CROPS AND SOIL IMPROVEMENT 



4. To furnish plant-food directly to growing 

 plants. Even when it has been produced from 

 the soil supplies alone, there is great gain because 

 the growing crop must have immediately available 

 supplies. Many of the plants used in providing 

 humus for the soil are better foragers for fertility 

 than other plants that follow, sending their roots 

 deeper into the subsoil or using more inert forms 

 of fertility. 



The Legumes. Any plant that grows and 

 rots in the soil adds to the productive power of 

 the land if lime is present, but plants differ in 

 value as makers of humus. There are only ten 

 essential constituents of plant-food, and the soil 

 contains only four that concern us because the 

 others are always present in abundance. If lime 

 has been applied to give to the soil a condition 

 friendly to plant life, we are concerned with three 

 constituents only, viz. nitrogen, phosphoric acid, 

 and potash. The last two are minerals and can- 

 not come from the air. They must be drawn 

 from original stores in the soil or be obtained from 

 outside sources in the form of fertilizers. The 

 nitrogen is in the air in abundance, but plants 

 cannot draw directly from this store in any appre- 

 ciable amount. The soil supply is usually light 



[38] 



