CHAPTER II. 



SOIL CONDITIONS AFFECTING PLANT GROWTH. 



IN order to put limits to our discussion, it is necessary at the 

 outset to state what soil factors influence plant growth and 

 how, in general, the influence is manifested. We shall, there- 

 fore, in the present chapter bring together certain of the results 

 obtained by plant physiologists which are indispensable for the 

 proper study of the subject. 



It has been shown that the five following soil factors pro- 

 foundly affect the growth of plants : 



1. Water supply. 



2. Air supply. 



3. Temperature. 



4. Supply of plant nutrients. 



5. Various injurious factors. 



The plant may be affected in two general ways in the 

 amount of growth (i.e. the total amount of dry matter formed), 

 or in the habit or other characteristic of growth. The former 

 is susceptible of quantitative investigation, the latter is not, or 

 only with difficulty ; it has, therefore, proved less attractive to 

 investigators. 



The Effect on the Amount of Growth. 



I. The Study of Single Factors. 



Of the five factors concerned plant nutrients are, on the 

 whole, the easiest to investigate quantitatively. The general 

 relationship between the supply of a given plant nutrient and 

 the amount of dry matter formed was demonstrated by Hell- 

 riegel at Dhame in the eighties of the last century. Barley 



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