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



301 



days for magnesium. It would appear that there is actually present in 



the water-soluble condition at the period of greatest depletion more than 

 enough potassium, calcium, and magnesium to supply the entire subse- 

 quent withdrawals of the crop at the same rate, and that any renewal or 

 increase of these amounts should be unnecessary if the plant is capable 

 of utilizing the entire supply present. 



Table III. 



-Comparison of water-extractable matters in soil I (cropped) with daily draft 

 by plant 



It is evident that nitrate and phosphate are in a class by themselves, 

 in that the supply must be renewed at a much more rapid rate than any 

 of the other ions; but, inasmuch as this soil was highly productive, it 

 must have possessed the power to replenish the soil solution as required. 

 But we have shown heretofore that there is very little difference in the 

 absolute amounts of solutes between good and poor soils under a crop of 

 barley. It follows that figures of the same order of magnitude as those 

 just stated would be obtained by similar computations from the poor soils. 

 We conclude, therefore, that in soils in any degree fit for agricultural 

 purposes (with the possible exception of very abnormal types, such as 

 peat, etc.), the potentially soluble matters are sufficient in amount to meet 

 the requirements of crops; but it does not follow that the plant is capable 

 of drawing upon this supply at the concentrations corresponding to 

 these amounts. 



