Texas Tech University, 1999. Figure 4. Effect of N:P2O5 ratio on micronaire, Reiter, et al., Texas Tech University, 1999.
Fluid Journal 1996-1998
Fluid Journal 2002-2004