Green manuring puts humus into the soil and that makes it an important farm practice, because humus is absolutely essential for the best growth of crops. Humus may be added in another way, by applying barnyard manure, but that method alone will not keep the soil from losing fertility from year to year. Even though it is saved with the utmost care, the manure produced on the farm will not return to the soil all the fertility removed by the crops. Circular No. 9, "Farm Manures," Iowa Agricultural Experiment Station, shows furthermore, that there are certain unavoidable losses in the storing of manure and that they may involve not only the valuable mineral constituents, but also the organic matter. As agriculture is now generally practiced in Iowa and elsewhere, the soil is being gradually worn out.

It is clear, therefore, that some other means must be employed to keep up the humus content of soils and this may be accomplished by turning under green manure crops.