INCREASING THE DEPTH OF SOIL DISTURBANCE INCREASES YIELDS OF DIRECT DRILLED WHEAT ON THE SANDPLAIN SOILS OF WESTERN-AUSTRALIA

We evaluated on sandplain soils in the northern wheatbelt of Western A ustralia in 1990 and 1991 the effect on wheat yields (Triticum aestivu m) of increasing the depth of soil disturbance (4-40 cm) while direct drilling. These sandplain soils are easily compacted and eroded by win d, but yields after direct drilling with tined implements are frequent ly poorer than yields after cultivation.Direct drilling with a tined s eeder was compared with: (i) direct drilling with a modified seeder on which the front 3 rows of cultivating tines could be lowered relative to the rear 3 rows of seeding tines; and (ii) with direct drilling fo llowed immediately by deep ripping. Progressively increasing the depth of disturbance reduced soil strength and generally increased grain yi elds of wheat. Effects on yield were more pronounced in the wetter yea r of 1991 on an experimental site that had not been previously ripped; responses of 32 kg/ha for each centimetre increase in depth of distur bance were recorded. The highest yields followed deep ripping (3429 kg /ha), and the lowest yields followed minimal soil disturbance using na rrow (5 cm) seeding tines working to seeding depth only (2144 kg/ha). These results confirmed that it is possible to use a modified direct d rilling technique (thus retaining the advantages of timely sowing afte r rainfall and lowered risk of wind erosion) and raise yields above th ose normally expected from direct drilling on these soils.