3o6 Journal of Agricultural Research voi. xii, no. 6 



3 and 9. For these soils to have supplied the requirements of a good 

 crop and at the same time to have maintained their general level of 

 concentration would have necessitated that more bases come into solu- 

 tion. It is not impossible that they would have responded to the demand 

 for bases, just as they would probably have responded to the demand 

 for phosphate. The important difference between these poor soils and 

 the remaining soils is that the latter would never have been called upon 

 to furnish larger amounts of bases than the uncropped soil shows ability 



to supply. 



CRITERIA OF FERTILITY 



The growth of a crop reduces the average nitrate content of soils to a 

 comparatively uniform level in soils of all degrees of productivity (see 

 Tables IV and V). The figures for nitrate in uncropped soils are always 

 higher than the known withdrawals by plants, but may not always be 

 sufficiently high to supply these withdrawals and certain other inherent 

 losses. The relative ability of soils to meet these losses may be inferred 

 from the amounts of nitrate in the uncropped soils. 



Except in a few soils containing large amounts of soluble phosphate, 

 the growth of a crop does not reduce this ion to any considerable extent. 

 (The small differences shown approach the magnitude of experimental 

 error.) Furthermore, the figures do not show that good soils possess the 

 power of renewing the soluble phosphate more rapidly than the poor 

 soils. Figures for soluble phosphate can not be considered to reflect the 

 relative power of soils to supply the plant. 



The growth of a crop reduces the basic-ion content of soils, but the 

 amounts remaining are still far in excess of the crop requirements. It is 

 not improbable, however, that their concentrations are in some cases 

 falling below the optimum requirements of plants. If this be the case, 

 comparison of the basic solutes of cropped soils may indicate deficiency 

 in this respect. The differences between the basic ion contents of cropped 

 and of uncropped soils are usually, but not always, greater than the 

 demands of crops. These differences therefore express the relative 

 power of soils to supply the crop requirements and to maintain the 

 concentration equivalent to the amounts shown by the cropped soil, 

 without drawing upon greater amounts of solutes than the uncropped 

 soil indicates capacity to supply. 



Test op criteria. — If we arrange our soils in the order in which they 

 possess the various characters to which attention has been called, we 

 may be able to bring out salient differences between good and poor soils. 

 The order in which the soils occur in each character is frequently deter- 

 mined by very small variations in absolute amount of solutes, but it must 

 be remembered that these are based on considerable numbers of analytical 

 determinations and are probably not vitiated by experimental errors.* 



1 Stewart, G. R. Op. cit. 



