The effect of irrigation on barley root architecture, yield and water-use efficiency in vertic texture contrast soils

Water availability is an ever increasing issue for agricultural production around Australia. Australia‘s south-eastern high-rainfall zones (annual rainfall between 500 and 900 mm) allow for extensive cropping. In Tasmania, this often requires supplemental irrigation to maximise yields. Texture contrast soils occupy 80% of southern Australian agricultural regions and can be difficult to irrigate due to hydraulic complexities, particularly with a vertic, clay subsoil. This thesis assesses how strategic irrigation can be used to overcome the complexities of texture contrast soils to improve grain yield and water-use efficiency (WUE) of barley through maximising root depth and distribution. Detailed root spatial data and growth rates were used to present a method to improve the simulation of barley growth and development on texture-contrast soils in the high rainfall zone. Barley (cv. Gairdner) was grown under waterlogged, optimal irrigation and rainfed conditions with five replicates on a texture contrast soil in southern Tasmania. Plants were sampled three times through the season for yield and yield components. Following harvest, 1 m\(^2\) pits were excavated and root number, soil moisture and soil strength were measured on horizontal soil faces to a depth of 110 cm. Volumetric soil moisture was recorded in each treatment throughout the growing season with a Sentek EnviroSCAN to a depth of 110 cm. Soil, plant and weather data were collated to parameterise the crop simulation model APSIM for the calculation of WUE. Increased root depth significantly improved grain yield and WUE. Maximum rooting depth was greatest under optimal irrigation and shallowest under rainfed conditions. Increased root depth was associated with improved grain yield. Grain yield was greatest under optimal irrigation, followed by the waterlogging and rainfed conditions, respectively. Optimal irrigation had the greatest WUE. Even though the rainfed conditions lead to the poorest yield, WUE was greater than the crop subjected to waterlogged conditions. Increasing the frequency and amount of irrigation led to waterlogging of the A horizon, which is a potential issue in texture contrast soils. The abrupt change in texture means there is a large contrast in the permeability of the two soil horizons. The low permeability of the B horizon and the low water holding capacity of the A horizon makes the soils very prone to waterlogging, particularly under irrigation. Although the soil in the waterlogged treatment had a lower penetration resistance, root depth was shallower than for the optimum treatment. The default capacity of APSIM to simulate barley grown on Tasmanian vertic texture contrast soils was relatively poor and parameters such as yield and root growth were overestimated. This was addressed by revising the root exploration factor and root water extraction parameters of APSIM, based on detailed root density curves. Strategic irrigation of barley improved grain yield, rooting depth and distribution in vertic, texture contrast soils. A better understanding of root-soil interactions can be used to develop more effective irrigation to increase yields and water-use efficiency of grain crops in these hydraulically complex soils.