ABSORPTION AND MOVEMENT OF WATER 105 



damp to the touch because of the considerable amount of 

 water held in the capillary state between the soil particles. 

 This can be removed by applying material possessing 

 stronger capillary attraction, for instance, blotting or fil- 

 ter paper, which will quickly become damp by withdrawing 

 water from the soil capillaries.* After soil has been dried 

 as thoroughly as possible by removing the capillary water 

 through stronger capillary attraction, water will still be 

 retained in the hygroscopic state, held on the surfaces of the 

 soil particles themselves. To overcome the attraction of the 

 soil particles and to remove the last traces of water much 

 greater force must be employed. In order to make soil 

 absolutely dry it must be taken away from its natural 

 position and exposed to some powerful dehydrating influ- 

 ence, e. g. concentrated sulphuric or glacial phosphoric acid- 

 in a desiccator, or, more simply, to heat in an open vessel. 

 Another means of demonstrating the very considerable force 

 by which soil particles attract and hold water and the sub- 

 stances in solution in it, is by filtering a dilute solution of 

 some convenient copper salt or of Fuchsin through soil. 

 The last trace of copper or of color will be removed from 

 the solution, and the filtrate may be successfully used for 

 the culture of plants, though the original copper solution 

 would have been poisonous. 



The amounts of water occurring in the hydrostatic, capil- 

 lary, and hygroscopic states in soils will vary with their 

 composition, with the fineness of the particles, and with their 

 compactness on the surf ace. f It is essential that plants 

 growing in coarse soils which drain rapidly, and in re- 

 gions where no rain falls during the growing season, 

 should be able to supply themselves with water even after 

 the hydrostatic and capillary waters have been entirely re- 

 moved from those layers of the soil traversed by the roots. 

 To accomplish this it is absolutely necessary that the plant 

 should be able to exert sufficient force to draw into its own 



* It must, of course, be noted that the cellulose walls of the capillaries 

 in filter paper also imbibe water. 



t See Year Book U. S. Dep't Agriculture, 1897, p. 129, report by M. 

 Whitney, and also other papers on Soils published by U. S. Dep't Agriculture. 



