RESTRICTED GROWTH OF LUPIN AND WHEAT ROOTS IN THE SANDY A HORIZON OF A YELLOW DUPLEX SOIL

Yellow duplex soils are the dominant soil type in the cropping region of Western Australia, but crop yields on these soils are often variable and below potential. We are seeking to understand the causes of the spatially variable crop growth, and a preliminary study conducted in 1988 at a site east of Beverley indicated that variable shoot growth was associated with variable early (< 6 weeks after sowing) root growth in the sandy A horizon. The present study aimed to identify the constraints to early root growth by locating the position in the A horizon where root growth becomes restricted and measuring the local soil properties. In poor growth areas, root penetration slowed markedly at about 15 cm (about 2 weeks after sowing), while in the good growth areas roots continued to grow downwards at about 8 mm day-1 for lupin and 4 mm day-1 for wheat. The soil was a particularly difficult environment for root growth, with generally low pH, low K and B, low porosity and aeration and high strength. Roots in poor growth areas appeared to experience greater mechanical impedance than in the good areas, most likely due to much lower clay contents leading to higher frictional resistance to particle movement or less stability of soil pores created by past roots or fauna.