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.

It is known that PO4 is retained by soils through ligand exchange, i.e., inner sphere complexation, but the mechanism for SO4 adsorption at the mineral‐water interface has been in debate. By studying the effects of ionic strength on ion adsorption, it is possible to distinguish between inner and outer sphere ion surface complexes. This study was conducted to evaluate ionic strength effects on SO4 and PO4 adsorption on γ‐Al2O3 and kaolinite at varying solution pH (3–11), and to infer SO4 and PO4 adsorption mechanisms at the mineral‐water interface. The adsorption of SO4 on γ‐Al2O3 and kaolinite decreased monotonically with increasing solution pH and was markedly reduced by increasing the concentration of background electrolyte. On the other hand, PO4 adsorption on γ‐Al2O3 and kaolinite increased from pH 3 to 4 and decreased from pH 6 to 11, with an adsorption plateau between pH 4 and 6. Effects of change in ionic strength on PO4 adsorption varied with pH. At low pH, PO4 adsorption demonstrated a slight decrease with increasing ionic strength, whereas at high pH, PO4 adsorption increased slightly with increasing ionic strength, resulting in a crossover point where there was no ionic strength effect. The triple‐layer model (TLM) was applied to model the adsorption of SO4 and PO4 with both inner and outer sphere complexes using the FITEQL 3.1 computer program. Sulfate adsorption was better modeled by assuming outer sphere complex formation, while PO4 adsorption was better modeled by assuming inner sphere complex formation.

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In the ascorbic acid‐molybdenum blue method for measurement of soluble P, the relationship between color intensity and P concentration in solution and color stability is greatly affected by organic and inorganic ligands such as oxalate, citrate, tartrate, or F. The objective of this study was to evaluate the effect of these ligands and to optimize the conditions for determination of inorganic P in the presence of the interfering ligand. The critical concentrations of the ligand at which P recovery was significantly decreased were 1.5 mM for oxalate, 3.0 mM for citrate or tartrate, and 10 mM for fluoride. The interference of the ligand with P determination was overcome by excess amounts of ammonium molybdate (AM) added before the color developing reagent (CDR). The critical molybdate/ligand molar ratios (excluding the amount of molybdate added in the CDR) required to completely eliminate the interference of the ligand were 1.12 for oxalate, 0.84 for citrate, 0.56 for fluoride, and 0.34 for tartrate. The presence of ligand and excess amount of AM did not affect the linearity of color intensity against P concentration but did alter the slope, suggesting that the modified method is reliable for P determination in the presence of an interfering ligand provided that the standard curve is prepared in the same matrix as the sample.