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 pr esent 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 t he soil was near field capacity, root proliferation and earthworm acti vity 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 i nter-particle bonding was weak. Root proliferation, soil porosity and clay inter-particle bonding all reached a maximum at about 2.2% C. Wit h 3-4% C present, few roots were seen, bonding remained strong, but th e soil was again compacted. It is suggested that with increasing C con tent, carbohydrate gel progressively bonds portions of the mineral mat rix together. Eventually, probably mainly as a result of earthworm act ivity, 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 resistan ce of the soil. Root growth is also inhibited at low C contents, becau se of the high bulk density. Maximum root proliferation occurred in pe rmanent banks raised around mature trees, where the banks were wetted by capillarity every 2-3 weeks.