THE EFFECT OF PLOWING DEPTH AND SEEDBED PREPARATION ON CROP YIELDS, WEED INFESTATION AND SOIL PROPERTIES FROM 1940 TO 1990 ON A LOAM SOIL IN SOUTH EASTERN NORWAY

The effect of ploughing depths, seedbed preparation and nitrogen ferti lisation on a loam soil (fluvaquentic humaquept) were investigated in a factorial field experiment on a loam soil at Angstroms in southeaste rn Norway. The experiment was established in the autumn of 1939, reorg anised in 1962 and was completed in the autumn of 1990. The working de pths for ploughing (12, 18 and 24 cm) and seedbed cultivation (4, 8 an d 12 cm) were constant during the whole research period. Different nit rogen application rates (50, 100, 150 kg ha-1) and seedbed preparation implement treatments (S-tine vs. rotary cultivator) were superimposed on the experiment in a factorial configuration in 1962. Since 1962 th e main crops have been barley (Hordeum vulgare L.), oats (Avena sativa L.) and ley (timothy (Phleum pratense L.) and red clover (Trifolium p ratense L.)).The yield loss for shallow ploughing (12 cm) was most pro nounced in years with high weed infestation and at low nitrogen applic ation rate (50 kg ha-1). Shallow harrowing with an S-tine cultivator ( 4 cm, one pass) significantly reduced cereal yields compared with more intensive and deeper cultivation. The soil structure measured from 19 68 to 1987 appeared to be slightly better with regard to plant growth requirements after shallow (12 cm) compared with deep (24 cm) ploughin g. Bulk density was lower and the porosity greater in the 7-11 cm laye r after shallow compared with deep ploughing. The modulus of rupture i ncreased significantly with ploughing depth. The amount of organic mat ter at the 0-40 cm depth was not influenced by ploughing depth. Howeve r, the content of organic matter in the surface layer was greater afte r shallow ploughing. Mineralisation of organic matter was estimated to be about 36 kg N ha-1 year-1 greater for a nitrogen application rate of 50 kg ha-1 compared with 150 kg ha-1. In the 0-40 cm layer the amou nt of organic matter on average decreased by about 1650 kg ha-1 year-1 from 1944 to 1987.