MANAGEMENT OF THE SOIL 31 



for the escape of the surplus carbonic acid that is 

 constantly being formed in the soil from the decay 

 of organic matter. 



This free interchange of gases bet^Yeen the soil 

 and the atmosphere is necessary, not only for the 

 normal growth of roots, but for the carrying on of 

 all those manifold biological activities already re- 

 ferred to on which the fertility of the soil so largely 

 depends. Tillage, therefore, not only kills weeds and 

 provides an abundant supply of oxygen to the roots, 

 but it actively promotes nitrification and all those 

 other complex chemical changes induced by the 

 growth of bacteria by means of which vegetable 

 matter and other potential plant foods in the soil be- 

 come soluble and available for the growth of plants. 

 This unfortunately is not an unmixed benetit. Sol- 

 uble plant food that is not at once absorbed by the 

 roots is in danger of being washed or leached away 

 by the rains. This, in fact, is constantly taking 

 place. The fact should never be lost sight of that 

 while tillage is absolutely necessary for the produc- 

 tion of good crops, it also rapidly depletes soil fer- 

 tility. When land is constantly devoted to tilled 

 crops, active measures must at the same time be taken 

 for adding in some manner to its supply of vegetable 

 matter and other plant foods, or its fertility will be 

 rapidly exhausted. This is especially true of tropi- 

 cal or subtropical countries where these soil activities 

 continue unchecked throughout the year. 



Finally and perhaps most important of all, tillage 

 aids materially in conserving the moisture of the 

 soil during dry weather. The fertility of a soil 



