58 SOILS AND MANDEES 



Sources of Soil Moisture. The moisture of soils is 

 derived principally from the rain, snow and dew precipi- 

 tated upon them, but they also gain some by means of their 

 hygroscopic and deliquescent properties. Soils lose water 

 by percolation and evaporation, but some is retained and 

 capillary phenomena cause it to be transferred from one 

 place to another. The amount of water precipitated 

 depends wholly, and the amount evaporated partly, on the 

 climatic conditions. The gain and loss of water in other 

 ways depends chiefly upon the physical properties of the soil, 

 though, in a sense, they are affected by the climate too. 



Precipitation. In this country the mean annual rain- 

 fall varies from about 25 to 50 inches, but in some 

 localities it is much more and in others less. One inch 

 of rain is equal to about 4*7 gallons per square yard, or 

 over 100 tons per acre. Much of the water precipitated 

 in the form of snow never penetrates the soil at all ; it 

 melts at the surface and runs off. A much larger amount 

 of dew is formed on the herbage than on the soil itself ; 

 what does fall on the soil probably remains on the surface 

 and is soon evaporated. 



Hygroscopy. Dew is formed only in a saturated atmo- 

 sphere, but soils can also absorb water vapour from a non- 

 saturated atmosphere by hygroscopic and deliquescent 

 action. The former is a purely mechanical process, quite 

 independent of the nature of the soil or its constituents. 

 All gases exhibit a tendency to become denser when in 

 contact with solid surfaces, and are therefore absorbed 

 in large quantity by porous bodies which present a great 

 extent of surface in small volume. The bleaching of 

 organic colouring matters by charcoal and the ignition of 

 hydrogen jets by spongy platinum are familiar illustra- 

 tions of this phenomenon. Soils act in a similar manner, 

 especially those of finer texture. In virtue of their great 

 internal surface they possess in a high degree the power 



