Effect of drying on concentrations of extractable micronutrient cations in soils

Abstract

Soil testing for micronutrients such as Cu, Zn, Fe and Mn involves extracting a "potentially plant-available" fraction of micronutrients from the soil. This is achieved with the aid of a wide variety of reagents which are often either chelating agents (e.g. ethylenediaminetetraacetic acid [EDT A] and diethylenetriaminepentaacetic acid [DTPA]) or weak acids (eg 0.1M HC1). Soil samples are normally air-dried and sieved ( <2 mm) prior to laboratory soil testing or other chemical analysis. Despite this the drying of soils, even at ambient temperature, can greatly increase the extractability and some nutrients such as phosphate and mineral N (Haynes and Swift, 1985; Sparling and Ross, 1988). Such increases can largely be explained by the release of phosphate and nitrogen from soil micro-organisms killed by desiccation (Sparling et al., 1987; Sparling and Ross, 1988). However, several workers have observed increases in DTPA- extractable Cu, Zn, Mn and Fe following air-drying (Shuman, 1980; Legget and Argyle, 1983) which could not be attributed to release of micronutrients from the microbial biomass (Sparling and Berrow, 1985). The cause of such increases in DTPA- extractable micronutrients is not known and neither is it known whether such increases will occur when other commonly employed micronutrient extractants such as EDTA or HC1 are used. This study investigates the effects of drying soils on levels of EDT A- DTPA- and HC1- extractable micronutrients.