Landforming a red-brown earth dash effects of topsoil removal and redistribution on irrigated crop production

Abstract

Landforming is the use of earthmoving machinery and laser controlled land planes to improve the efficiency of irrigation layouts. Duplex soils with shallow A horizons are vulnerable to the exposure of subsoil in this process, and production from areas where topsoil has been removed is substantially less than from normal soil. A landformed area with a large range of depths of cut (soil removed) and fill (extra soil deposited) gave a linear decrease in dry matter (DM) yield/ha of maize (Zea mays) from 23.7 to 11.2 t/ha, and of oats (Avena sativa) from 13.9 to 7.5 t/ha with increasing depths of topsoil removal, but no progressive yield increase with added soil, so that overall production was estimated to be decreased by 16%. Whole plant N concentration was 24% lower, plant Zn 33% lower and soil Zn 43% lower from areas where topsoil was removed. Ameliorating the exposed subsoil with ripping, gypsum and compensatory N and P fertiliser produced only 71% of maize DM compared with normal soil (16.5 v. 11.7 t/ha). However, the replacement of 75 mm of topsoil completely restored yield. In a pot experiment which removed many of the physical limitations of the field, maize yields from subsoil and topsoil were similar when Zn was added to the subsoil in addition to N, P and K. Although Zn deficiency can limit maize growth on exposed subsoil, soil physical characteristics are a major cause of reduced yield and these were not eliminated in the field by amelioration. The cost of topsoil replacement during landforming may be less, in the long term, than amelioration plus cumulative production losses. The importance of topsoil in maintaining the yield potential of cropping land should not be underestimated in landforming designs for irrigation.