Chemistry and transformation of boron in soils of Kerala

Abstract

Sixty five representative soil samples from 23 agro ecological units under five agro ecological zones of the state were collected and characterised with respect to the available boron status for the present study on “Chemistry and transformation of boron in soils of Kerala”. The samples were characterised for pH, EC, CEC, PBS, exchangeable cations, total sesquioxide and available nutrient status (organic carbon, P, K, Ca, Mg, S, Fe, Mn, Zn and Cu) along with available boron.. Among these soils 18 per cent of samples which included samples from southern and northern coastal sandy plains were deficient in boron (< 0.5 mg kg-1) while the acid saline soils of Pokkali and Kaipad have recorded highest levels of available boron (> 3.0 mg kg-1). The availability of boron was found to increase with soil acidity and electrical conductivity. Thirty one soil samples selected from 65 characterised samples representing all the AEUs and ensuring variability in physico-chemical properties and available boron were subjected to fractionation of soil boron.

The different fractions, readily soluble, specifically adsorbed, organically bound, oxide bound and residual boron accounted a mean status of 0.59, 0.86, 1.79, 2.31 and 94.45 per cent of total boron respectively in soils of different AEUs in Kerala. All the fractions except residual boron were significantly and negatively correlated with soil pH, and positively correlated with electrical conductivity. The oxide bound and residual boron were highly positively and significantly correlated with the content of sesquioxide and clay and the organically bound boron was significantly correlated with organic carbon content indicating the influence of organic matter, oxides and hydrous oxides dominated amorphous clay minerals of tropical soils in retention and solubility of boron.

Among the six soils included for adsorption studies (lateritic soil from Pattambi, wet land soil from Elavampadam, sandy soil from Onattukara, black cotton soil from chittoor, Kole land soils from Thrissur and Kayal land soil from Kuttanad), only two soils ie. Onattukara and Kayal land soils followed Langmuir adsorption isotherms at 250C. The adsorption data for all the soils except Onattukara sandy soil could be fitted in to Freundlich adsorption isotherm both at 25 and 400C. The amount of boron adsorbed at unit equilibrium concentration ( Freundlich K: KF) decreased with increase in temperature from 25 to 400C. The intercept of Quantity-Intensity curve decreased to negative values at higher temperature indicating desorption of boron from the sites when solution concentration fell below equilibrium concentration. The decrease in free energy with a positive enthalpy and a positive entropy in Pattambi, Elavampadam, Onattukara and Kole soils revealed that the adsorption of boron were spontaneous and endothermic. The reverse was the case with black soils of Chittoor and Kole land soils where adsorption was exothermic and non spontaneous.

The incubation experiment conducted for a period of three months with different levels of organic matter and borax in three soils ie. Onattukara sandy, lateritic upland and low land of Elavampadam revealed the clear role of organic matter in adsorption, retention and release of boron. Either organic matter or borax independently could increase available boron substantially after one month of incubation and maintained almost a constant level during the further course of incubation. However higher levels of organic matter reduced the available boron after one month incubation as a result of adsorption by organic matter. At highest level of organic matter and borax (15 t and 12 kg ha-1 respectively), adsorption of boron was highest.

The field experiment to optimize the boron nutrition for rice in low land lateritic soils in central Palakkad plains (Elavampadam) revealed the clear role of boron in improving the productivity of tillers, number of grains per panicle, thousand grain weight and ultimately yield. The response curve fitted to find out the optimum dose of boron for grain yield showed that application of borax @ 5.5 kg ha-1 would result in yield improvement to the tune of 1 t ha-1. The fractionation of soil boron at panicle initiation and harvest stage indicated that both readily soluble and specifically adsorbed boron in soil increased with the levels of borax up to 6 kg ha-1 while the other fractions increased linearly with the levels of borax up to 12 kg ha-1. The correlation studies and path analysis clearly indicated that plant absorption of boron mainly take place from readily soluble, specifically adsorbed and organically bound boron in that order due to the existence of dynamic equilibrium between these fractions.