Chapter X. 



THE BALANCE BETWEEN SUPPLY AND REMOVAL OF 

 MINERAL PLANT NUTRIENTS. 



The mechanism of the solution and transport of mineral nu- 

 trients developed in the preceding pages makes it of interest to 

 determine the relation between the possible or probable supply 

 of mineral plant nutrients and crop demands over large areas. 

 The inquiry can be formulated more specifically : Is the move- 

 ment of mineral plant nutrients towards the surface soil equal to 

 or in excess of the removal by drainage waters and garnered 

 crops? Satisfactory data are yet wanting for anything like 

 exact computations, but approximate figures are available which 

 appear sufficient for the present purpose. 



The rainfall (R) can be considered as disposed in three por- 

 tions, the fly-off (/), the run-off (r), and the cut-off (c). Stat- 

 ing this as an equation, 



The cut-off can be resolved into the portion (a) seeping through 

 the soil to ultimately join the run-off, and the portion (&) re- 

 turning to the surface to ultimately join the fly-off. Stated as 

 equations, 



R=f+r+a+b 



In other words, the rainfall can also be' considered as made up 

 of the fly-off, the capillary water of the soil and the drainage 

 from the area. According to Murray, 1 Geikie, 2 Newell, 3 and 

 others, the drainage water for humid areas, or such an area as 

 the United States as a whole, would be between 20 and 30 per 

 cent, of the rainfall, the major portion coming from seepage 

 water rather than surface drainage. Assuming the higher fig- 



1 On the total annual rainfall on the land of the globe, and the rela- 

 tion of rainfall to the annual discharge of rivers, by Sir John Murray, 

 Scot. Geog. Mag., 3, 65-77 (1887). 



2 Textbook of Geology, by Sir Archibald Geikie, p. 484 (1903). 



3 In Principles and conditions of the movements of ground water, by 

 F. H. King, Ann. rept. U. S. Geol. Surv., 19, II, 59-294 (1897-98). 



