EFFECTS OF LONG-TERM HERBICIDE USE, IRRIGATION AND NITROGEN-FERTILIZER ON SOIL FERTILITY IN AN APPLE ORCHARD

In 1972 an experiment was set up to investigate the long-term effects of herbicide, irrigation and two rates of nitrochalk fertiliser application on soil fertility in a Cox's Orange Pippin apple orchard. Samples taken in 1986 showed that uncultivated soil which had been maintained bare by herbicide had much lower organic C, total N and extractable K and Mg concentrations than soil which had been maintained under grass. Extractable P concentrations were lower in soil under grass than in soil under herbicide. In the absence of grass, soil pH was slightly lower than in its presence. All these effects were much greater at depths above 7.5 cm than below. Irrigation of the grass slightly increased organic C and total N levels at 0-7.5 cm compared with unirrigated grass but had no effect on extractable P, K and Mg. Increasing the fertiliser rate from 63 to 189 kg N ha-1 had no effect on organic C, total N, extractable P and K. Yet, throughout the soil profile, extractable Mg concentrations were greater at the low than at the high N fertiliser rate. In a seedling growth test on soil taken from the orchard in 1988 (and confirmed to be free from residual herbicide), apple seedlings grown in soil which had previously been under grass grew significantly better than those in soil which had been bare. These differences were ascribed to a greater rate of N mineralisation in the soil formerly under grass. The results of this trial indicate that to safeguard soil fertility it is necessary to maintain a grass cover in the orchard. In addition, fertiliser application on newly planted trees should be adjusted to take account of the presence or absence of grass in the previous soil management treatment.