Chemical Constituents 



solution. The first six are constituents of the 

 Uving stuff itself, the remaining four each play their 

 several parts, potassium assisting sugar formation, 

 iron being essential in the formation of the green 

 colouring matter, and so on. If either of these 

 chemical elements is present in too small quantity in 

 a soluble form in the soil the growth of the plant is 

 interfered Avith. If certain of them are present in 

 too great quantity as compared with the others 

 either part of the plant is encouraged to develop at 

 the expense of the other parts, or parts are injured or 

 destroyed. If the soU solution is too strong, absorp- 

 tion of water by the plant is retarded or actually 

 prevented. Only when the soil solution contains 

 sufficient but not excessive quantities of soluble 

 salts combining the essential chemical elements can 

 water and earth-salt absorption go on normally and 

 the plant be suppUed with the requirements for 

 healthy Ufe. The rate of absorption of water and 

 earth-salts is to a large extent governed by the rate 

 at which each constituent of the solution is used by 

 the plant, or, in the case of the water, got rid of. 



Ideal conditions for absorption by the root are 

 provided when the water supply is arranged so that 

 every soil particle is kept bathed in a weak solution 

 of all the earth-salts which the plant requires, while 

 the spaces between the particles are filled with air, 

 when the soil temperature is sufficiently high, for 

 the rate of absorption rises with the increase of 



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