CARBON CONTENT AND BULK-DENSITY OF AN IRRIGATED NATRIXERALF IN RELATION TO TREE ROOT-GROWTH AND ORCHARD MANAGEMENT

This paper investigates why there are usually very few feeder roots present in the surface soil of irrigated orchards in the Goulburn Valley, as this may be reducing fruit yields. Nine orchards were selected to cover the range of soil and water management. In each orchard, when the soil was near field capacity, root proliferation and earthworm activity were noted and a core taken. Where the C content of cores was 1% or less, few roots were present, the soil was severely compacted and inter-particle bonding was weak. Root proliferation, soil porosity and clay inter-particle bonding all reached a maximum at about 2.2% C. With 3-4% C present, few roots were seen, bonding remained strong, but the soil was again compacted. It is suggested that with increasing C content, carbohydrate gel progressively bonds portions of the mineral matrix together. Eventually, probably mainly as a result of earthworm activity, the mineral matrix becomes embedded in gel. Then, although the gel itself retains about 4.6 g H2O per g of C at 10 kPa suction, none is available for root growth, because of the high penetration resistance of the soil. Root growth is also inhibited at low C contents, because of the high bulk density. Maximum root proliferation occurred in permanent banks raised around mature trees, where the banks were wetted by capillarity every 2-3 weeks.