Hoagland — 6 — Plant Nutrition 



trol. Following a search of the literature, it was de- 

 cided that a useful service might be rendered to plant 

 nutrition through a systematic and well-controlled in- 

 vestigation of soil solution phenomena. For this pur- 

 pose 13 soils in large lots were assembled at the cen- 

 tral laboratory. These soils were selected from diverse 

 locations in California and with reference to particular 

 soil types. After thorough mixing, the soils were 

 placed in duplicate tanks holding about a ton each. 

 These soils are still in course of study after twenty- 

 seven years. Barley crops have been grown on the 

 soils during this period except for certain years in 

 which some soils were allowed to lie fallow. Only 

 distilled water has been added to the soils and drain- 

 age has been prevented. The water is added in such 

 amounts as to avoid an excess. No crop residues or 

 nutrient salts have been returned to the soils, save 

 for the finer parts of the root system which could not 

 be removed. {See plate 1). 



Chemical studies of many types have been made 

 on the soils and on the crops produced. I suppose 

 that no other soils held under similar control have 

 received such extensive examination over so long a 

 period of time. I am now concerned with only a few 

 aspects of these experiments, especially those relating 

 to the soil solution. The first objective was to follow 

 the seasonal and secular trend of changes in the com- 

 position of the soil solution and the effects of cropping 

 on its composition. For some time the only technique 

 available was that of making water extracts of the 

 soils, but later a procedure was developed (BURD 

 and Martin, 1923), following a suggestion of F. W. 

 Parker (1921), for obtaining solutions from the soil 

 at moisture contents approximating the general range 

 of moisture contents held by the soil as the crops 

 grew. 



The general procedure was to pack the soil care- 

 fully in a brass tube, then to place over the soil a 

 column of water and subject the system to air pres- 



