Root growth in subsoils of Oxisols is often precluded because the roots of many crops are sensitive to soil acidity. This increases plant stress during dry periods when available soil water in the limed surface layers is exhausted. The low soil CEC and high rainfall in Central Brazil suggest that soluble calcium salts might leach below the plow layer and reduce soil acidity. To test this possibility, soil columns, simulating field profiles, were constructed in the laboratory and various Ca salts were mixed in the 0 to 15‐cm layer. Addition of 1,200 mm water to 2,000 kg/ha Ca added as CaCl2, CaSO4, and CaCO8 caused Ca movement to 180, 75, and 25 an, respectively. Calcium sulfate decreased Al saturation and increased soil pH at depth. Several field experiments were sampled to determine the effect of previous treatments on the movement of Ca + Mg. Three to four years after application of gypsum, in ordinary superphosphate, subsoil pH and Ca + Mg status were increased, and Al saturation decreased at depths as great as 75 to 90 cm. Zea mays L. roots growing in the improved subsoil environment were able to take up water and withstand droughts.
High crop yields on P‐deficient Oxisols are not possible without application of relatively high amounts of fertilizer P. Correction of P deficiency can be accomplished by an initial broadcast application of a high rate of P or by lower rates of P applied as a band treatment or combination broadcast and band treatment. The residual effect of such P applications is influenced by the rate and method of application. A field study was established on a Haplustox in the Cerrado of Contral Brazil to evaluate the residual value of applied P as measured by corn yields and soil P analyses.Comparisons were made of the residual‐P effectiveness where a total of 140, 280, and 560 kg P/ha were applied initially as one broadcast application or, in equal increments as banded applications, to the previous four crops.With a total application of 140 and 280 kg P/ha, the residual P from the banded applications gave greater corn yields and P uptake than the initial broadcast applications. At the 560 kg P/ha rate, where yields were nearly maximum, there were no differences in yield due to the application method.Soil test P levels indicated a considerable residual effect from the higher rates of broadcast P. Fractionation of soil P indicated a conversion with time of more soluble forms of P to less soluble forms. Estimates of P requirements from P. sorption curves suggested a residual efficiency of 40–60% of the previously broadcast fertilizer P. Although clayey Oxisols have a high P requirement for maximum yields, there is a considerable residual effect of fertilizer P which needs to be included in estimates of the efficiency of P fertilization.
Phosphorus deficiency and high P sorbing capacities are a major limitation to intensive cropping of many soils in the Cerrado area of Central Brazil. A field study comparing rates and placement of P for corn (Zea may L.) was conducted to determine initial and long‐term P fertilization requirements. Rates of broadcast P were 70, 140, 280, and 560 kg P/ha and banded rates were 35, 70, and 140 kg P/ha. In addition, combination treatments of 140 or 35 kg P/ha broadcast and banded rates of 35 kg/ha per crop were used.Broadcast treatments gave greater yields than band treatments at the same rates for the first crop. At the end of four crops, however, total yields and P uptake were very similar for broadcast and band treatments in which the same total amount of P had been applied. Marked residual effects were observed with the higher rates of broadcast P.A greenhouse study was conducted in which a given amount of P was mixed with varying proportions of the total soil volume. Corn growth and P uptake were a function of both the concentration of P in the portion of the soil fertilized and the percent of the total soil volume that was fertilized. These results tend to explain the increase with time in relative yields of the banded treatments.The best method for applying P to these high adsorbing soils appears to be an initial broadcast application of 140 kg P/ha and a band application of 35 kg P/ha to each crop. This treatment maintained the available soil P at the critical soil test level and produced 80 to 85% of the maximum yield.
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