Root growth and nitrogen uptake of carrot, early cabbage, onion and lettuce following a range of green manures

An experiment was performed to study the significance of rooting depth of four vegetable crops on their utilization of green manure nitrogen (N). Rates of rooting depth development were estimated as approximately 0.2, 0.7, 1.2 and 1.2 mm day °C^?1 for onion, carrot, lettuce and cabbage, respectively. At harvest, onion and lettuce were found to be shallow‐rooted with final rooting depths of only 0.3 and 0.6 m, respectively, whereas carrot and cabbage reached rooting depths of at least 1.1 m. The two deep‐rooted vegetables increased their N uptake by 46, 24 and 7 kg N ha^?1 when following winter‐hardy legumes, non‐hardy legumes and rye, respectively; the equivalent responses by the two shallow‐rooted crops were 23, 9 and 15 kg N ha^?1, respectively. Thus the deep‐rooted crops used the legume N more efficiently but the shallow‐rooted crops made better use of N left by the non‐legume rye crop. These interactions between green manure type and vegetable crop N response are the result of the dual effects of the green manures: biological N fixation by the legumes, and the variable ability of the green manure crops to concentrate available N in the topsoil. Before shallow‐rooted crops, the ability of rye to concentrate N in the topsoil may be as important as the N fixing ability of legumes.