Ammonium Chloride Solution as an Alternative Laboratory Procedure for Exchangeable Cations in Southern Brazilian Soils

Most Brazilian soil-testing laboratories use Mehlich 1 and 1.0M potassium chloride (KCl) solutions as extractants for the determination of phosphorus (P), potassium (K), and sodium (Na) and for exchangeable calcium (Ca), magnesium (Mg), manganese (Mn), and aluminum (Al) in agricultural soil samples. Other laboratories use a combination of exchangeable ionic resin and KCl procedures. With recent adoption of the inductively coupled plasma (ICP-OES) in routine soil-testing laboratories, soil extraction with 1.0M ammonium chloride (NH4Cl) became an alternative due to the possibility of determining all exchangeable elements in one run (Ca, Mg, K, Mn, Na, and Al), leaving determination of phosphorus (P) with Mehlich 1 or exchangeable ionic resin. To evaluate the performance of the NH4Cl solution, an experiment was carried out with thirty-seven samples of soils representative of the southernmost state of Brazil, Rio Grande do Sul. Four extraction solutions [Mehlich 1 at soil/solution ratio of 1:10 and 1.0M ammonium acetate (NH4OAc), 1.0M KCl, and 1.0M NH4Cl at soil/solution ratio 1:20] were used with three different shaking times (5, 30, and 60min). Correlation coefficients among all methods were high. Mehlich 1 did not perform well against NH4OAc and NH4Cl, despite the high correlation coefficients, with values consistently lower for K, even when the time of extraction was increased from 5 to 30 or 60min. However, for concentrations less than 0.30cmolkg(-1) (i.e., in the range of K deficiency), both solutions performed similarly. Calcium and Mg increased with time of shaking. Comparable values of exchangeable Ca, Mg, and K, as well as of Al and Mn, were obtained with 1.0M NH4Cl with 60min shaking and the standard procedures of 1.0M NH4OAc and 1.0M KCl. The determination of Al by traditional titration/back-titration of the 1.0M KCl solution gave slightly greater results compared to ICP-OES obtained using extraction with 1.0M NH4Cl. The results indicate that for Ca, Mg, Mn, and Al, it is possible to replace the traditional 1.0M KCl extraction with 1.0M NH4Cl solution, with 60min shaking time and a soil/solution ratio of 1:20.