Undersowing Legume Crops for Green Manuring of Lettuce
Sorensen, J.N. | Thorup-Kristensen, K.
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.
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