THE BIOLOGICAL CONDITIONS IN THE SOIL 237 



ing that maize fails to complete its growth in water cultures 

 containing all the recognised nutrient salts if these are chemi- 

 cally pure, but it grows normally as soon as tap water is intro- 

 duced. No combination of added salts has as good an effect 

 as the tap water (Table LXIL). 



Table LXII. — Growth of Maize in Water Cultures. Maze, 1915. 



Culture solution containing compounds of boron and aluminium (tap water) 43 



„ ,, „ „ „ „ ,, ,, (distilled water) 24 



„ ,, „ „ ,, ,, aluminium and arsenic . 16*5 



„ „ „ „ ,, „ „ arsenic and iodine 36*6 



A considerable amount of work has obviously to be done 

 before the problem can even be clearly stated. And against 

 all this is the fact, abundantly demonstrated by Dr. Brenchley, 

 that barley and peas make full growth in nutrient solutions 

 containing only the ordinary " essential " foods (54^). 



The Concentration of the Soil Solution. 



It is well-known that plants cannot tolerate too con- 

 centrated a solution, otherwise their osmotic relationships are 

 seriously disturbed. The cell sap of the plant root exerts an 

 osmotic pressure of 7-20 atmospheres (p. 222). In ordinary 

 agricultural soils the soil moisture has a concentration of the 

 order of 01 to 0'2 per cent, and an osmotic pressure of about 

 o*2 to I atmospheres (p. 125). In some soils, however, the con- 

 centration and the osmotic pressure of the soil solution rise 

 much higher, and in the alkali soils it becomes too high 

 for plant growth. Gola (108) has proposed to classify 

 habitats on the basis of concentration of soil solution, his 

 groups being — over 2 per cent. ; 2 to 0*5 per cent. ; 0*5 to 0'2 

 per cent. ; below o "2 per cent.^ The scheme is interesting and 

 promising : it would be improved by taking into account the 

 composition of the solution which at higher concentrations 

 is of great importance (p. 78) and by stating the limits in 

 terms of osmotic pressures. 



^ The total concentration of spring and river waters is of the order of o*i to 

 o'3 per cent. 



