Undersowing Legume Crops for Green Manuring of Lettuce

The objective of this experiment was to investigate the potential of several species of undersown legume crops to deplete soil nitrate and to accumulate nitrogen (N). A further objective was to measure the effects of the legume crops as a green manure for a succeeding early lettuce crop. In two years, different undersown legume crops were compared with a control treatment where no crop was undersown. In November, at the end of their growing season, the legume crops reduced the content of mineral nitrogen in the top 1 m of the soil to one third of what was available when no crop was undersown. The N uptake in shoots and roots was in the range of 100–160 kg ha−1. The legume crops were incorporated at the beginning of April the succeeding year. In May, the content of soil mineral nitrogen was 3–4 fold the content in November. The main increase was seen in the top 0.25 m profile of the soils in which the legume crops had been grown. In July, the N mineralized from the incorporated legume crops and from other soil organic matter was estimated to be 36 and 38 kg ha−1, respectively. The yield of lettuce was increased by 20–45% when grown after legume green manure crops compared with the control treatment. The yield level was comparable to that obtained in conventional horticulture.