THE CONSTITUTION OF THE SOIL 77 



together into compound particles. How far the properties of the ulti- 

 mate particles are modified by this union we cannot say, but no very 

 profound alteration seems to take place in the sands and silts because 

 the properties of the separate fractions, deduced by correlation methods 

 from studies of numerous soils, agree tolerably well with the properties 

 revealed by direct experiments on the fractions themselves. The finer 

 particles are more changed, the result being to minimise the effects of 

 their smallness. Thus, while the limits within which the properties of 

 a soil fall are determined by the ultimate particles, a considerable 

 variation is possible within these limits through the formation of com- 

 pound particles. 



It is unfortunate that so little is known about the compound par- 

 ticles, because they play a great part in determining the relationships 

 between soil and plant growth. They can be disintegrated by various 

 cultivation methods, such as ploughing the soil when wet, or by allow- 

 ing the stock of organic matter and calcium carbonate to fall too low, 

 and when this has happened the "clay " properties become emphasised, 

 so that the soil loses its fine crumbly state and is very apt to become 

 sticky when wet, and to dry into a hard cake through which young 

 plants can only force their way with difficulty. The compound par- 

 ticles can be re-formed by careful cultivation and by adequate additions 

 of organic matter and calcium carbonate, but the process may take 

 years, nor can it be hastened until it is better understood. 



The reader cannot fail to have noticed how many of the important 

 soil properties are due to colloids. The formation of these compound 

 particles, the absorption of soluble manures, the retention of water 

 (in part), the swelling of the soil when wet and its shrinkage when dry, 

 are all colloidal phenomena. If we regard the mineral particles as the 

 skeleton of the soil we must look upon the colloids as clothing it in 

 many of its essential attributes. How the colloids are arranged in the 

 soil is not known, but the simplest view, and one in accordance with 

 all the facts, is that the mineral particles, especially the fine silicate 

 particles, are coated 1 with a colloidal complex containing silica, alu- 

 mina, ferric oxide, alkaline bases and phosphoric acid derived from 

 the weathering of the rock material and the so-called humus. These 

 various components are not in true chemical combination, but in a state 

 of absorption, or solid solution. The complex is decomposable by 

 changes in temperature, concentration of the soil solution, etc., but it 

 decomposes continuously and not in the fer saltern manner of ordinary 

 chemical reactions./ It can interact with various solutions, absorbing 



1 See also (88) and (89). 



