PHYSICAL CONSTITUTION OF SOIL 3 



If the particles composing a soil are not all of one size, but 

 are a mixture of large and small, then the proportion of 

 interspaces in the whole volume is diminished, the small 

 particles fitting in between the large ones. If in the system 

 of solid spheres with closest packing, a second set of spheres 

 is introduced, exactly fitting into the spaces between the 

 larger ones, the proportion of the interspaces will be reduced 

 from 25-95 to 6-76 per cent, of the total volume. If the 

 process is repeated by the introduction of a third set of still 

 smaller spheres, exactly fitting the remaining spaces, the 

 proportion of interspace will fall to 1-76 per cent, of the 

 total volume (Soyka, Wollnys Forsch. der Agrikulturphysik, 

 1885, i). Any mixture of particles of different size and all 

 soils consist more or less of such mixtures tends therefore 

 considerably to reduce the space available for air or water. 



If, on the other hand, the particles composing a soil are 

 themselves porous, the volume of the interspaces may be 

 considerably increased. It is indeed obvious, that if spheres 

 of sponge were substituted for the solid spheres in the systems 

 represented in Figure I, the proportion of unoccupied space 

 in the total volume would become very much greater. The 

 particles of quartz sand in a soil are non-porous, but particles 

 of limestone are generally porous, and the particles of decayed 

 vegetable matter (humus) are highly porous. A fertile soil 

 also largely consists of compound particles, made up of 

 fragments of various sizes, held loosely together; such 

 particles act as large particles in the soil, but are themselves 

 porous. The proportion of empty space in a mass of soil 

 may also be greatly increased by appropriate tillage operations, 

 the object of which is to bring the soil into a state of loose 

 powder, thus increasing its bulk. The condition of a fertile 



B 2 



