76 DRY FARMING 



nodules on the roots of peas, vetch or the true clovers. 

 The groups are absolutely distinct. 



The Factors of Soil Fertility. 



52. The Measure of Fertility. — The measure of the 

 fertility of the soil is the crop it produces or can be made 

 to produce. This is not the result of any one factor, but 

 of a combination of factors. 



The physical make-up of the soil determines in part at 

 least the extensiveness and effectiveness of the root- 

 system of the plant. It is concerned with the ability of 

 the soil to absorb and retain moisture. It affects the 

 tilth, the air supply and the warmth of the soil. 



The chemical composition of the soil determines its 

 potential possibilities. Since we know the kinds and 

 amounts of plant-food elements that our crops require, 

 and since chemical analyses will tell us the total amounts 

 present in any particular soil, we have an index to the 

 fertility of that soil. "We have a means of discovering the 

 deficiencies, and when confirmed by actual field tests, 

 this is the safest guide to good practice. 



The bacteria are concerned with the liberation (mak- 

 ing available) of plant food. This is brought about 

 through the organic matter supplied. The value of 

 organic matter is therefore apparent. It furnishes the 

 food and energy for the bacteria which when thus sup- 

 plied make plant food available (soluble) for the plant. 

 In addition the actively decaying organic matter is the 

 source of humus in the soil. 



Permanency in agriculture depends upon the control 

 by man, in so far as is possible, of the factors essential 

 to the production of crops. The stock of materials in the 



