Effect of cropping on the physical fertility of krasnozems

Abstract

The physical properties of krasnozems and euchrozems in Australia have received little attention due to the widespread perception that they are 'good' soils. We present data from three locations which are representative of these soil types in the South Burnett region of southern Queensland. At each location, paired sites were chosen where continuously cropped and uncropped land existed side by side. The effect of continuous cropping on physical fertility, relative to that of soil in virgin condition, is assessed and related to published data from other locations where available. Continuous cropping has reduced infiltration rates from 80 mm h-1 or more to values as low as 25 mm h-1. Originally low bulk densities ranging from 0.6 Mg m-3 at the surface to 1.3 Mg m-3 at 70 cm depth have been increased to corresponding values of 1.0 and 1.4 Mg m-3. Local maxima of 1.3 Mg m-3 occur between 15 and 30 cm depth. Penetration resistance has been increased markedly at low water contents as a result of increased cohesion, with an uncropped soil having values <2.5 MPa over the available water content range comapred with values of >6 MPa in the continuously cropped soil. Organic carbon in the surface soil has been reduced from 4-5% to 1% and aggregate stability under rain has been reduced, the % aggregates <0.125 mm diameter increasing from 10% to 20% or more. Soil water storage has been reduced together with crop water extraction, with these effects attributed to a loss of water storage pores in the top 30-50 cm of the profile and restricted root development. The problems of rehabilitating these soils are briefly discussed. Continuous cropping under dryland conditions has greatly reduced the physical fertility of krasnozem and euchrozem soils and made them more susceptible to variation in seasonal conditions. Available data suggest degradation is also occurring on krasnozems used for intensive vegetable production. Methods of rehabilitating physical fertility of degraded krasnozems are required, as are farming systems which are more sustainable in the long term.