EFFECT OF SEASON AND CROP GROWTH IN MODIFY- 

 ING THE son. EXTRACT 



By Guy R. Stewart/ 

 Assistant Chemist, California Agricultural Experiment Station 



HISTORICAL REVIEW 



The first studies on the water-soluble material of soils were inspired 

 by the results of the investigations of the absorption of plant nutrients 

 carried on by Thompson (55)^ and by Way (60). Their epoch-making 

 discoveries in the year 1850 aroused a widespread discussion of the 

 manner in which the essential compounds might be held in the soil. 



Liebig (59) was greatly impressed by this work and carried out a 

 series of investigations in which he studied the absorption of calcium 

 phosphate and potassium sulphate. He concluded that, since the 

 phosphate and potassium radicals were so readily absorbed by soils, 

 very little of these essential nutrients could be present in the soil water. 

 The results of lysimeter and drainage studies confirmed him in his belief, 

 and he proposed the theory that the plant roots must be able to draw 

 nourishment directly from the soil particles. At the same time Grouven 

 (22) had analyzed the extract from three soils obtained by percolating 

 6,000 c. c. of distilled water through 2,000 gm. of soil. He attempted 

 to relate these figures to the amount of material that would be brought 

 into solution by the season's rainfall. 



Eichhom {14) also studied a soil near Bonn, Germany, in an attempt 

 to obtain a solution which would approximate the moisture existing in 

 the soil. He added 36.5 per cent by weight of water, and allowed it 

 to stay in contact with the soil for 10 days. He concluded that the 

 soil contained all water-soluble nutrients necessary for raising a crop. 



This view was severely criticized by Wunder (63), who held with 

 Liebig {39) that the soil could not furnish sufficient water-soluble ma- 

 terial. Schumacher (52) repUed to Wunder (65), upholding Eichhorn's 

 views. Gradually, through the work of Peters (45), Jarriges (31), 

 Ulbricht (38), Hoffman (28), Wolff (62), and Cossa (13), it became 

 evident that the water extract from soils contained the major plant 

 nutrients. 



1 The writer desires to make acknowledgment of the assistance of the Division of Soil Technology, of 

 the California Experiment Station, in selecting the soils employed in the investigation and in perform- 

 ing the physical analyses reported in Table II. The writer is also indebted to Mr. A. W. Christie, of 

 this station, for the performance of ammonification and nitrification studies and to Messrs. H. E. BiUings, 

 A. W. Christie, and J. C. Martin, of this Station, for assistance in portions of the analytical work per- 

 formed in 1916. 



* Reference is made by number (italic) to " Literature cited," p. 364-368. 



Journal of Agricultural Research, Vol. XII, No. 6 



Washington, D. C. ^^^- "• ^9i8 



I- Key No. Cal.— 16 



(31O 



