8o SOIL CONDITIONS AND PLANT GRO WTH 



thirty-six different combinations of equal total concentra- 

 tion. 



The extent of the injurious effect depends on the concen- 

 tration : at 0"i atmosphere osmotic pressure none of the 

 solutions proved injurious to seedlings ; at a higher concen- 

 tration seven caused severe and four slight injury ; fifteen 

 caused various degrees of productiveness without injury. 



The specific injury caused in these circumstances by mono- 

 potassic phosphate and by sulphate of ammonia to soy beans 

 was studied by Shive (262^) and by Wolkoff (316) respectively. 



The amount of growth produced by well-balanced solutions 

 also depends on the total concentration, increasing up to a 

 certain point. 



The optimum ratio of nutrients for a given stage of plant 

 development alters with the concentration ; it is not the same 

 at 0"i, 175, and 4 atmospheres. But it is not affected by 

 the nature of the medium ; it is the same in sand as in water 

 culture. So also it is independent of variations in the 

 moisture content of the sand, being the same for degrees of 

 moistness varying from 40, 60, to 80 per cent, of the water- 

 retaining capacity of the sand. But it is not constant for 

 the whole range of growth of the plant, being different in 

 seedling and ripening stages and different for the growth of 

 " top " and of roots. 



It must not be supposed, however, that the physiological 

 balance is a rigid ratio: Hoagland and Sharp (136^) obtained 

 satisfactory growth with a wide range of mixtures so long 

 as the total supply and concentration of essential elements 

 was adequate. Maze has adduced evidence that physiological 

 balance is an important factor in soils and is affected by 

 calcium carbonate and sometimes by humus (196). 



Absence of Injurious Substances. 



We have seen that many salts have a toxic effect if given 

 alone to the plant, but for our purpose we need consider only 

 those causing injury in presence of other compounds. Two 

 cases arise in practice : some substances are injurious even in 

 small quantities, others only in excess. 



