THE BIOLOGICAL CONDITIONS IN THE SOIL 103 



considerable time, often over the interval between one shower and an- 

 other. In sandy soils the pores are larger and the water movements 

 more rapid. 



3. The soil colloids absorb water without, however, holding it very 

 strongly. It loses all power of movement and no longer forms an 

 actual free film, but it still appears to be available for the use of micro- 

 organisms and plant roots. 



These facts explain the law empirically set out many years ago by 

 Schiibler (253) that the moisture content of a soil is a function of its 

 structure. A sandy soil soon becomes wet, but dries again rapidly. 

 Its large pores allow rapid percolation of the free water ; its relatively 

 small total surface (a consequence of the large size of its particles) holds 

 a proportionately small amount of water ; it possesses but little col- 

 loidal material to absorb and retain water. Addition of easily de- 

 composable organic matter increases the amount of colloid and thus 

 increases the water-holding capacity ; addition of clay increases the 

 colloids and the total surface, and also partially blocks up the pores, 

 the two last effects being due to the smallness of the clay particles. 

 Under equal conditions of water supply, clay soils and soils rich in 

 organic matter are, therefore, much moister than sandy soils : illustra- 

 tions are given in the Table on p. 6^ and in Table XLII. 



Table XLII. — Moisture Content of Sandy, Loamy, and Clay Soils at Woburn 

 Lying not Far Apart and Under Approximately Equal Rainfall Conditions. 

 Russell.* 



We can gain a better idea of the meaning of these results by trans- 

 lating them into volumes. The soil is a porous mass, and a large part 

 of it is not occupied by solid matter at all but by air and water. Com- 

 parison of the true specific gravity determined by the specific gravity 

 bottle with the apparent density obtained by weighing a block of soil 

 of which the in situ volume is known, shows that the solid matter forms 

 50 to 65 per cent, leaving Jo to 35 per cent, of pore space. Organic 

 matter increases the pore space in consequence of its " lightening " 

 action (p. 66). 



' The determination is made by drying at 40° C. 



