220 SOIL CONDITIONS AND PLANT GROWTH 



with the specific gravity of the mass — called the apparent den- 

 sity — obtained by weighing a block of soil of which the volume 

 in situ is known, shows that the solid matter forms 50 to 65 

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

 matter increases the pore space in consequence of its " lighten- 

 ing" action (p. 130). This is shown in Table LVIII. 



Table LVIII.i — Pore Space, Water Content, and Air Content of Certain 

 Soils. Russell. 



In attempting to elucidate the phenomena of the water 

 supply of the soil the most significant fact is that the soil 

 has a certain, but not an indefinite, capacity for holding water. 

 Two consequences follow. The plant is never able to obtain 

 all the water present ; it wilts and dies when it has taken all 

 that its root cells can absorb, in spite of the fact that some 

 water is left behind in the soil. At the other extreme, a soil 

 is not able to hold more than a certain amount of water 

 against the force of gravity, and any excess rapidly drains 

 away. It does not appear that water once passed into the 



1 Driest periods of 1909 and 1910. During the abnormal drought of 191 1 

 the numbers fell to 6 and 8 for the first two soils. 



