THE CONSTITUTION OF THE SOIL 75 



with acid and alkali to break up the compound particles completely, 

 the other made on the untreated soil, where the breaking up is only 

 partial : 



Disintegration Disintegration 



complete. incomplete. 



Fine gravel .... -3 -2 



Coarse sand .... 8-3 8-6 



Fine sand .... 3*2 2*9 



Silt .... 8-6 I2-I 



Fine silt .... ii'2 13*3 



Clay . . . . 43-2 36-3 



The existence of these compound particles puts out of the question 

 any complete quantitative interpretation of a mechanical analysis. The 

 properties of a soil are not the sum of the properties of the separate 

 fractions clay, fine silt, silt, etc. because in a normal soil these frac- 

 tions, which we may regard as the ultimate particles, are largely bound 

 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 



6* 



