The growing demand for the determination of several analytes in a large number of soil samples explains the increasing interest in developing automatic methods of soil analysis. This paper reports the use of flow injection analysis (FIA) to perform automatically the simultaneous determination of the pairs K/Ca, Ca/Mg, Na/K, and Na/Mg in soils. A dialysis unit was used to divide the sample plug into two different streams and also to create large dilution capacity. After appropriate treatment inside the manifold, the resulting plugs were directed to two detectors placed in parallel, thus allowing bicomponent analysis. Atomic absorption and flame emission spectrometry were used as detection processes. The results obtained for an average of 25 soil samples by the developed FIA systems were in good agreement with those provided by the reference methods, i.e., the relative deviations were no larger than 3%. Sampling rates from 80 to 120 determinations per hour, and relative standard deviations below 3%, were obtained.