Changes in surface soil properties of vertisols under dryland cropping in a semiarid environment

Abstract

Changes in the properties of Vertisols in the Walgett area, north-western New South Wales, under the present continuous cropping system were investigated. Soils (0-0.1 m) from four locations which had been cultivated for 8-50 years were collected and a range of physical, chemical and biological properties were measured. Adjacent native pasture soils which had never been cultivated were used for comparison. Associated with a substantial reduction in organic matter levels (up to 40%), the cultivated Vertisols were found to have higher pH, lower extractable (Bray No. 1) phosphorus, lower structural stability and lower biological activity. The increase in pH of soils under cultivation (1 : 5 soil: water) varied from 0.5 units (8.3-8.8) in a grey clay under 20 years of cultivation to 1.5 units (7.0-8.5) in a brown clay which had been cultivated for 50 years. The higher pH may be the cause of lower extractable phosphorus found in the cultivated soils. Lower biological activity was detected in all the cultivated soils when compared with adjacent pasture soils. Added carbon and nitrogen substantially increased the biological activity of all the cultivated and pasture soils; however, in three of the four soils, carbon dioxide production of the amended cultivated soil remained lower than the corresponding amended pasture soil. For Vertisols, a pasture phase may be important in preventing alkalinization as well as in maintaining P availability, in addition to the commonly recognized benefits such as maintaining soil structure and nitrogen levels.