138 Physics of the Soil. 



least 2 per cent, of moisture yet available. The loamy 

 clay of (145), and given in the table, has about the same 

 texture as that of the reddish clay in the table of (161) and 

 it will be seen that its per cent, of moisture under the corn 

 was also about the same as that computed. 



164. Available Soil-Moisture Affected by Jointed Structure 

 in Clay Subsoils. The tendency of clay subsoils to shrink 

 and become divided into small cube-like blocks greatly di- 

 minishes the available moisture in them. This shrinkage 

 not only often results in breaking rootlets in two but when 

 new rootlets form they advance most readily through the 

 fissure planes and are not able to place themselves in the 

 most favorable relations with the soil to permit capillarity 

 to bring the moisture to the rootlets. It is because the 

 sandy soils and loams seldom develop the structure referred 

 to and because the rootlets and root hairs are able to secure 

 a more uniform distribution throughout them as well as 

 because of the larger size of their grains that plants are able 

 to drain their moisture down to so low a per cent. 



165. Available Soil-Moisture Increased by Open Structure. 

 When soils are in any way left with a loose open struc- 

 ture, as happens with deep plowing and especially with 

 good subsoiling, not only is the ability of the loose soil to 

 retain moisture increased but a larger proportion of this 

 retained water becomes available to the crop. A larger 

 amount of water is retained because when perfect capillary 

 connection with the unstirred soil below, is broken, surface 

 tension opposes rather than aids gravity in producing per- 

 colation and spaces too large to remain full of water other- 

 wise are able to retain it. 



When the soil is open and loose the case is quite different 

 from that resulting from shrinkage referred to in (164), 

 for in this case the roots and root hairs are better able to 

 enter the separated portions and, as the moisture films are 

 thicker, the moisture is more readily gathered. 



