Effects of rundown in soil hydraulic condition on crop productivity in south-eastern Queensland - a simulation study

Declining soil organic matter levels because of cropping have been shown to reduce crop growth and yield, but the effects of changing infiltration and soil hydraulic properties on crop productivity have not been widely evalua ted. Cropping systems in south-eastern Queensland have, in the past, involv ed intense tillage, trafficking with heavy machinery, and changed organic m atter cycling, affecting soil aggregation, permeability, water-holding char acteristics, and organic matter. The aim of this paper is to determine how important infiltration and soil hydraulic condition has been to the water b alance, crop growth, and yield in the past, and may be in the future if man agement is not changed. Change in physical and chemical condition of the 5 most commonly cropped soils in south-east Queensland (Sodosols, Vertosols w ith less than or equal to 55% clay, Vertosols with >55% clay, Red Ferrosols and Red Chromosols/Kandosols) was measured over 0-70 years of cropping and estimated up to 200 years. The APSIM model was used to predict effects of changing soil condition in a rain-fed, fertilised, wheat-summer fallow crop ping system with intense tillage. Decline in infiltration, restricted inter nal redistribution of water, and increased evaporation reduced water supply to the crop, causing simulated yield to decline by 29, 38, 25, 17, and 13% for the 5 soils, respectively, after 50 years of cropping. Gross margin de clined at a faster rate, falling by 36, 50, 40, 20, and 21%, respectively a fter 50 years because of increasing fertiliser requirement to compensate fo r declining soil fertility. Crop productivity on most soils continued to st eadily decline as period of cropping increased to 200 years. To arrest or r everse this downward trend, it is likely that substantial changes to curren t cropping systems will be needed, including reducing tillage and trafficki ng, and improving organic matter levels.