Root and shoot response of field-grown lettuce and broccoli to a compact subsoil

Abstract

The direct effect of subsoil compaction on the root and shoot growth of lettuce (Lactuca sativa ‘Classic’) and broccoli (Brassica oleracea L. Italica ‘Bushido’) was investigated in field experiments. Possible secondary effects of the compact subsoil were minimised through careful supply of water and nutrients to the soil. Roots were sampled at 3 times to determine the timing and extent of root growth into the compact subsoil. The compact treatment had a distinct layer of high strength soil at 0·17-0·35 m, maximum soil strength of 3·1 MPa at 0·27 m. In the loosened treatment, deep ripping reduced the maximum subsoil strength to 1·9 MPa at 0·35 m. The roots of both crops penetrated the compact subsoil approximately half-way through the vegetative growth phase of the shoots. Root growth was restricted by the compact subsoil, with root length densities 60-75% lower than in the loosened subsoil. As a result, only 6-13% of the total root system was present in the compact subsoil. The reduced root length in the subsoil was compensated for by increased root growth in the topsoil, for the broccoli crop only. The compact subsoil had no effect on the shoot growth or yield of either vegetable crop when water and nutrientswere well supplied. This may have been due to the small proportion of roots growing in compact soil and/or the advanced vegetative growth stage of the shoots when the roots grew into the compact subsoil. Consequently, under dield conditions, a compact subsoil had no direct effect on vegetable crop shoot growth or yield.