IMPROVEMENT OF THE SOIL 79 



possess habits of growth in a wild state that make 

 a careful study of their cultural characteristics and 

 possibilities exceedingly desirable. It is altogether 

 probable that in the future many more of them 

 will be utilized for soil-improving purposes than 

 at present. As has already been pointed out, the 

 subject of green manuring is of great importance in 

 the tropics where the stock of vegetable matter in 

 the soil is so quickly destroyed, but it is here that 

 the subject has attracted least attention and its pos- 

 sibilities for good are only beginning to be realized. 

 Commercial Fertilizers. — While the subject of green 

 manuring with leguminous crops is of the utmost 

 importance to Southern agriculture, and while it is 

 possible by this means alone to keep the soil in good 

 mechanical condition, and to supply it with a large 

 amount of nitrogen, it does not add to the total 

 quantity of potassium and phosphoric acid, those other 

 equally important food elements. With even the 

 richest soils these will at length become exhausted, 

 and it will be necessary to supply them in the form 

 of commercial or, as they are often called, chemical 

 fertilizers. Immense quantities of these fertilizers 

 are sold annually in the South, and in only too many 

 cases farmers have come to depend on them exclu- 

 sively as the means of increasing their crops. This 

 is extremely unwise. In the first place, it is poor 

 economy. It is always unwise to buy that which 

 can be produced cheaper at home. In the second 

 place, soils manured year after year with only com- 

 mercial fertilizers soon lose their vegetable matter, 

 become difficult to work, and suffer excessivelv 



