COMPARISON OF THE NEUBAUER BIOASSAY WITH SOME CLASSICAL AND UNIVERSAL EXTRACTANTS FOR DETERMINING PLANT AVAILABLE-P IN CULTIVATED SOILS

Four universal and three classical chemical extractants were tested against the Neubauer seedling P test to determine their sensitivity to the P status in a wide range of cultivated soils in Switzerland. Eighteen soil samples with varying chemical and physical properties were treated with 0, 20, 40, 80, and 160 ppm of N and K in order to determine the best nutrient status for the growth of winter rye seedlings. The bioassay plants took up an average of 167 kg/ha of P in the control soils during 17 days. Improvement of the soil nutrient status by the addition of N, K, and Mg resulted in an additional P uptake of 0-115 kg/ha of P. The tested soil did not show a low level of available P according to the Neubauer seedling method. The amount of P extracted by means of chemical methods decreased in the following order: Egner - Riehm, Na2EDTA, NH4OAc - EDTA, Mehlich No. 3, Morgan - Wolf, water, CaCl2. However, the best correlations between soil P test values and bioassay P were found for the CO2 saturated water extractant, followed by Mehlich No. 3, NH4OAc - EDTA, 0.01 M CaCl2, Egner - Riehm, Na2EDTA and Morgan - Wolf in decreasing order. Improved soil nutrient status significantly increased the correlation coefficients for only four of the seven tested chemical methods: water, NH4OAc - EDTA, Mehlich No. 3, CaCl2. The very high correlation coefficients of NH4OAc - EDTA, Mehlich No. 3 and 0.01 M CaCl2 versus the Neubauer P indicate that these three methods may be used with advantage as compared to the CO2 water extractant in a wide range of tested soils.