New extractant for soil test evaluation of phosphorus, potassium, magnesium, calcium, sodium, manganese and zinc

Abstract The objective of this investigation was to evaluate a new soil test extractant which would allow simultaneous extraction of several plant nutrients over a wide range of soil properties and eliminate the present need for grouping of soils. The new extractant is composed of 0.2N NH4Cl‐0.2N HOAc‐0.015N NH4F‐0.012N HCl at approximately pH 2.5. It was compared to Bray 1 (0.025N HCl‐0.03N NH4F), Olsen (0.5M NaHCO3, pH 8.5), DA (0.05N HC1–0.025N H2SO4), 0.075N HCl and DAP (DA + 0.008N NH4F) for extractable P. The uptake of P by millet grown in the greenhouse on six taxonomically different Ultisols at two rates of lime and six levels of P was correlated with the chemically extractable P. In addition, K, Mg, Ca, Na, Mn and Zn extracted by the new method from 122 acid to calcareous soils from the U.S. and Jamaica were compared to those extracted by DA, while K, Mg, Ca and Na extracted by the new method were also compared to those extracted by neutral, normal ammonium acetate. The ratio of extractable P by 0.075N HCl/DAF was used to classify soils into groups of < 0.34, 0.34 ‐ 0.66 and > 0.66. Phosphorus uptake by millet was highly correlated with the P extracted by the new extractant, Bray 1, and DA, followed by Olsen. The predominant form of P in these 6 soils was Al and Fe‐P and the 0.075N HCl/DAF ratio was < 0.34. Phosphorus desorbed by the new extractant was highly correlated with Bray 1, followed by DA and Olsen on 72 acid soils with 0.075N HCl/DAF extractable P ratio of < 0.34. It was also highly correlated with all extractants on a group of 26 soils with 0.075N HCl/DAF ratio > 0.66 and was correlated only with Bray 1 and Olsen on a group of 24 soils where the latter ratio was 0.34 ‐ 0.66. Mean extractable P was lowest for the acid group of soils and highest with the neutral to basic group of soils for all extractants. Deviation between the extreme groups were ± 9, 12.5, 14 and 49 mg P/dm3 with the new extractant, Bray 1, Olsen and DA. respectively. Hence, the new extractant was highly comparable with Bray 1 and Olsen with soils having a wide range of differing properties. Potassium, Mg and Ca in the new extractant were highly correlated with those extracted by neutral N NH4OAc and DA. Extraction efficiency of these nutrients was essentially the same for NH4OAc and new extractant, while DA was less efficient, notably for K. Tentative relationships between levels of P, K, Mg, Mn and Zn obtained by the new extractant and crop response to these nutrients, with special reference to percent Ca and base saturation, have been presented.