10 THE FERTILITY OF THE SOIL [CH. 



in the last instance on the energy stored up in the 

 organic matter, in other words, on sunshine that was 

 caught years ago by plants long since dead. 



Tliere is reason to suppose that the four gi'eat 

 constituents of the soil — the inert fi^agments of sand, 

 the reactive clay, the calcium carbonate and the 

 organic matter — are not merely lying alongside of one 

 another in the soil. The evidence indicates that the 

 colloidal constituents form a jelly-like coating over 

 the inert particles, and this jelly contains much of 

 the food of plants and of bacteria ; it may be likened 

 to the nutrient jelly of the bacteriologist. 



The soil mass is not solid throughout but is full 

 of pores like a sponge. In a compact arable soil not 

 more than 60 — 70 % of the volume is soil material, 

 the remaining 30 or 40 % being empty space ; in 

 a pasture soil the proportion of empty space is even 

 greater. This pore space is at times completely filled 

 with water, but more usually air is also present : at 

 Rothamsted often to the extent of 10 7o of the volume, 

 leaving 25 7o filled with water. The water is not 

 pure but contains in solution carbonic acid, nitrates, 

 carbonates and other salts of calcium, magnesium, 

 etc. ; it is held in the soil partly by surface attractions 

 and partly by the colloids. 



We may thus think of the soil as a porous mass 

 made up of a hard framework plastered over with 

 a jelly containing chemically active substances, plant 



