An examination of some acid soils at Batlow, New-South-Wales

Abstract

This paper has two components. Firstly it evaluates a set of data for the apple-growing soils of the Batlow district. Secondly it makes some comments on the use of silver thiourea for the determination of exchangeable Al and Mn and the estimation of exchangeable Al by titration of 1 M KCl suspensions. A group of 21 profiles from soils used for apple-growing in the Batlow District were examined in the laboratory. It was found that many of the soils were very acidic, especially in the subsoil. Dilute CaCl2 extracted as much as 50 µg g-1 of Al and considerable exchangeable Al was extracted by silver thiourea. Exchangeable Ca levels were usually adequate for plant growth, hence low levels in apple fruit may reflect the adverse effect of Al on Ca uptake by the plant. Potassium levels were low and may be marginal in some cases and Mg tended to be low in the surface. Principal component analysis showed 77% of the variation in the data was accounted for by three independent components, one involving pH, exchangeable Ca and Al, one involving K and Mg and one involving exchangeable Mn. It was demonstrated that silver thiourea does not replace Al as effectively as the basic exchangeable cations. Also, thiourea reduces Mn so that low values will be obtained for exchangeable Al in soils with much easily reducible Mn because of the increase in pH that accompanies reduction. Silver thiourea appears to overestimate exchangeable Al in some soils with very low cation exchange capacity. Titration of clay suspensions in 1 M KCl is a satisfactory and reliable way of measuring exchangeable Al that also enables an assessment of pH-dependent charge. The method used by the USDA Soil Survey Staff, viz. equilibration of soil samples with 1 M KCl for 16 h, filtration, and determination of Al also gives reproducible results and is taken as the standard. Successive leaching with portions of 1 M KCl tends to give low values for exchangeable Al.