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R.D. Connolly and D.M. Freebairn

Agricultural Production Systems Research Unit, PO Box 102, Toowoomba, Qld 4350

The productivity of cropping systems is often influenced by the physical state of surface soil. Farm operations change the structure of surface soil, and problems with workability, poor emergence and lower infiltration associated with soil structure degradation are commonly observed by land managers. While recognised as an important issue, the consequences of soil structure decline on productivity are not easily defined or measured. Using a combination of direct measurement and computer simulation, this study characterised the consequences of declining soil structure of the 0-20 cm deep soil on long-term runoff and crop production for several soils in south-east Queensland.


Measurements of infiltration characteristics were made on 18 soils in south-east Queensland. The infiltration capacity of uncropped, cropped and previously cropped soil placed in a pasture rotation were measured using rainfall simulators and disc permeameters. The cropping system model, PERFECT (1), linked to the infiltration model, SWIM (2) and modified to represent soil structure conditions, were used to simulate the effect of declining soil structure on wheat yield. Input parameters for the simulation model were derived from measured data. Two management strategies were simulated - a continuous cultivation strategy, and a medic/grass pasture rotation. Long-term change in soil structure in several contrasting soil types was simulated. An economic analysis of simulated yield was used to compare the productivity of the management treatments.


The measured infiltration capacity of more than 80% of the soils surveyed declined with cropping, but the rate and magnitude of loss of soil infiltration capacity varied with soil type. Reduction in infiltration was caused by increased surface sealing of the surface soil (0-10 cm deep), and decreased conductivity of sub-surface soil (10-20 cm deep) due to plough and wheel track compaction. A pasture phase improved the infiltration capacity of degraded soil, though at least three to five years of vigorous pasture appears necessary to effect any lasting improvement in soil infiltration capacity. Long-term simulations of cropping were used to compare the relative productivity of the management treatments. Continuous cropping led to reduced yields and economic returns on all soils. A pasture rotation was able to maintain yield at higher levels than that possible with continuous cultivation. The profitability of a pasture rotation, however, varied from soil to soil. The effects of climate, soil water holding capacity and soil structure status need to be considered before deciding on management strategies.


1. Littleboy, M., Silburn, D.M., Freebairn, D.M., Woodruff, D.R. and Hammer, G.L., 1989. PERFECT: A computer simulation model of Productivity Erosion Runoff Functions to Evaluate Conservation Techniques. Qld Dept. of Primary Industries Bulletin QB89005, Brisbane, Australia.

2. Ross, P.J., 1990. SWIM - A simulation model for soil water infiltration and movement. CSIRO Div. of Soils, Townsville, Queensland, Australia. 59 pp.

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