68 ILLINOIS ACADEMY OF SCIENCE 



SOIL MOISTURE AND PLANT SUCCESSION 



GEORGE D. FULLER, UNIVERSITY OF CHICAGO 



The direct source of the water supply of plants being the 

 moisture in the soil, the amount of this moisture is evidently of 

 the highest importance to vegetation. This has been recognized 

 by many leading ecologists and phytogeographers, but very 

 little data have been made available as to effects of definite 

 quantitive amounts of this moisture upon the vegetation or 

 of the amount and range of the soil moisture in different plant 

 associations. This has been largely due to the difficulty in 

 relating the amount of soil moisture to the production of vege- 

 tation. It is clear that there can be no direct relation between 

 the percentage of water present in soils and the amount avail- 

 able for plant growth, for a sandy soil with 15 per cent of 

 moisture is at or near saturation, while a stiff clay with 15 per 

 cent of water is so dry that all plants wilt in it, even with a 

 humid atmosphere. 



Efforts have been made to establish a standard by which the 

 actual water content of soils could be related to plant produc- 

 tion. Clements 1 determined the amount of water remaining in 

 soils when pronounced wilting occurred, and regarding this as 

 non-available, termed it the e chard, while the difference be- 

 tween the amount actually present in the soil and the echard 

 was the available water, or chresard. Livingston recognized 

 that the water-holding capacity of soils varied and had a fairly 

 constant relation to the soil moisture conditions. Then Briggs 

 and McLane 3 determined the moisture equivalent of soils by 

 the application of a centrifugal force of 1000 times that of 

 gravity thus providing a method of measuring and comparing 

 the retentiveness of different soils for moisture acted upon by a 

 definite force. This had the advantage of being measured in 

 absolute terms, and of being reproducible within narrow limits 

 of error. It remained for Briggs and Shantz to refine the 

 methods of determining the percentage of water in soils when 

 permanent wilting occurs in such a plant as the standard 

 Kubanka wheat, giving the wilting coefficient, and further to 

 show that a constant relation existed between the moisture 

 equivalent and the wilting coefficient; that is, 



moisture equivalent=wi\t\ng coefficient 

 L84 



They also clearly demonstrated the fact that plants continue 

 to take water from the soil long after the wilting coefficient is 

 reached. The writer, believing that none of the water absorbed^ 



