EFFECTS OF SOIL DISTURBANCE ON WEED SEEDLING EMERGENCE AND ITS LONG-TERM DECLINE

The long-term effect of different depths of soil cultivation on weed s eedling emergence from naturally occurring populations of weed seeds i n the soil was examined in four experiments on land previously in past ure, raspberry canes or intensive vegetable production. At approximate ly monthly intervals, weed seedlings were counted and then killed with contact herbicides, after which plots were cultivated to 250 or 10 mm , or were left undisturbed. The treatments were continued for 7 years. One experiment was then discontinued whilst the previously uncultivat ed plots on the other three were cultivated to 150 mm at approximately monthly intervals for a further 4 years in one experiment, and for 8 years in the other tsvo. After the first year, very few seedlings emer ged in the uncultivated and shallow cultivated plots, and seedling num bers declined slowly in the deep-cultivated plots. Under repeated deep cultivation, seedling emergence of almost all species declined expone ntially. Different species declined at different rates, with Rubus ida eus L., Plantago lanceolata L., Veronica arvensis L. and Ranunculus sp p, being the most rapidly declining group. Rates of decline for indivi dual species were similar to those observed in Europe. Juncus bufonius L. behaved differently from the other species, and showed no decline in seedling numbers. In the initial 7-year period, 28 000 weed seedlin gs per m(-2) emerged from the deep-cultivated plots on soil previously cropped with vegetables. Over the same period, less than 11 000 seedl ings emerged in shallow-cultivated plots, and just over 4000 in uncult ivated plots. In the second phase of the experiments, fewer seedlings of most species emerged than in the first phase, and the decline in nu mbers of Coronpous didymus (L.) Sm, seedlings was slower.