THE COLLOIDAL PROPERTIES OF SOIL 



171 



cause in a normal soil these fractions, which we may regard 

 as the ultimate particles, are largely bound together into com- 

 pound particles. How far the properties of the ultimate par- 

 ticles 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 experi- 

 ments 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 par- 

 ticles, a considerable variation is possible within these limits 

 through the formation of compound particles. 



TABLE XLVI. EFFECT OF COMPLETE DISINTEGRATION OF SOIL BY ACID AND 

 ALKALI ON THE RESULTS OF MECHANICAL ANALYSIS OF A CLAY AND A 

 LOAM. 



It is unfortunate that so little is known about the com- 

 pound particles, 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 plough- 

 ing the soil when wet, or by allowing 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 be- 

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

 which young plants can only force their way with difficulty. 



