PHYSIOGRAPHIC FACTORS 169 



When the maximum water holding capacity, the field capacity, 

 or capillary capacity, and the dry weight of an undisturbed sam- 

 ple are known, it is relatively simple to calculate pore volume, air 

 capacity, volume weight, and specific gravity of soil material. 162 



The moisture equivalent denotes the water content of soil that 

 has been subjected, usually for thirty minutes, to a centrifugal 

 force of one thousand times gravity in a soil centrifuge. Its deter- 

 mination is simple if equipment is available. Within limits, it bears 

 a constant relationship to certain other soil moisture values or, at 

 least, suggests what these values should be. Its ratio to field ca- 

 pacity is near unity, but the relationship is least constant with 

 coarse-textured soils. In many soils the moisture equivalent is 1.84 

 times as great as the water left in those soils when plants wilt. Un- 

 available water can, therefore, be approximated from the moisture 

 equivalent. The ratio of moisture content to moisture equivalent 

 (relative wetness) can be used to make comparisons between soils 

 or soil strata of different textures where moisture content alone 

 would mean little in terms of plants because of variation in avail- 

 ability. 



The permanent wilting percentage should be considered as the 

 moisture content of the soil at the time when the leaves of plants 

 growing in that soil first become permanently wilted. Because it 

 has not always been so considered, there have been various other 

 terms (wilting point, wilting coefficient, wilting percentage) ap- 

 plied to the concept, and not all investigations have produced the 

 same results. Briggs and Shantz 35 first emphasized the importance 

 of this soil moisture condition to plant growth and called it the 

 "wilting coefficient!' Their procedure was to grow seedlings in 

 glass tumblers of soil sealed with a mixture of paraffin and vase- 

 line. When the leaves wilted and did not recover overnight in a 

 moist chamber, the moisture content of the soil was determined 

 by oven drying at 105° C. and calculated as a percentage of the 

 dry weight. It is generally agreed that permanent wilting marks 

 the soil water content at which absorption becomes too slow to 

 replace water lost by transpiration. 



Briggs and Shantz came to the conclusion that soil texture alone 

 determines moisture content at which plants wilt permanently, 

 regardless of the species, their condition, or the environmental 



