304 Journal of Agricultural Research voi. xn, no. 6 



(and presumably their concentrations) to the same general level in both 

 good and poor soils. It seems probable that, when certain minima are 

 reached, the plant can not absorb sojutes, and subsequent withdrawal 

 then depends upon the capacity of the soil to elaborate additional solutes 

 as rapidly as the plant requires them. The figures themselves, however, 

 can not give a direct measure of this capacity, but small variations may 

 possibly reflect important differences in the rate at which solutes are 

 elaborated by different soils. 



EquivaIvEncs of bases. — If we compare the figures for the individual 

 basic ions in good and poor soils (both in the cropped and uncropped 

 condition), we find numerous instances in which some one or more ions 

 are lower in the good soils than in the poor soils. These discrepancies 

 need not concern us if we regard the figures merely as a means of getting 

 at the power of soils to replenish the soil solution. In fact, this power is 

 more likely to be reflected by the aggregated soluble matters than by 

 individual constituents, and preferably by the aggregate of those con- 

 stituents of similar chemical properties. We propose, therefore, to con- 

 solidate our data for the basic ions in one figure (by addition) for subse- 

 quent consideration. If we further consider all of our data with reference 

 to the seasonal requirements of a good crop, we can perhaps indicate why 

 some soils are more productive than others. 



In Table II we gave the amounts of the chemical elements found in a 

 good crop of barley produced on soil i , computed to parts per million of 

 soil. These figures were 42.50 p. p. m. of nitrate, 11.90 p. p. m. of phos- 

 phate, 18.00 p. p. m. of potassium, 2.9 p. p. m. of calcium, and 2.98 p. p. m. 

 of magnesium; or in round numbers, 42 p. p. m. of nitrate, 12 p. p. m. of 

 phosphate, and 24 p. p. m. of total bases. We assume these figures to 

 represent the probable requirements of a good crop of barley on all soils 

 and propose to examine our data with reference to the capacity of the 

 various soils to meet this requirement. In Table V we combine all 

 pertinent data available. 



Nitrate. — The figures for excess in the good and medium soils show 

 an extraordinary discrepancy between the amounts of nitrate in the 

 uncropped soil and the amounts found in the cropped soil plus the with- 

 drawal of a good crop. We can not satisfactorily account for this dis- 

 crepancy, which appears to be ca,used in part by an inhibition of nitrifi- 

 cation due to the presence of the crop, but which also represents an actual 

 loss of nitrate from the cropped soil. Since the soils were kept in tight 

 containers, there could be no loss from drainage; and neither denitrifica- 

 tion, reduction to ammonia, nor the possible loss of ammonia by way of 

 the plant appeals to us as an adequate explanation. We can only sug- 

 gest that the presence of a crop may cause such a change in the biological 

 environment of the soil that the nitrogen of nitrates is stored in insoluble 

 (protein) forms in the soil. For our present purposes, however, it is 

 only necessary to point out that such losses appear to be a necessary 



