1. It is suggested that the striking failure of winter wheat grown in rotation with two summer crops of tares or mustard on the sandy soil the Woburn Experimental Station is due to the production of nitrate and ammonia from the green manures at times when the wheat is unable to use them efficiently and to the consequent loss of nitrate in the drainage. Owing to its low C: N ratio the nitrogen in tares nitrifies very rapidly and the loss by leaching is very great. Mustard, on the other hand, reduces the winter loss, but the nitrogen present in the mustard and that absorbed in the decomposition of the excess carbon compounds are liberated too slowly to be utilised efficiently by the wheat and much of the nitrate subsequently produced is also lost by leaching.