8 FUNDAMENTALS OF AGRICULTURE. 



stave. If the length of this stave is increased then 

 the next shortest stave will determine the capacity ^of 

 the barrel, and so on until all are of equal length. 

 The same relation holds among the factors determin- 

 ing the fertility of the soil. 



Humus. We must now consider some of these im- 

 portant factors. One of the most important is known 

 as humus. This is a black waxy or tar-like substance 

 formed by the partial decomposition of vegetable 

 matter in the soil and constituting a coating around 

 soil grains. Humus is important in binding the sands 

 together, so as to give them greater coherence and in 

 part prevent their being blown by the wind as would 

 otherwise be the case. It also greatly increases the 

 moisture-holding capacity of the soil, since this sub- 

 stance can hold two or three times its weight of mois- 

 ture. On heavy clay soils it has the effect of lessening 

 the tenacity with which the grains of soil are held 

 together in lumps and so improves the tilth. More- 

 over this humus contains a great deal of nitrogen which 

 by going through a chemical process caused by the 

 bacteria in the soil is changed into the form of a salt 

 of nitrogen or nitrate, which can be absorbed by the 

 plant. It is, therefore, of the utmost importance that 

 the supply of humus in the soil be maintained. 



Sources of Humus. Now humus is chiefly formed 

 from the fine roots of grasses and exists in large 

 amounts on prairie soils where these grasses have been 

 growing for thousands of years. In the soil kept cul- 

 tivated it is being continually burned out, and the best 

 method by which it can be handled or even maintained 

 is by rotation of crops including the growing of grasses 

 a part of the time either for pasture or hay. Green 

 manuring crops add somewhat to this as does stable 

 manure, but to a very much less extent than the fine 

 roots of grasses. 



Chemical Reaction of Soils. Another important 

 matter with reference to the condition of soils is what 

 is known as their chemical reaction, that is, as to 



