♦ 78 



SOIL CONDITIONS AND PLANT GRO WTH 



Boron, given as boric acid, was found by Dr. Brenchley (53^) 

 to increase the growth of peas, but not of barley, in water 

 cultures at concentrations of about 1/100,000; above this 

 point harmful effects were produced. In Voelcker's pot ex- 

 periments (290) even I part in 200,000 of soil proved toxic 

 for barley, but at lower concentrations there was a slight 

 stimulating effect. More recent experiments are recorded by 

 Cook and Wilson.^ 



Physiological Balance. 



It is not only necessary to supply all the essential nutrient 

 substances to the plant, in addition there must be maintained 

 some kind of proportion between the various salts ; this is 

 spoken of as the physiological balance. Plant physiologists 

 have long recognised that single salts of potassiurn, magnesium, 

 sodium, etc., are toxic to plants, while a mixture of salts is 

 not. Calcium salts are by much the most powerful reducers 

 of this toxic effect. Thus Kearney and Cameron ii^^d) found 

 that a root of Lupinus albus was just killed when immersed in 

 '00125 N magnesium sulphate solution (7 parts per 100,000), 

 but the effect was modified by added salts, as shown in 

 Table XXV. 



Table XXV. — Effect on Various Salts in Reducing the Toxicity or 

 MgSO^. Kearney and Cameron (145a). 



Hansteen found that the toxic effect of potassium salts 

 used singly was overcome even when so little lime was 



added that the ratio ^^^-^ = -^ — . Osterhout found (220a) 

 KgO 840 ^ ' 



that Vaucheria sessilis lived for three weeks in distilled water, 



1 F. C. Cook and J. B. Wilson, Journ. Ag. Research, 1918, 13, 451. 



