32 



THE ELEMENTS AND THE SOIL 



Texture of the soil. 



The size and shape of the particles of which the soil is composed 

 determine its texture. The arrangement of the particles determines 

 its structure^ as " loose," " open," " mealy," " friable," " cloddy," 

 " porous," " hard," " compact," " retentive," " leachy." 



The texture determines the amount of soil-surface exposed to roots, 

 and to a great extent the quantity of moisture that the soil may hold. 



The size and form of the particles determine the number in a given 

 volume of soil. It has been estimated by Whitney that a gram of soil 

 contains 2,000,000,000 to 20,000,000,000 soil particles. The number of 

 particles per gram of different soil types is approximately as follows : — 



Early truck . . . . 

 Truck and small fruit 



Tobacco 



Wheat 



Grass and wheat . . 

 Limestone . . . . 



1,955,000,000 



3,955,000,000 



6,786,000,000 



10,228,000,000 



14,735,000,000 



19,638,000,000 



Owing to the fact that a soil is made up of particles, there is between 

 them a certain amount of space that is occupied by air or water ; this 

 is kno^vn as the " pore space." In ordinary soils the pore space varies 

 from a little over 50 per cent in the finest clay soils to about 25 or 30 

 per cent in coarse sands of uniform texture. 



Soil Water 



Water occurs in the soil in three forms : (1) Gravitational or hydro- 

 static water ; (2) capillary water ; (3) hygroscopic water. 



Amount of water used by various crops in producing a ton of dry-matter (Stevenson) 



