Soil management for irrigated vegetable production. II. Possible causes for slow vegetative growth of lettuce associated with zero tillage

This work explores the consequences of using zero-tillage for irrigated vegetable production. The research is based on the hypothesis that, under the well-watered and adequately fertilized conditions prevailing in commercial vegetable growing, the reduced root density expected under zero-tillage should not limit vegetative growth. To test this hypothesis, irrigated lettuce crops were grown under a range of soil physical conditions on a hardsetting sandy loam soil. Crops were grown during the spring, autumn and summer, in order to compare zero-tillage and cultivation treatments under conditions of low, medium and high evaporative demand. Lettuce yields under zero-tillage were reduced relative to those under cultivation for each of these crops. However, calculations based on a model which describes the movement of water to roots show that the root length measured in the zero-tillage treatment greatly exceeded that required to meet the potential evaporative demand in the field. A possible explanation for the reduced growth is given, based on an inhibitory root signal which has been shown under controlled conditions to reduce shoot growth in hard soil even when the leaves are well supplied with water and nutrients. When lettuce was grown under zero-tillage, but following a crop of subterranean clover (Trifolium subterraneum L.) which senesced in summer to form an in-situ mulch, yield was as high as in cultivated soil. The soil near the surface in this treatment was not only moister and physically weaker than that from the conventional zero-tillage treatment, but also likely to contain biopores which would provide zones of low root impedance in an otherwise hard soil.