Hoa gland 



— 10 — 



Plant Nutrition 



solution studies were made on many other soils in 

 California and elsewhere, and it has become apparent 

 that soil solutions can often be much more dilute than 

 those of our original set of soils, and still plants will 

 not necessarily fail to absorb adequate amounts of 

 nutrient ions. Thus, the concept of "supplying power" 

 and the interrelation of the solid to the liquid phase 

 of the soil became considerations of paramount signifi- 

 cance and they are so today. 



1000- 



250 



UNCROPPED 



8 12 16 



WEEKS FROM PLANTING 



24 



Textfigure 1, — Effect of cropping on the soil 

 solution as reflected in total solids of water extracts 

 of the soil at various periods of crop growth. (From 

 HOAGLAND, 1918). 



In these experiments prominence must be given to 

 the nitrate ions which undergo the greatest changes 

 in concentration in the soil solution.* With continuous 

 cropping the capacity of the soils to produce barley 

 crops greatly diminished, so that most of the soils 

 are now at a low level of production. Primarily re- 

 sponsible for the decreases in yields seems to be the 

 increasingly inadequate supply of nitrate nitrogen, 

 although the possibility exists that other limiting 

 factors may be entering, particularly, in certain soils. 



*Nitrate ions may be selectively absorbed by plants and 

 replaced in the solution by bicarbonate ions. Also see later dis- 

 cussions. 



