Legumes as a nutrient source for Iceberg lettuce (Lactuca sativa crispa)

A study carried out in Uppsala, Sweden, compared three legumes: red clover (Trifolium pratense), lucerne (Medicago sativa) and yellow sweet clover (Melilotus officinalis) as a green manure for an early and late crop of Iceberg lettuce (Lactuca sativa crispa). Differences between the treatments were only apparent in the early crop of lettuce. With the early crop, the dry matter content of the leguminous green manure was more significant in effect on yield than its nutrient content. Red clover and lucerne provided approximately the same level of nutrients as yellow sweet clover, but despite this gave lower yields of Iceberg lettuce than the yellow sweet clover. A possible reason for this was that the higher amount of dry matter supplied by the yellow sweet clover had a positive effect on soil structure and thus improved lettuce rooting and water uptake. The results indicate that at farm level, reliability of biomass production is more important in choice of a green manure crop than chemical and morphological properties of the species.