FERTILIZERS 129 



Humus. Opinion as to the value of humus to plants has, pen- 

 dulum like, swung to extremes. According to the early alche- 

 mists, decaying animal and vegetable substances yielded their 

 spirits to the new plants. Many of the earlier chemists be- 

 lieved that the larger part of the materials entering the growing 

 crop was supplied by humus. The net result of the combined 

 labors of DeSaussure, Boussingault, Dumas and Liebig on this 

 problem was to demonstrate that plants obtain most of their 

 food from the air, and particularly that part which was sup- 

 posed to be furnished by humus. Subsequently to this, humus 

 was supposed to have a low value, but it is now known to per- 

 form many functions of the greatest consequence in plant 

 growth. A certain amount of humus is essential to the proper 

 physical condition of the soil. Besides influencing tilth, per- 

 meability and weight of soils, it facilitates drainage and pre- 

 vents baking and cracking. Humus increases the power of the 

 soil to absorb and retain moisture and renders it more friable 

 and mellow. It supplies nitrogenous plant food and aids in 

 making mineral plant foods and fertilizers more available and 

 effective. It also lessens the danger of the winterkilling of 

 wheat, and it furnishes food for the myriads of bacteria which 

 live in all fertile, aerated, moist and warm soils. 



The best method of keeping an adequate humus supply in the 

 soil is to grow clovers and grasses in the crop rotation and plow 

 under all plant refuse. Leguminous inter- or cover-crops can 

 sometimes be grown advantageously after the main crop of the 

 year is gathered, such crops being plowed under in the fall 

 or spring for the purpose of supplying humus for the next 

 crop. These inter-crops also tend to prevent plant food from 

 leaching out of the soil between crops. Among the best humus 

 furnishing crops to be grown thus are soy beans and cowpeas, 

 but even rye may be used. 



As to the amount of humus contained in dried soils, those from 

 the Red river valley contained 4.82 per cent, those between 

 the Snake and Pelouse rivers, 6.4 per cent, those near Walla 

 Walla, Washington, 4.8 per cent, and those of Missouri 4 per 

 cent. Many of the soils of the south are deficient in organic 

 matter. The native prairie soil of the Red river valley was very 

 rich in humus, but the amount has been greatly depleted by 

 continuous wheat growing. The soil humus can be increased 



