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Fluid Journal : Fluid Journal 1993-1995
3 Fluid Journal Fall 1994 uptake will reflect only soil differences. Soils/treatments. We collected ten surface soils representing those on which cotton is grown in Louisiana, Arkansas, and Mississippi. Initially available phosphate, as determined by Bray 2 extraction, varied from a medium level (82 ppm) to a very high level (518 ppm) among the soils. Solution fertilizers were sprayed onto the soil in the grades shown in Table 1. Soil and fertilizer were thoroughly mixed. Sources. Fertilizer sources were ammonium polyphosphate (11- 37-0) as a phosphate source, UAN (32-0-0) as a nitrogen source, and ammonium thiosulfate (12-0-0-26S) as a sulfur source. Potassium hydroxide (47% K2O and Sequestrene zinc (14.2% Zn) were the other source materials used. P soil supply After a 21-day incubation, treatments were analyzed for amount of phosphate in soil solution. Soil solution was displaced with a displacement column method. Amount of phosphate adsorbed on the solid phase of the soil was determined with anion exchange resin, Buffer power and effective coefficients for phosphate then were calculated for each treatment. Addition of starter fertilizer increased both solution and solid phase phosphate for all soils in this study. The increase in both solution phosphate and solid phase phosphate tended to be a function of the total amount of phosphate in the starter, rather than the N:P2O5 ratio. An obvious exception was the concentration in solution displaced from treatments that had complete starter fertilizer applied. Sulfate in this material had an acidifying effect; therefore, additional phosphate was released into solution. Since phosphate in solution is the most readily available form, this may affect phosphate uptake. It's worthwhile to note, however, that solid phase phosphate was not affected by sulfate addition. Predicting uptake Soil supply data for phosphate were used in the Barber mechanistic model to predict phosphate uptake by young cotton plants (Table 2). The relative increase in phosphate uptake due to starter applications was calculated for each treatment. Effect of the starter was directly related to the relative increase in solution phosphate concentration after fertilizer application. Although total uptake was greatest for Commerce silt loam, the relative increase in phosphate uptake was less, due to the greater inherent solution phosphate concentration. The relative increase in phosphate uptake was greatest for the Gigger-Gilbert complex, which had less inherent solution phosphate. Considering previous research on effectiveness of phosphate placement, these results can be expected. However, total phosphate required to improve cotton growth and yield may be greater than can be supplied by the Gigger- Gilbert com-plex, even though available phosphate (as predicted by the model) was increased by starter application. In this case, benefit from starter application would be more effective use of applied phosphate, rather than a significant yield increase. Nutrient interactions In order to determine overall effect of starter applications, interactive effects were also considered. Data in Table 3 show the effect of starter application on concentrations of P, K, S, and Zn in soil solution. The N:P2O5 ratio obviously affected the concentrations. Since nutrients in soil solution are those most readily available to plant roots, availability of other elements also was affected by the starter applications. As expected, highest concentrations were measured in treatments with complete starter applications. Dr. Kovar is assistant professor and Dr. Funderburg is extension specialist at Louisiana State University. !
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