SUPPLY OF NUTRIENT SALTS 63 



on plant growth : the more concentrated the solution the 

 better the growth. They obtained the same result in 

 water cultures. But Stiles (1915) has shown for water 

 cultures of barley that concentrations of the nutrient 

 solution of I "8, 0*36, o'i8, and 0*09 parts per thousand 

 produced approximately equal crops if the solution were 

 changed sufficiently frequently to maintain the supply ; 

 the lower concentrations are of the same order as that of 

 the soil solution. The conditions in water culture and in 

 soil are of course very different, but it is significant that 

 in the simpler conditions the plant can thrive in an extremely 

 dilute solution, if the amounts of the essential salts are 

 sufficient (cf. also Brenchley, 19 16, and Stiles, 1916). 

 An exceptionally weak soil solution, however, generally 

 indicates a soil poor in important minerals, and has, 

 therefore, a starving effect due to actual lack of one or 

 more of the necessary salts. The effects of such starva- 

 tion may be seen especially in crop plants and weeds 

 growing in light sandy soils ; the shoots are stunted, 

 the leaves small, and premature flowering and fruiting 

 occur. In such conditions water supply may also play a 

 part. Highly concentrated soil solutions are typical of 

 saline soils of coastal regions and deserts, and of alkaline 

 desert soils. The injurious effects of these on ordinary 

 plants may be in part due to high osmotic pressure, but 

 other factors — excess of toxic salts, extreme alkalinity, and 

 disturbed water relations — are important. Gola has pro- 

 posed a classification of plant communities based on the 

 osmotic pressure of the soil solution ; the account by 

 Cavers (1914) should be consulted. 



Balance 0! Essential Salts. — To ensure normal growth 

 there must be an adequate supply of compounds yielding 

 the seven essential elements : nitrogen, phosphorus, 

 sulphur, magnesium, potassium, calcium, and iron. Silicon, 

 chlorine, and sodium are always present both in soil and 

 plant but are not essential, though silicon may play an 

 important part in the formation of mechanical tissue. 

 Some of the essential elements produce injurious effects 



