EDAPHIC OR SOIL FACTORS: PHYSICAL 213 



Determinations of soil moisture should be made at intervals of a 

 week throughout the entire vegetative season and plotted graphically. 

 Their ecological significance is much greater when records for a series 

 of consecutive years are available. 



Water Capacity. — The determination of the water capacity of the 

 soil is somewhat more complex than the measurement of the actual 

 water content, but it is more significant ecologically. The water 

 capacity is the amount of water which a soil is capable of retaining 

 against gravity (Ramann, 1911; Hilgard, 1914). Methods for such 

 determinations are described by Ramann, Hilgard, Siegrist (1930), and 

 others. Burger (1922, p. 47) gives the following directions: 



With a 1,000-cc. steel measuring cylinder a sample of undisturbed mature soil 

 is carefully removed and saturated in the water bath for 24 hr. After saturation, 

 any soil protruding over the mouth of the cylinder, due to expansion, is leveled 

 off and the sample is weighed after excess water has been permitted to drip off for 

 2 hr. Then the sample is broken in pieces and dried at 100° to 120°C. until a 

 constant weight is reached. The difference in weight represents the water content 

 of the saturated soil; 



_ a er con en ^ ^^^ _ ^^^^j. capacity percentage by weight; 



The best expression for the water-holding power of a soil is given by 

 the statement of water capacity in percentage of volume. 



The water capacity is not identical with the pore volume of the 

 mature soil. The larger pores contain gravitational water which is not 

 included in the determination of water capacity. 



Interesting comparisons between the pore volume and the water 

 and air capacity of the soil of different dry grasslands of Bohemia are 

 given by Khka (1929) . The water capacity of the soil of the Festucion 

 vallesiacae is always high, hardly ever falling below 50 per cent, while 

 the air capacity is low. 



Water capacity is a function of grain size and structure, on the one 

 hand; and of the expansible substances (humus, clay) present, on the 

 other. Humus soils have an extraordinarily high water capacity. 

 From Nitzsch's (1925) investigations it appears that water capacity 

 increases with increasing pore volume, i.e., with increasing looseness of 

 the soil, up to a certain maximum determined by local conditions. 

 Beyond this maximum it decreases very noticeably. 



While the available water actually present in the soil is dependent 

 upon precipitation and evaporation, and therefore subject to con- 

 siderable fluctuations, the water capacities of the various soil layers of a 

 plant community are very constant. Their values may usually be 

 established rather accurately by a single examination. In a dry 



