THE SOU.. 37 



dissolved and then reprecipitated on the surface of hundreds 

 of soil particles. There is a great difference in the avail- 

 ability of the potash in a soil particle, and the same potash 

 after it has been weathered and fixed on the surface of 

 many soil particles. When available plant food is spoken 

 of what is meant is : ( 1 ) the plant food actually dissolved 

 in the soil water ; (2) the fixed plant food slowly soluble in 

 the soil water and in the root juices ; (3) the food (manure) 

 which will dissolve on coming into contact with water. 



THE PHYSICAL PROPERTIES OF THE SOIL. 



If not more important, the physical properties of a soil 

 are at least quite as important as its chemical properties. 

 Many soils well stocked with all the essential plant foods, 

 are far from fertile, simply because their physical proper- 

 ties are unfavourable for plant growth. After glancing at 

 some of the general properties of soils, the soil in its 

 relation to heat, air, and water will be considered. The 

 object of the mechanical analysis of soil, is to separate 

 the mineral particles into classes of various sizes, and 

 thereby to ascertain the "Texture" of the soil. In this 

 separation, the finer particles are obtained by suspension 

 in water and subsequent settling, the remaining coarser 

 particles being passed through special sieves. The mi- 

 neral particles of all soils vary considerably in size, from 

 the large grains of coarse sand to the finest particles 

 of clay. In every soil, particles of various sizes are 

 found ; in sandy soil there is usually a small proportion 

 of the finest clay material, and in most clay soils a 



