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Fluid Journal : Fluid Journal 1993-1995
3 Fluid Journal Fall 1993 700 600 500 400 300 200 100 0 Pounds Lint/A Irrigated Dryland 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 Years Figure 2. Historical cotton lint yield for irrigated and dryland production on the souther n High Plains of Texas. from planting to first flower is greater than 60 percent. A final plant density of 25,000 to 30,000 plants per acre is the optimum for this water supply. Each plant should be capable of producing 6 to 7 mature fruit with this water supply, resulting in 400 to 500 pounds of lint per acre. If plant density exceeds 60,000 plants per acre, production capacity per plant drops to 2 fruit per plant, resulting in yields in the 250- to 300-pound per acre range with the same water supply (Table I). Scheduling/fertility Under irrigated conditions, irrigation scheduling and fertility management become critical considerations. Considerable evidence is emerging that suggests "high frequency/lower volume" applications of irrigation water can increase yield and water-use efficiency compared with "less frequent/higher volume" applications. In our own work, we have found that a 3- to 6-day frequency replacing 75 to 80 percent potential ET produces the highest yields (Table 2). These types of application frequencies and volumes can be readily achieved using center pivot sprinklers and a volume equivalent of 3 to 3.5 gallons per minute per acre. For instance, a 125- acre center pivot pack-aged for 450 gallons per minute can apply one inch per acre every 5.2 days at 100 percent efficiency. Although 100 percent application efficiency probably can't be achieved, efficiencies greater than 90 percent are possible. Nutrient requirements In order to maximize water-use efficiency of the cotton production system, adequate nutrient supplies must be provided to allow the plant to produce maximum yields within limits of the water supply. It takes about 4 inches of water to get the plant big enough just to begin to reproduce. For each inch of water above 4 inches, approximately 50 pounds of lint (by- product of seed production) can be produced. For each pound of lint, approximately 1.6 pounds of seed are produced. Nitrogen. Cottonseed is approximately 25 percent protein, which requires about 35 pounds of N per bale of lint (500 pounds) just to produce seed (Table 3). N requirement for vegetative material is another 15 pounds per bale. Therefore, if each inch of water is capable of producing 50 pounds of lint, N required to support this production is 5 pounds per inch of water. We use this guideline for N management under both irrigated and rainfed conditions. Preplant N applications are based on the stored soil water supply at a rate of 5 pounds N per inch of stored water. Supplemental N is applied based upon rainfall and irrigation provided. We want 90 percent of the total applied N to be in the soil by first flower (usually 60 days after planting), such that we apply N during the season at the rate of 10 pounds of N per inch of water supply up to first flower. We then stop soil N applications if dryland or apply at the rate of 5 pounds per inch under irrigated conditions for an additional three weeks (peak flower period). N applications based on water supply optimize N-use efficiency. Phosphorus. Due to the calcareous nature of soils in Texas and on the southern High Plains in particular, phosphorus is rapidly tied up as calcium phosphate and reaches an equilibrium with the soluble pool, which can't be influenced to any great degree. We use a maintenance approach to phosphorus fertilization. Each bale of cotton removes about 20 pounds of P2O5. We recommend replacing the phosphorus removed by last year's crop. Zinc. Clay loam soils average 0.5 to 0.7 ppm extractable zinc, whereas sandy soils (loamy fine sand) average 0.2 to 0.3 ppm. We find a foliar application of zinc to be more effective than a soil application of ZnSO4. We recommend about 0.1 pounds of Zn per application, with the first application during early square development (6- to
Fluid Journal 1996-1998