Characterization of Kerala soils into fertility classes with respect to available P and K extracted by a common extractant

Abstract

A laboratory study was undertaken to evaluate the suitability of Mathew’s triacid extractant (0.06 N H2SO4 + 0.06 N HCI + 0.05 N oxalic acid) for the combined extraction of available P and available K in soil. The suitability of this triacid extractant was confirmed on a large number of soil samples. Precise relationships between triacid K and NH4OAc K were established. The ten fertility classes for available P and K currently followed in the soil testing laboratories are based on Bray-1 P and NH4OAc K. By making use of suitable regression equations the class intervals in terms of triacid P and triacid K values have to be formulated for the purpose of fertilizer recommendation. The use of this extractant can simplify the work in soil testing procedures thus enabling considerable savings of time and labour. A large number of soil samples was collected representing the entire state of Kerala. The available phosphorus of soil was extracted by Bray No.1 solution (1:10 soil solution ratio with an equilibration period of 5 min) and by Mathew’s triacid extractant (1:10 soil solution ratio with an equilibration period of 30 min). Phosphorus in the extract was then determined colorimetrically by the chlorostannous reduced molybdophosphoric blue colour method in HCI system. The available potassium of the soil was determined by neutral N NH4OAc with a soil solution ratio of 1:5 and an equilibration period of 5 min as well as by the triacid acid method. Potassium in the extract was determined flame photometrically. Soils were also analysed for organic carbon, pH and EC. Suitable correlation was worked out between Bray-1 P and triacid P. A linear regression equation was fitted to predict triacid P values from Bray-1 P values. Suitable relationship was also established between NH4OAc K and triacid K. The linear regression model worked out between NH4OAc K and triacid K was used to predict triacid K values from NH4OAc K. Most of the soils were acidic and non-saline. The pH showed a negative correlation with Ec and organic carbon content of soil. Soils varied in the content of organic carbon from 0.04 per cent to 4.41 per cent but in general was rich with a mean organic carbon content of 1.14 per cent. Electrical conductivity was positively correlated with organic carbon content of soil. A positive correlation was observed between organic carbon and available K. Mineralisation of organic matter contributed significantly to the increasing acidity of soil. The available P estimated by Bray-1 was found to range from 0.46 to 370.30 ppm whereas triacid P ranged from 1.73 to 462.50 ppm. The triacid extracted larger amounts of available P than the Bray-1. A significant correlation was observed between triacid P and Bray-1 P (r = 0.9575**). By fitting the regression equation y = 1.15x, where y represented triacid P and x represented Bray-1 P, the triacid P values of the ten fertility classes have been worked out. The available K as estimated by neutral N NH4OAc ranged from 10.0 to 425.0 ppm whereas triacid K ranged from 4.0 to 154.0 ppm. Triacid extracted lower amounts of available K (42.38 ppm) than the neutral N NH4OAc K (105.23 ppm). Although triacid showed less efficiency for releasing K from soil it was found to have high correlation with neutral N NH4OAc (r = 0.9235**). The linear regression equation of the form y = 0.44x was worked out where y represented triacid K and x represented NH4OAc K. This was employed to redefine the ten fertility classes in terms of triacid K values. The increase in precision obtained by the relationship between P estimated by Bray 1 and triacid as well as K estimated by neutral N NH4OAc and triacid by grouping the soils into separate textural classes was only marginal. The present study therefore confirmed the suitability of the triacid for estimation of both available P and K in the soil of Kerala. The revised class intervals for the ten fertility classes followed in the soil testing laboratories of Kerala have been formulated in terms of triacid P and triacid K values which will serve as a guide for giving fertilizer recommendations for various crops.