HUMUS 87 



ing less organic matter. Humus also helps to fill the otherwise too 

 large air space in loose, open, sandy soils, thus preventing too rapid 

 leaching and holding the moisture nearer the surface, where it can be 

 utilized by plants. Humus acts as a sponge in the soil. It is one of 

 the best known absorbents of water, and hence its presence adds 

 greatly to the water-holding capacity of soils. In Minnesota, *Snyder 

 found that a native soil contained 3.97 per cent of humus and had a 

 water-holding capacity of 62 per cent, while a soil cultivated for 23 

 years, but otherwise similar, contained 2.59 per cent of humus and 

 had a capacity for water of only 54 per cent. 



The very rapid depletion of the fertility of the soil by continuous 

 cultivation of one crop is also largely due to the resulting loss of 

 humus. Humus influences fertility in two ways : 



(i) By supplying nitrogen directly, and 



(2) By helping to make the mineral elements soluble. 



It is from the humus that all crops except legumes must obtain 

 their supply of nitrogen. While nitrogen is no more essential to the 

 growth of corn than some other soil elements, it is the one which is 

 required in the largest amount, and is the one most easily lost from 

 the soil. Throughout the corn belt, it is much more often the supply 

 of nitrogen than that of any other element which limits the crop pro- 

 duction. In Minnesota it has been found that "The loss of nitrogen 

 from four grain farms amounted to from three to five times as much 

 as that removed by the crops. This loss was due to the rapid decay 

 of the humus and the liberation of the nitrogen which forms an essen- 

 tial part of the humus." At this same Station, *when corn was grown 

 continuously on the same plot for 12 years, the loss of nitrogen 

 amounted to 1,400 pounds, or 18 per cent of the total amount orig- 

 inally present, and the waste of humus corresponded to that of nitro- 

 gen. The yield of corn was much less than that grown on similar 

 plots, but in a rotation. By its direct action in rendering the minerals 

 of the soil soluble and available to plants, humus performs a most 

 important function and greatly influences the crop yields obtainable. 

 **A large part of the mineral supplies of a fertile soil are found chem- 

 ically combined with humus, and it is chiefly in this form that they 

 are used by the crops. Thus, the loss of humus by continuous crop- 

 ping places another check on crop yields, for no matter how large the 

 natural supply of minerals in a given soil, they are useless to growing 

 crops until rendered soluble. 



''Minnesota Bulletin, ITo. 89. 

 **Minnesota Bulletin. No. 94. 



