THE PHYSICO-CHEMICAL INVESTIGATION OF SOILS. 



339 



action of plant-roots in general upon the soil, even if we could 

 give that action the same time (a growing season) and op- 

 portunity afforded them in nature by the root-surface. We 

 are forced to proceed empirically; and among the numerous 

 solvents suggested for the purpose of soil extraction, that of 

 Dyer, already mentioned, viz., a one per cent solution of citric 

 acid, making allowance for such neutralization as may occur 

 in the soil, has seemed to the writer to give results most largely 

 in agreement with cultural experience. Walter Maxwell has 

 recommended aspartic acid in lieu of citric, as approaching 

 nearer to practical results, at least with sugar cane. 



According to the investigations of Dyer, on Rothamstead 

 soils of known productiveness or manurial condition, it ap- 

 pears that when the citric-acid extraction yields as much as 

 .005% of potash and .010% of phosphoric acid, the supply is 

 adequate for normal crop production, so that the use of the 

 above substances as fertilizers would be, if not ineffective, at 

 least not a profitable investment. These figures refer to the 

 ordinary field crops of England and to soils originally fertile 

 and well supplied with lime. It can readily be foreseen that 

 under other climatic and soil conditions, different figures may 

 have to be established. So far as the writer's experience goes, 

 however, the above figures are very nearly valid for the arid 

 climates as well ; only the figures obtained for arid soils are 

 usually far in excess of the above minimum postulates. Fig- 

 ures for lime and nitrogen are given in chapters 8 and 19. 

 But the results obtained with the highly ferruginous soils of 

 Hawaii show that under such conditions, figures far exceeding 

 the minimum ones established by Dyer nevertheless coexist 

 with need of phosphate fertilization. 



