SOIL SANITATION 297 



successful farming and bringing the soil to 

 its highest capacity of crop production." 



In addition to the injurious substances to 

 be found in soils, there are others, such as 

 creatinine, which seem to exercise a positive 

 tonic action on crops. Among these are many 

 nitrogenous substances which seem to be the 

 result either of root excreta of growing plants, 

 or the result of the decomposition of green 

 manures, barnyard manure, and many other 

 organic compounds used as fertilizers. Legum- 

 inous crops are especially rich in some of these 

 rare compounds, and the isolation of these 

 substances from soils growing legumes adds 

 another argument to the sum of benefits of a 

 practice thousands of years old. 



As to the origin of this class of compounds, 

 both toxic and tonic, little definite is known. 

 It is certain that several of the compounds 

 are the result of root excretions. Others re- 

 sult from the breaking down of organic debris 

 present in the soil independent of that left by 

 crops. Still others are the result of the debris 

 left by roots of plants, and whether or not, in 

 this connection, each family of plants actually 

 does leave in the soil residues inimical to its 



