Improved soil and irrigation management for forage production 1. Site establishment and soil physical properties

Red-brown earths (Red Sodosols and Red Chromosols) are the dominant soil type used for irrigated forage production in northern Victoria. However, these soils have dense, clay-textured subsoils that can limit productivity. Soil modification may reduce these limitations through improving subsoil structure. In a field experiment on a Red Chromosol near Kyabram, we compared 2 systems of soil modification involving deep ripping and gypsum application (with and without subsurface drainage) with an unmodified control. Two irrigation frequencies were imposed. This paper reports on the site establishment and the soil physical responses to soil modification. Later papers detail the forage yield responses and plant–soil–water relations. Initially, both soil modification treatments improved the physical condition of the subsoil. For example, the bulk density of the modified and the drained subsoils was reduced to about 1.48 Mg/m3, compared with 1.71 Mg/m3 in the control. Soil strength was reduced in the modified and drained treatments between depths of 0.10–0.40 m at most soil water contents. In the subsoil, the modified and drained treatments had smaller soil aggregates with about 60% by weight being less than 20 mm diameter compared with 39% from the control treatment. Subsoil hydraulic conductivity, at 10 mm water tension, was 12 mm/h in the control, 33 mm/h in the modified treatment and 71 mm/h in the drained treatment when measured 1 year after the treatments were imposed. These improvements in soil physical properties generally persisted for the 2 years of measurements.