CORRELATIONS OF CATION EXTRACTANTS IN COSTA-RICAN SOILS

Modified Olsen (MO) and N KCl are the extractans used in Costa Rica to diagnose soil fertility. Our laboratory used Universal Soil Extractants to speed analysis and to obtain the best correlations between relative yield and nutrient level in the soils. We evaluated the aforementioned, Mehlich 3 (M3), Modified Morgan (MM) and Ammonium Acetate (AA) in four soil orders (Ultisols, Vertisols, Inceptisols, and Andisols). For calcium (Ca), all reagents gave highly significant correlations and in some cases AA, M3, and KCl showed an essentially one-to-one relation. The regression slope of 0.9 was for KCl-Ca and M3-Ca when all soils were considered as a population. KCl-Ca and MM-Ca regression gave an average slope of 0.94. When KCl and M3 extractable magnesium (Mg) were regressed, we obtained highly significant correlations in all orders. The same behaviour was found for KCl-Mg and AA-Mg. MM showed poor correlations with the other extractants. All reagents gave highly significant correlations for potassium (K). The regression between MO-K and M3-K showed a perfect one-to-one relation. The average regression slope indicated that MM extracts 55% less K than MO. AA extracted 13% more than MO in these soils. We concluded that M3 can be used instead of KCl and MO in Costa Rican soils without exception for the extraction of cations.