Feb. II, 191S Water Extractions of Soils and Crop Production 309 



from the uncropped soil taken alone do not take into account the fact 

 that the solutes in the cropped soils can not be reduced below certain 

 minimum and probably variable limits. 



In the present paper we have dealt entirely with chemical criteria 

 because they afiford the most convenient expression of the results of the 

 activities of the soil, chemical, physical, and biological. We do not 

 wish to be understood as minimizing the importance of biological stu4ies 

 because we regard living organisms as being the most important single 

 agency, through the formation of nitrates and carbonic acid, in modify- 

 ing the soil solution. While biological studies have a most important 

 bearing on the proper treatment of soils, the resultant effects of all 

 activities upon these heterogeneous mixtures can only be developed by 

 the actual growth of crops and observation of their effects. 



