The relationship of soil properties to the growth of barrel medic at Condobolin, New South-Wales

Abstract

Poor growth of barrel medic (Medicago truncatula) has been observed in established pasture on the red duplex and gradational soils of the semi-arid sheep and wheat growing areas of western New South Wales. Red-brown earth soils were examined from three paddocks on Condobolin Research and Advisory Station in conjunction with a visual assessment of the growth of medic. The soils were sampled at 20 m intervals along three line transects in each of three paddocks with different cropping histories. Bray-P; pH, Mn and Al extracted by 0.01 M CaCl2; exchangeable plus soluble Na, K, Ca and Mg; and pH in a 1:5 soil/water suspension were measured at four depth increments at each sampling point. Although the pH in CaCl2 was often less than 5.0, in general no Al was extracted from the 0-5 cm soil layer. Classification of the soils on medic growth alone showed medics were absent from the most acid soils. However, there was considerable within class variability. A better approach was provided by classifying the soils by means of a Euclidean distance/flexible sort of scores from the first principal component. This classification showed that the poorest medic growth was associated with the most acid group with considerable amounts of Al soluble in 0.01 M CaCl2 in the 10-20 cm horizon. Medics grew best in soils with moderate pH values between 5 and 6 in CaCl2 where there was no soluble A1 and moderate levels of exchangeable Ca and Na and soluble Mn. Medics appeared also to grow less well in more alkaline soils where there was very little soluble Mn. There were no differences between classes in available P. There were differences between paddocks; the uncultivated paddock being the most acid and the paddock with the longest cropping history having more Bray-P, more exchangeable Na and more exchangeable Ca at 10-20 cm. The uncropped paddock became more acid and the base status was lowered by natural soil processes and its condition probably indicates the status of many such soils in the district.