130 Physics of the Soil. 



jurious, such as black alkalies and possibly toxic principles 

 developed by the roots of plants or soil organisms or during 

 their decay. 



153. Capillary Water. It is in this condition or quantity 

 in the soil from which crops and soil organisms chiefly de- 

 rive their supply of water, and the right amount at all 

 times is therefore very important. It is in the capillary 

 water, too, that most of the plant foods derived from the 

 soil are held in solution and with it moved to the plants as 

 needed. When the texture of the soil is right the capillary 

 water simply surrounds the soil grains and soil granules 

 as a thin sheet which is continuous where the grains are 

 nearly or quite in contact, but there are always open spaces 

 through which the air may circulate and supply the needs 

 of roots and soil bacteria. 



If the soil is puddled and the granules broken down then 

 the surface films on the smaller soil grains come so nearly 

 in complete contact that there is insufficient room for air 

 to diffuse and plants cannot thrive in it. 



154. Hygroscopic Water Moisture in this form possibly 

 plays an important part in the actual solution of plant food 

 from the soil and fertilizer grains because it is this portion 

 which lies in immediate contact where the action must take 

 place ;4)ut if this is true it can only do its work rapidly 

 when capillary water is also present to carry away from 

 the dissolving surfaces the products which are being 

 formed. 



Polished surfaces do not as readily rust as those which 

 have become tarnished or otherwise roughened. When a 

 steel knife blade has become a little rusty the rusting then 

 goes on much more rapidly, possibly because each particle 

 of rust becomes invested with its film of hygroscopic mois- 

 ture, and when these lie against the fresh metal the water 

 can have a greater thickness and permit a more rapid move- 

 ment of the compounds formed, away from the corroding 

 surface. 



