MINERAL PLANT NUTRIENTS 77 



of phosphoric acid (PO 4 ) are carried into the sea annually from 

 the United States, while from 48,00x3,000 to 100,000,000 tons 

 of potassium and 18,000,000 to 40,000,000 tons of phosphoric 

 acid are being carried towards the surface of the soil. If it be 

 assumed that an average of one ton per acre of dry crop con- 

 taining one per -cent, potash and 0.6 per cent, phosphoric acid 1 

 be removed from the entire area of the United States, then the 

 annual loss from this source would be 24,000,000 tons of po- 

 tassium and 14,000,000 tons of phosphoric acid. Consequently, 

 there is an ample margin between the losses by cropping and 

 seepage waters, and the supply of capillary waters. It is true 

 that cases exist where the production of vegetable matter is much 

 greater than a ton to the acre, productions of five tons or even 

 more being on record. But such cases occur only where the 

 water supply is also greater, either through natural rainfall or 

 artificial irrigation; and it should also be borne in mind that the 

 production of so large a mass of green crop involves a consid- 

 erable drawing power on the water in the soil in addition to the 

 evaporation which would take place at the surface under ordi- 

 nary conditions. In other words, the plant would then be play- 

 ing no small part in drawing to itself its needed supplies of water 

 and dissolved mineral nutrients. 



The question may be asked, if the processes outlined above 

 are generally operative, why accumulations of soluble mineral 

 substances are not usually found at the surface of the soil. As 

 a matter of fact such accumulations do occur normally when the 

 evaporation at the surface is relatively large, that is, under arid 

 conditions. And under humid conditions it appears to be a gen- 

 eral rule that the surface soil contains more readily soluble or 

 absorbed mineral matter than do sub-soils. 2 No great accumula- 



1 Estimated from Wolff's tables, How crops grow, by Samuel W. 

 Johnson, 1890, appendix. 



1 See, for instance : Investigations in soil management, by F. H. King, 

 Madison, Wis., 1904, p. 62 et seq. This tendency towards a higher content 

 of absorbed soluble mineral matter in the surface soil has been amply 

 confirmed by other experiments. It has been advanced as an argument 

 against the assumption that the hydrolysis of the soil minerals is a 

 reversible process. But as pointed out elsewhere in the text, many of 

 the soil minerals can be made in the wet way at more or less elevated 

 temperatures and the more rational explanation is simply that at ordinary 

 temperatures the rate of formation is exceedingly slow. 



