Feb. II, 1918 Water Extractions of Soils and Crop Production 



299 



(uncropped) soils, both good and poor soils are reduced to the same 

 general level at the time the crop is growing. The condition of soils under 

 crop is epitomized in Table I. 



Table I. — Water extractable matters in soils^^ under crop. Average during period ^ of low 



nitrate content 



[Results expressed as parts per million of soil] 



o For a detailed description of soils, see Stewart, G. R. Op. cit. 



b Period covered: Soil 7, twelfth to eighteenth week after planting; soils 12 and 14, eighth to eighteenth 

 week after planting; all others, tenth to eighteenth week after planting. 



We observe in Table I a general uniformity in the magnitudes of the 

 various solutes present at the period of depletion in soils of varying pro- 

 ductive capacity. Apparently crops first reduce the supply of solutes 

 until these approach a comparatively low and (with a few exceptions) 

 fairly uniform level. Subsequent withdrawals must further reduce the 

 existing supply or be made good by new material coming into solution. 



WITHDRAWAL BY CROPS 



In the second year's experiments reported by Stewart we installed 

 eight additional containers of soil i , planted to barley, and harvested and 

 analyzed the crop from time to time. This particular soil was chosen 

 because it was known to be one of our most productive soils and the 

 amounts withdrawn by the crop would presumably represent the usual 

 draft to be expected from the crop under consideration. The data 

 obtained are presented in Tables II and III. In scrutinizing these data 

 it must be remembered that we are not attempting to show the require- 

 ments of plants or their nutritive habits, but merely their effect on the 

 soil. 



If we now compare these data with the water-soluble content of the 

 soil, we may develop the approximate relation between the demand of 

 barley and the supply in a productive soil. The relation is expressed in 

 the column headed "Ratio." Literally interpreted, the ratio for each 

 Ion expresses the number of day's supply contained in the water extract 

 during the several periods covered. Thus, in the case of nitrate we see 

 that there was never less than a nine days' supply in the soil during any 

 period of the growing season. If the plant is capable of absorbing all of 



