302 NATURE STUDY AND AGRICULTURE 



of humus. It not only improves the physical condition of 

 the soil but adds some of the most important plant foods, 

 phosphorus, nitrogen, and potassium. It is the most 

 perfect of all fertilizers. 



One thing more is done in many places to keep up the 

 fertility of the soil, and that is the use of commercial fertil- 

 izers. This we have already discussed in our study of soil 

 chemistry. Just what plant materials may be lacking in 

 any soil can be told only by testing the soil, not in the 

 laboratory but in the field. Any boy can have a fertilizer 

 plot in which he can determine whether the application of 

 commercial fertilizers will increase the productiveness of 

 the soil. See suggestion for home experiment. 



To sum up the whole story then. In order to increase 

 the fertility of the soil we must see to it that the soil is kept 

 in good physical condition by proper tillage, drainage, and 

 the addition of humus ; and that the plant materials that are 

 removed by the plants to manufacture their products be 

 returned in good measure in the form of stable manures, 

 leguminous plants, green manures, or commercial fertil- 

 izers. 



Farmers' Bulletins: Beneficial Bacteria for Leguminous 

 Crops, No. 214; Soil Inoculation for Legumes; Bureau of 

 Plant Industry, No. 71; Cowpeas, No. 89; The Soybeans 

 as a Forage Crop, No. 58; Alfalfa or Lucerne, No. 31; 

 Alfalfa Growing, No. 215; Drainage of Farm Lands, No. 

 187; Practices in Crop Rotations, No. 289; Relation of 

 Sugar Beets to General Farming, No. 320. 



