Model experiment on the effect of a plough pan on crop yield under differing conditions of soil moisture availability

A field experiment conducted over two years examined the effect of increasing depth of cultivation from 1 to 40 cm on crop yields in a generally compacted soil where a plough pan had developed. The test crop was sugar beet in the first year and peas in the second; both crops were grown under irrigated and unirrigated conditions. Measurements showed that the physical condition of the uncultivated profile to a depth of 40 cm was unfavourable for root growth, but that the plough pan represented a greater constraint. The weather in the two years differed markedly: 1976 was much drier, and 1977 wetter, than average. Under conditions of moderate stress, i.e. 1976 irrigated and 1977 unirrigated treatments, when maximum soil moisture deficits (SMD) of 60 and 137 mm, respectively, were achieved, cultivation to 40 cm gave maximum yield. Under conditions of low moisture stress (1977 irrigated, maximum deficit 40 mm), cultivation to 10 cm was adequate. The SMD reached 250 mm in the 1976 unirrigated crop and here cultivation to 10 or 20 cm gave maximum yield. Neutron probe soil moisture measurements showed that the ability of the crop to extract water from lower horizons was restricted in compact soil conditions.