The significance of (surface) physical and chemical properties in determining soil surface condition of red earths in rangelands

Abstract

The surface layers of a red earth near Cobar, N.S.W., have been classified into four condition classes by using attributes that are readily observable in the field. These classes provide a reliable estimate of potential dry matter production. Soil samples from the three depths 0-1, 1-5 and 5-10 cm of each class were analysed for various physical and chemical properties. Soil surface resistance was measured in the field. Field measurements of infiltration were carried out at supply potentials of +10 and -40 mm by using the disc permeameter. Differences in physical and chemical properties of the classes occurred mainly in the 0-1 cm layer. Below 1 cm there was very little difference in properties between the four classes. The 0-1 cm depth of the class 1 soils (most productive) contained significantly (P=0.05) higher amounts of organic matter, available nitrogen and exchangeable magnesium than the class 4 soils (least productive). Class 1 soils had higher aggregate stabilities in the 0-1 cm layer and lower soil surface resistance than class 4 soils, indicating more favourable soil physical conditions for seed germination and emergence. Aggregates of all classes underwent considerable dispersion in water when subjected to mechanical stress. At a supply potential of +10 mm, class 1 soils had a significantly (P=0.05) higher infiltration rate than class 4 soils. However, at a supply potential of -40 mm, there was no difference in infiltration rate between classes. Scanning electron microscope (SEM) micrographs showed that cryptogamic material on the surface of class 1 soils resulted in a porous, well aggregated structure compared with the dense, compacted structure of class 4 soils. The differences in physical and chemical properties support earlier micro-morphological evidence that the classes 1-4 essentially represent an erosional sequence from no erosion (class 1) to strongly eroded (class 4). The importance of these physical and chemical properties of different soil classes in determining productivity of rangeland soils is briefly discussed.