A study was made of the nitrogen fixing systems of various forage legumes and their rotational effect on yield of subsequent cotton crops, as well as of yields of the different legumes. Phillipesara, clitoria and lubia out-yielded all the others. Nitrogen yields of the various forage legumes showed that they are active fixers of atmospheric nitrogen, but yields of cotton following the various legumes showed wide seasonal variability and were neither significantly nor consistently different in different seasons. On the other hand, the increase in cotton yield due to fertilizer nitrogen was highly significant, indicating a comparatively small end-of-season contribution of cut legumes to soil nitrogen. It was therefore difficult to differentiate between the legumes on the basis of any consistent ability to influence yields of following cotton, suggesting that the choice of a legume in the rotation should depend primarily on economic considerations.Legumes have long been recognized as beneficial rotational crops, because they have the ability to fix atmospheric nitrogen in their root nodules and make it available both to themselves and to other crops. In the Sudan, forage legumes were very early recognized as useful in old rotations, designed at a time when artificial fertilizers were not known and practices such as the use of fallows and legumes were employed to maintain soil fertility. Rotations with unfertilized cotton have for long shown the value of fallows preceding cotton and the beneficial effects of a legume in the rotation (Burhan and Mansi, 1967), but more emphasis was placed on the importance of legumes as fertility builders and soil improvers than as producers of fodders. More recently, evidence has been presented to indicate that, with the introduction of nitrogenous fertilizers, cropping of the Gezira could be considerably intensified without reducing cotton yields (Jackson and Burhan, 1968). Accordingly, the number of fallows was substantially reduced so that new crops could be included, but legumes continued to be valuable constituents of most rotations in the irrigated areas. With the recent need for diversification of cropping and the integration of livestock into the farming system, the value of fodder crops is becoming more obvious.Although the manurial effect of legumes, as a group, is generally accepted, information on the relative value of different legumes as rotation crops, their nitrogen fixing ability, and their effect on following cotton is still very scanty. The experiments reported in this paper were, therefore, initiated with the primary objective of studying the nitrogen fixing systems of various forage legumes and their rotational effect on the yield of fertilized and unfertilized cotton planted immediately after the legumes, or with an intervening fallow year. The experiments also served to compare the yield performance of the different legumes.
