The effects of very deep ploughing and of subsoiling on crop yields

1. Deep tillage, namely, ploughing to a depth exceeding 12 in., or subsoiling to a depth of about 18 in., increased crop yields on about half the fields in which an experiment was made. Unfortunately, it was not possible to recognize what was the difference in soil properties between the 50% of fields that responded appreciably to deep tillage and the remainder whose yields were either unaffected or sometimes reduced by deep tillage, though, as one would expect, sands were normally less responsive to deep tillage than the heavier soils.2. On the clay soils, deep ploughing in autumn tended to give a surface soil that dried out up to several weeks earlier in the spring than land that was shallow ploughed. Subsoiling did not cause the surface soil to dry out in the same way. Only rarely did the subsoil brought up by deep ploughing give difficulty in working down to a seedbed, and probably in as high a proportion of fields the subsoil worked easier than the surface soil.3. On the clays and loams, the response of potatoes to deep tillage was more marked on fields that were heavily manured than on those lightly manured.4. Sugar beet normally gave a higher yield if its potash and phosphate was spread on the land before ploughing, even if this was in the autumn, than if it was applied in the seedbed. The exceptions to this result were for soils heavier than sandy loams if ploughing depths exceeding 12 in. were used, for the yield of beet was, on the average, depressed on these soils if the potash and phosphate was ploughed in compared with it being worked into the seedbed.5. Deep ploughing tended to give better control of many perennial weeds, and often of annual weeds than shallow ploughing. It is suggested that much of this benefit could be obtained when ploughing to 9–10 in. deep if ploughs were used that cut a furrow considerably wider than the conventional 10½–11 in. furrow, so allowing more complete inversion of the furrow slice.