INTRODUCTION xiii 



air, moisture, and temperature within the soil are quite as 

 essential for vigorous plant growth as are the corresponding 

 conditions in the atmosphere above. 



Different crops, and even different varieties of the same 

 crop, demand quite different degrees of moisture or dryness 

 in the soil to bring them to perfection. The most favourable 

 proportion of water varies even in the different stages of a 

 plant's growth. The temperature of the soil required for 

 various purposes is also very different. A knowledge of 

 these facts is quite essential for deciding what crops should 

 be grown upon a particular soil, and what treatment the 

 soil should in each case receive for their successful culture. 



The texture of the soil also largely determines the availa- 

 bility of the plant food which it contains. The surface 

 presented to the action of the roots is far greater when a soil 

 is in a condition of fine tilth, than when the same soil is 

 consolidated, or contains unbroken clods, The supply of air 

 and water, and the condition as to temperature, also largely 

 determine the intensity and character of the chemical pro- 

 cesses which take place in the soil, and by which plant food 

 may be either produced or destroyed. 



The movements of salts in a soil are to a large extent 

 determined by physical actions, and on the extent and 

 direction of these movements the effects produced by these 

 salts will greatly depend. The fertilizing action of a saline 

 manure, or the injurious effect produced by the accumulation 

 of salts in alkali lands, is thus largely governed by physical 

 conditions, and these conditions come more or less under the 

 farmer's control when he is fully acquainted with their nature. 



In most cases a good deal may be done to improve the 

 physical conditions of a soil, and render it more suitable for 



