In order to determine the quantities of rnineral elements in soils that are available to plants, biological methods of measurements such as that described by Neubauer and Schneider, have been used. This particular method has been widely employed for study of soils of temperate climates. However, very little was known as to the application and value of this biological method for study of tropical and subtropical soils. The purpose of the experiments described in this paper was evaluate the efficiency of the Neubauer method, using rye and rice plants, for determining available potassium in samples of the four principal soil types of the State of São Paulo. The soil types studied were Arenito Bauru, Massapé-salmourão, Terra roxa legítima and a Humus soil. The Arenito Bauru soil is a sand soil, with 50 - 60% of sand (> 0,2 mm and < 2 mm) and 2 - 10% of clay (< 0,002 mm). The Massapé-salmourão soil is a soil type with 30 - 50% of sand, 20 - 30% of clay and it generally has micas as primary mineral. The Terra roxa legítima soil is a soil which contains 10 - 20% of sand, 35 - 40% of clay and it is originated from basalts. The Humus soil presents a variable composition with a high content of organic matter. The results obtained from the biological tests were compared with the amounts of exchangeable potassium determined by chemical analysis. When rye was used as a test plant it was found that the relative amount of potassium extracted by the plants varied from 83 to 114% in Arenito Bauru, 30 to 81% in Massapé salmourão, 22 to 84% in Humus soil. In the tests where rice plants were used the amounts of available potassium extracted varied from 68 to 107% in Arenito Bauru, 52 to 147% in Massapé salmourão, 4 to 76% in Terra roxa legítima and from 72 to 122% in Humus soil. The values greater than 100% signify that the test plant extracted a portion of the fixed potassium not exchangeable. These data show that the rice plants in general had a greater capacity for extraction of potassium than did rye plants. Within the 35 average results obtained from measurements with rye plants only 2 were found to have absorbed more than 24 mg of K2O. This amount of potassium was considered by Neubauer and Schneider, to indicate that the soil is provided with sufficient potassium. The results from the tests using the biological method then indicate that in 33 out of 35 cases studied the soils lacked potassium. This, however, is contrary to results obtained from field experiments on these soils.