The literature which deals with the question of excretion of nitrogen by leguminous plants is reviewed especially with reference to accounts of experiments in which no excretion was obtained.

Experiments made at the Wisconsin experiment station prior to 1936, primarily concerned with the associated growth of pea and oat mixtures, were uniformly negative with respect to benefit to the oat plants through the association.

Experiments carried out at the Biochemical Institute in Helsinki, Finland, with a pea and barley mixture were in general positive, although poor development of the plants because of unfavourable weather were accompanied by little or no excretion.

On duplicating the Helsinki experiments at Wisconsin, once more both positive and negative results were obtained, although the development of the plants was excellent.

The occurrence of excretion appears to depend on many complicated circumstances some of which have been defined, while others are at present unknown. Factors which insure a uniform development of the plant cultures and especially a not too rapid growth of the leguminous plant appear to be important; as yet no universally successful method of achieving this type of development has been determined. It is concluded that although there can be no doubt with, regard to the existence of excretion, knowledge of the factors concerned is still insufficient to allow duplication of the results consistently at all experiment stations. Until such knowledge becomes available, application to practical agriculture can remain only an attractive possibility and not an established fact.