8 THE FERTILITY OF THE SOIL [CH. 



they mingle with the soil and begin to decay the 

 energy is liberated in the form of heat, and by the 

 time they are completely decayed they have given 

 out just as much heat as if they had been burned in 

 a bonfire. The original heap of mineral matter 

 contained no easily available store of energy ; the 

 mixture of mineral matter and plant residues on the 

 other hand does. The consequence of this addition 

 is very profound : life is now possible in the soil, and 

 there springs up a vast population of living creatures 

 all drawing on this accumulated store of energy, 

 flourishing so long as it holds out, and dying off when 

 it is exhausted. 



In our climate, and in humid climates generally, 

 the decay of the plant residues is not complete, at 

 any rate during the course of a few seasons, and some 

 of the products accumulate as dark brown or black 

 substances conveniently known by one name, humus. 

 These substances have certain physical properties 

 which they impart to the soil ; they are sticky, 

 they absorb and retain water, they swell when wet 

 and shrink when dry. In other words they are 

 colloids. Thus the third effect of organic matter on 

 the soil is to increase the amount of colloidal material, 

 but some of this is of entirely different character 

 from that already present. By far the most significant 

 of these effects, however, is the bringing in of stores 

 of energy because this constitutes the vital distinction 



