Root penetration and proliferation are directly related to enhanced nutrient and water uptake, and thus to increased crop growth and yield. However, few studies have reported root growth and its seasonal variation in irrigated summer crops in fine-textured soils such as Vertosols. The objective of this study was to quantify the effects of tillage methods and the summer crops cotton (Gossypium hirsutum L.), maize (Zea mays L.) and sorghum (Sorghum bicolor (L.) Moench.) on root density in the non-sodic surface and subsurface soil (0–0.5 m) and sodic subsoil (0.6–0.9 m) of irrigated Vertosols in northern New South Wales (NSW). Root growth of cotton, maize and sorghum was evaluated using a combination of core sampling, and minirhizotrons and an image capture system, in several experiments conducted from 2004 to 2012, near Narrabri, northern NSW. The experimental sites had Vertosol soils with average clay contents of 65 g 100 g–1 in the surface 1 m, with sodic horizons present at depth. Rooting depth of cotton was relatively shallow, with most roots in the surface 0.6 m. Subsoil (0.6–0.9 m) root growth of cotton was sparse under continuous cotton but was greater with a cotton–wheat rotation. Among cotton genotypes, surface root length density of a Bollgard® cotton variety was less than that of its non-Bollgard counterpart. Subsoil root growth of sorghum and maize ranged from moderate to high, and accounted for a significant proportion of the total length of their root systems. This may be because maize and sorghum were able to tolerate the waterlogged conditions in the sodic subsoils of these Vertosols.
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Vol. 66 • No. 2