44 THE SOIL AND THE SAP 



growing. The chalk and the sedimentary Hmestones which 

 are built up from the shells of minute marine organisms play 

 a considerable part in the composition of fertile soils. 



When we come to examine an ordinary garden soil we 

 shall find that silicates and broken-up limestone, chalk and 

 other rock-fragments, with certain phosphates, are mixed up 

 with a great amount of organic matter, a rich dark-coloured 

 material called humus. In course of time this is apt to spread 

 over our chalk hills and our clay and sandy formations. The 

 ideal soil is not composed of sand or clay or humus alone, but 

 contains a proper proportion of all three : the sand to ensure 

 porosity and a proper circulation of water, the clay to lend 

 firmness and to prevent too rapid evaporation, and the humus 

 to provide plant-food rich in nitrogen. The average grains 

 making up a soil are about two and a half times as heavy as 

 an equal volume of water, and the weight of a cubic foot of 

 soil varies from about 80 to 105 lbs., the sandy soils being the 

 heavier and those rich in humus the lighter. In ordinary soils 

 from one-third to one-half of the volume is yore-space, which 

 may be occupied by air or water according to circumstances, 

 depending largely on rain-fall and the effectiveness of the 

 drainage. The total surface area of the particles in one cubic 

 foot of a light loam is estimated at about an acre. Each granule 

 or particle of soil being surrounded by a pore-space, excepting 

 at those points at which it is in contact ^^dth other particles, 

 it follows that the air spaces, however contorted in size and 

 direction, must form continuous tubes or passages which 

 traverse the soil in all directions. 



The organic matter has two sources. One is decayed plants 

 and animals, which rot away and disintegrate in the soil. 

 A familiar form of this is leaf-mould with its "moist rich 

 smell of the rotten leaves," as Tennyson notes. That which 

 has been built up from the soil is returned to it as decaying 

 matter, "dust to dust." This decay is associated with a cer- 

 tain amount of heat, as one sees in a fermenting manure 

 heap ; and, in fact, as much heat is given off as would be given 

 off if the decaying material were burned, but of course the 

 process is much slow^er. It is this decaying organic matter 

 that helps to make life possible in the soil. 



