480 Agricultural Experiment Station, Ithaca, N. Y. 



How the soil liolds Us icater. 



That a proper understanding of the question may be reached, it 

 is necessary to have a knowledge of the conditions under which 

 water exists in the soil, and of the part it plays in the mysterious 

 operations of plant growth. Water may be in one of three forms, 

 — as free, capillary, or hygroscopic water. The free water of 

 the soil is that which flows under the influence of gravity. It 

 is the source of supply for wells and springs. It is not directly 

 used by plants and its presence in the soil within eighteen inches 

 of the surface is detrimental to the growth of most cultivated 

 crops. It is valuable, however, because it is the supply from 

 which capillary water is drawn. 



The capillary water does not flow by gravity. It is the direct 

 source of moisture for plants. It may be either drawn upwards 

 or it may pass downwards depending upon whether the soil is 

 drier at the surface or below. In time of droughts, the capillary 

 action of the soil may be sufiScient to raise the water through a 

 distance of five or six feet, its power in this respect depending 



directly upon its physical condition. If the 

 soil is coarse and cloddy and the particles are 

 not compact, then the water can not rise to 

 take the place of that which is carried off by 

 evaporation or used up by plants in their 

 growth. If, however, the soil is fine, in good 

 condition, and homogeneous, the water passes 

 ,,„ ,, ., , . freely and continuously to the surface. No- 



139.— The soil mulch. "^ *' 



tice the track of the horse on the plowed 

 ground, or the foot-print of the driver, and see how the moisture 

 comes directly to the surface, because the soil has been com- 

 pacted and there is intimate capillary relation between its parti- 

 cles. This moist surface shows that the water is passing off 

 from it into the air. This observation should teach a lesson. 

 The soil may be pulverized and made compact, but the capillary 

 pores near the surface must be enlarged by tillage so as to break 

 the capillary connections and stop the water in its upward 

 course, and thus force it to pass off through the tissues of the 

 plant. This loose surface stratum, two or three inches deep, is 

 the " soil mulch " (Fig. 139) of which so much has been said 



