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Nutritional investigations in cotton, Emerald Irrigation Area

D.E. Hibberd, W.D. Hamilton and N.G. Soley

Poor cotton growth had been experienced on soils of the Emerald Irrigation Area with resultant unsatisfactory low yields of lint in the order of 450 to 800 kg ha-1. The problem occurred predominantly on Open Downs (BUg) soil and to a lesser extent on scrub (Tb Ug) and alluvial (AUg) soil. Identification of deficient nutrients from plant symptoms remained uncertain. The suspected 4 deficient nutrients P, K, S and trace (Zn, Cu, Mn and Mo) were applied in a 2 factorial field trial on a previously low yielding BUg site using two replicates. Lint yields are presented in Table 1.

TABLE 1. Cotton lint yields (kg/ha) from BUg site, Emerald

The phosphate treatment gave a highly significant response in yield representing a massive increase of 82%. No other nutrient responded significantly. The soil P status (bicarbonate extraction) was 5 ppm P in the 0-10 cm depth with 2 ppm P down the profile. Phosphate was applied as super king (19.2% P) banded in the hill at 40 kg P ha-1. Table 2 shows that many of the soils sampled from 1975 to 1979 fall into the < 10 ppm bicarbonate P category. P deficiency in the area previously undetected may be widespread and would have been a major cause of the poor cotton growth.

TABLE 2. Bicarbonate P (ppm) status of soils, Emerald Irrigation Area 0-10 cm depth

The results are noteworthy in that phosphate responses in cotton hitherto had not been recorded on low P soils in Queensland (St. George and the Darling Downs) or in New South Wales (Warren Area).

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