62 PLANT PRODUCTS 



packed is subject to considerable control. If we assume, 

 for the sake of argument, that all the particles in the soil 

 are spherical, and that they are packed together with loose 

 packing, then the air space will amount to 47 per cent, of 

 the total. With close packing they will give 26 per cent. In 

 practice, however, such purely theoretical considerations have 

 little relationship to what actually occurs. The particles are 

 not spherical, and, at any rate in a temporary manner, they 

 put themselves into a condition known as " crumb " structure, 

 in which the particles have built themselves up into irregular 

 groups, with fairly large openings between groups of particles, 

 so that in fertile soils the vacant space filled with either 

 air or water amoimts to about 50 to 70 per cent. Where 

 there is much fibrous, half -decayed root, the openings of 

 the structure, and consequently the air and water space, 

 may be further increased. In the operation of tillage the 

 earth is broken apart and allowed to fall back gently, so 

 that the structure is much more open. Rolling will compact 

 the soil and decrease the air and water content. For the 

 growth and development of any root system space in the 

 soil is necessary, and the provision of this necessary space 

 is largely dependent upon tillage operations. The movement 

 of the water in the soil is much altered by variations in 

 the open space in the soil. 



A very important study in the physical properties of 

 soils consists in the consideration of the properties of colloidal 

 material that the soil contains. A rough distinction between 

 the colloids in the soil and the solid grains may be made 

 by stirring the soil up with water, allowing the grains to 

 settle for twenty-four hours, and pouring the muddy liquid 

 off. Some portions of the soil will practically never settle 

 in water, but may be made to do so by precipitating with 

 suitable agents. The addition of sodium carbonate will 

 increase the proportion of a soil that will not settle in water. 

 The addition of calcium sulphate will precipitate nearly 

 all the soil colloids. Any strong solution — sodium chloride 

 and sodium sulphate- — will also precipitate the colloids ; 

 super-phosphate, lime, basic slag, and farmyard manure all 



