RELATION OF THE SOIL SOLUTION TO THE SOIL 



EXTRACT 



By D. R. Hoagland, J. C. Martin, and G. R. Stewart 

 Division of Agricultural Chemistry, California Agricultural Experiment Station 



Modern views of soil fertility recognize the general principle that 

 plants derive their immediate supply of inorganic elements entirely from 

 the soil solution. It has also been proved that the soil solution is subject 

 to highly significant fluctuations. The concentration and composition 

 of the soil solution may undergo very great alterations as a result of 

 seasonal changes, crop growth, activities of microorganisms, rainfall, fer- 

 tilization, etc. The evidence supporting this point of view is now too 

 strong to admit of any doubt. It is justifiable to assume, therefore, that 

 further progress in the study of the soil as a medium for plant growth 

 will depend upon an increased knowledge of the soil solution, particularly 

 in its dynamic relations to the soil mass, to the plant and microorganisms, 

 and to the application of fertilizing materials. 



Experimental work on the soil solution immediately encounters a 

 formidable obstacle in the difficulty of separating from the soil the solu- 

 tion which provides nutriment to the plants. When the soil contains 

 moisture in percentages most suitable for plant grov/th, the solution is 

 held by the soil particles with such force that no ordinary means will 

 serve to effect a separation. This fact is well recognized, and various 

 attempts have been made to overcome the difficulties involved and to 

 obtain the soil solution in an unmodified state. The most important 

 developments in this phase of the work have been described by Morgan 

 (8) l and by C. B. L,ipman (7). It is yet too early to state any final con- 

 clusions based on data obtained by these methods, but their further per- 

 fection may lead to the attainment of most essential information. A 

 considerable advance in our ideas concerning the soil solution has already 

 resulted from the application of the freezing-point method to soils, as 

 first described by Bouyoucos and McCool (4). However, the study of 

 the soil solution in its relation to plant growth is so fundamental that it 

 should be approached from every possible angle, with the hope that 

 eventually we may possess an adequate understanding of the nature of 

 the nutrient medium in the soil and of the modifications produced in this 

 medium by various treatments. 



Since the soil solution is constantly undergoing modification, the inves- 

 tigator is often required to make numerous determinations at frequent 



1 Reference is made by number (italic) to " Literature cited," p. 394~395- 



Journal of Agricultural Research, Vol. XX No. 5 



Washington, D. C Dec. 1, 1920 



w Key No. Calif. -25 



(38l) 



