66 BULLETIN 1059, U. S. DEPARTMENT OF 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 waiter 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 thai 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 character 

 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, hut 

 particularly at its initiation, upon a proper balance between tempera- 

 ture and moisture, especially as these are integrated in the condition 

 of the surface soil. 



In the last analysis all other environmental condition- react more 

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

 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 m grains 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 «.f the individual 

 plant and of the society. 



OSMOSIS AS A FACTOR IX WATER ABSORPTl< »\ . 



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 



