GREEN MANURES 217 



the best results. The richness of the soil may be measured by 

 the amount of that plant food which is present in the smallest 

 degree; that is, the soil may have plenty of potash and phos- 

 phoric acid, but if it is weak in nitrogen, its value is no greater 

 than is its nitrogen content. Thus the addition of nitrogen, 

 in this particular case, would allow the other plant food to 

 act proportionally, and the result would be greater than that 

 due to the mere addition of the nitrogen alone. Any addition 

 of plant food to the soil is called enriching the soil. 



The materials used for enriching the soil are treated under 

 the various paragraphs concerning plant food. Yet we must 

 remember that humus is absolutely necessary to make plants 

 grow well. The best methods for increasing the humus con- 

 tent is by the use of green manures and barnyard manure. 



155. GREEN MANURES 



Crops which are grown for the purpose of being plowed 

 under, are called green manures. Cowpeas, crimson clover, 

 and various legumes are used for this purpose, and the result 

 is twofold : the supply of humus is increased, and the nitro- 



