Estimate of Potential Acidity Using the pH SMP Method in Soils in the West Paraguay

The substitution of the 0.5 mol.L-1 calcium acetate method to quantify the potential acidity by the use of Shoemaker, McLean, and Pratt Buffer hydrogen potential (pH SMP) is feasible in routine laboratories. This method replacement consists of eliminating operational and economic limitations that limit the number of daily determinations, such as: difficulty in distinguishing the turning point of the indicator during the titration, high consumption of reagents, extraction and rest of 16 hours. However, one of the premises to achieve this objective is the edaphological regionalization of the data to estimate with reliability, the results of acidity potential (Hydrogen + Aluminum - H+Al). The soils of the West Paraguay are acidic and one of the components of this acidity is the sum of interchangeable aluminum (Al3+) with the Hydrogen ions (H+), thus making the H+Al a chemical attribute of the soil directly related to the liming. This work aimed to relate the methods of pH SMP and that of calcium acetate 0.5 mol.L-1 (pH 7) aiming to define an equation that estimates the potential acidity of soils in this area, through the equilibrium pH of the suspension with the SMP solution. Forty-nine soil samples from the West Paraguay were submitted to regression analysis. Linear, quadratic, and exponential models were obtained. From the analysis of the coefficients of the models and behavior of their residuals, the exponential equation H+Al = 10743.6198.e(-1.2479 pH SMP) (R2 = 0.9753) was the one that best estimated the potential acidity of soils belonging to the West Paraguay.

Automated summary

The substitution of the 0.5 mol.L-1 calcium acetate method for pH SMP is feasible and eliminates operational and economic limitations. Regionalization of soil data is necessary for reliable estimation of acidity potential. The best equation for estimating potential acidity in West Paraguay soils was determined through regression analysis.