66 BULLETIN 1059, U. S. DEPARTMENT OE AGRICULTURE. 



SOIL MOISTURE AND SOIL QUALITIES. 



The subject of soil moisture is closely related to that of precipita- 

 tion. Since the physical and chemical properties of the soil are 

 closely linked up with moisture, it seems logical to consider all of 

 these subjects together as a question of water supply, after which 

 atmospheric conditions which particularly affect water losses may 

 be taken up. 



There is practically no question that water is the prime requisite 

 of all life, for without water the colloids could not exist. It is 

 hardly more true of plants than of animals that, besides possessing 

 water at any given time, they must be almost continually given new 

 supplies to make up for unavoidable losses ; but, with the exception 

 of aquatic species, plants are more at the mercy of the moisture of 

 the habitat than are animals, because they can not move to new 

 supplies — the water must somehow be brought within their reach. 



Ecologically, it is perhaps unsafe to say that moisture has more 

 to do with the establishment, development, and succession of a plant 

 society than any other condition, that is, that it controls the char- 

 acter of the plant society more directly. It is perhaps nearer the 

 truth to say that when the temperature conditions are about optimum 

 for a given plant or society, moisture determines success or failure 

 almost absolutely. Yet this does not express the situation, for in a 

 vast majority of cases the plant society must depend at all stages, 

 but particularly at its initiation, upon a proper balance between 

 temperature and moisture, especially as these are integrated in the 

 condition of the surface soil. 



In the last analysis all other environmental conditions react more 

 or less on the soil moisture, and the best measure of their influence in 

 this respect is found in a measure of changes in the soil moisture. It 

 is readily seen, therefore, that the direct measurement of the soil 

 moisture is of the utmost importance. 



The moisture content of the soil, whether expressed in grams per 

 kilogram of soil or cubic centimeters per cubic meter of soil, does not 

 give directly a measure of the rate at which it may be obtained by the 

 plant, because of the great variation in the moisture-withholding 

 powers of soils. This rate is obviously very important whenever the 

 atmospheric conditions are such as to cause heavy loss from the leaves, 

 and may often determine success or failure of the individual plant 

 and of the society. 



Osmosis as a Factor in Water Absorption. 



The rate of absorption is unquestionably dependent upon the simple 

 physical process known as diffusion, which is commonly called 

 osmosis when speaking of plants, since the mixing of the two liquids 



