THE BIOLOGICAL CONDITIONS IN THE SOIL 219 



The main facts in regard to water supply are as follows : 



1. A sandy soil is liable to great fluctuations in moisture 

 content, easily becoming very wet or nearly dry. Movement 

 from underground sources of supply is rapid, but except where 

 it is gravitational, i.e. due to seepage from higher ground, 

 it does not appear to extend far. A chalk soil behaves 

 similarly. The fluctuations are reduced and the tendency 

 to become dry is considerably lowered when organic matter 

 or clay is added to the sand. 



2. A heavy clay soil fluctuates proportionately much less in 

 water content, and its maximum and minimum contents are 

 both high. Water travels only very slowly : it is common to 

 see the soil of a field cracking with drought almost up to the 

 edge of a stream. 



3. Feat soils like clay soils have a high maximum and a 

 high minimum water content. They show in an exaggerated 

 degree a tendency seen in all soils a difficulty in becoming 

 remoistened after the soil has become dry. 



These relationships are well shown in Table LVII. 



TABLE LVII. MOISTURE CONTENT l OF SANDY, LOAMY, AND CLAY SOILS AT 

 WOBURN, LYING NOT FAR APART, AND UNDER APPROXIMATELY EQUAL 

 RAINFALL CONDITIONS. RUSSELL. 



For biological purposes a better idea of the meaning of 

 these results is obtained by translating them into volumes. 

 The soil is a porous mass and a large part of it is not occu- 

 pied by solid matter at all but by air and water. Comparison 

 of the specific gravity of the soil particles determined by the 

 specific gravity bottle the so-called " true specific gravity " 



1 This determination is made by drying at 40 C, 



