THE POTATO 217 



"Any farm crop which is grown for the sole pur- 

 pose of plowing it under to increase the supply of 

 organic matter in the soil is known as a 'green 

 manure.' Green manures affect the soil benefi- 

 cially in many ways. Some of the possible benefits 

 are: (1) The addition of vegetable matter or 

 'humus/ with its attendant beneficial effect upon 

 the physical and chemical properties of the soil. 

 (2) Increasing the nitrogen content of the soil by 

 fixation of nitrogen of the air, when leguminous 

 crops are used as the green manure. (3) Using 

 surplus available plant food which might other- 

 wise be lost. (4) Plant food from lower depths 

 may be brought nearer to the surface and made 

 available for subsequent crops. 



"The kind of crop which may best be used as a 

 green manure depends upon which one or more of 

 these beneficial effects is most desired. If the 

 addition of humus, or an increased supply of decay- 

 ing vegetation, is the only necessity, then any rank- 

 growing farm crop may be used. The more suc- 

 culent or juicy plants are best, as they decay much 

 more quickly and are more easily incorporated in 

 the soil. If, however, the supply of nitrogen in the 

 soil is small and its increase is either the chief 

 necessity or a desirable addition to the increased 

 humus content, then some leguminous crop must 

 be used, as no other farm crop has the power of 

 utilizing atmospheric nitrogen or of returning to 

 the soil any essential element of fertility which it 

 did not draw from it. If it is desired to bring up 

 from below some of the mineral plant food which 

 is present in deeper layers of soil, then a deep- 

 rooting crop should be used. 



'The legumes, or leguminous crops, are a group 

 of plants which aie characterized by growing their 



