LONG-TERM EXPERIMENTS WITH DIFFERENT DEPTHS OF MOLDBOARD PLOWING IN SWEDEN

With the main objective to produce a basis for advice to farmers conce rning optimal ploughing depth under various conditions, a series of he ld experiments were initiated throughout Sweden. At 19 sites on variou s soils (clay content 72-521 g kg(-1) organic matter content 21-89 g k g(-1)) mouldboard ploughing to about 15, 22 and 28 cm depth was repeat ed annually for up to 17 years. The total number of location-years was 241. Traditional farming had previously been practised at the sites, including annual mouldboard ploughing to 20-25 cm depth. Spring-sown b arley (Hordeum vulgare L.) and oats (Avena sativa L.) were the most fr equent crops but many other crops were grown less frequently. Crop res idues were generally returned to the soil; straw was chopped at harves t. Post-emergence herbicides were regularly used, generally resulting in an adequate control of annual weeds. However, the control of perenn ial weeds, particularly couch grass (Elymus repens L. Gould) was often inadequate. At ploughing depths of 22 and 28 cm, the mean crop yields were 2% and 3%, respectively, higher than at 15 cm. However, the resu lts varied considerably between sites. In soils with a high silt conte nt, the shallowest ploughing resulted in up to 10% higher yield than d eeper ploughing, provided the control of perennial weeds was adequate. The main reason seemed to be improved structural stability in the sur face soil because the concentration of organic matter in this layer be came higher the shallower the ploughing. In clay soils with relatively stable structure, as well as in sandy soils, the deepest ploughing re sulted in the highest yields, probably because of the deeper loosening . At sites where perennial weeds imposed problems, the weed control wa s better the deeper the ploughing, sometimes increasing the relative y ield after deeper ploughing by several percent as compared with shallo w ploughing. Most of this effect was obtained already at the intermedi ate ploughing depth. The results led to the following conclusions for Swedish agriculture. It may be profitable to plough sandy soils annual ly as deep as 30 cm, coarse sandy soils perhaps even deeper. In clay a nd clay loam soils, ploughing deeper than 20-25 cm generally cannot be recommended. In silty soils with an unstable structure, mouldboard pl oughing, if any, should be shallow (less than or equal to 15 cm), and perennial weeds should be controlled by other methods. (C) 1998 Elsevi er Science B.V. All rights reserved.