EDAPHIC OR SOIL FACTORS: PHYSICAL 



215 



quotient in humus soils always gives quite corresponding values, 

 Crump views the resultant average value from several determinations 

 as the most important constant for plant communities on humus soil 

 whose occurrence and distribution are conditioned by soil moisture. 

 Various striking cdaphic facies of the oak woods in Yorkshire examined 

 in February ga\e the moisture coefficients shown in Table 21 on page 

 214. 



The following data seem to show that the season of the examination 

 has no particular effect upon the results: 



Table 22. 



-Coefficient of Soil Moisture in the Sesleria coerulea Turf on 

 Limestone 



Time of examination 



Water (air 



dried), per 



cent 



Humus, per 

 cent 



Coefficient of soil 

 moisture, per cent 



April 



April 



August . . . 



Average 



31.4 

 15.4 

 22.3 



37.7 

 17.0 

 19.0 



0.80 

 0.90 

 1.17 



0.96 



Wilting Point and Wilting Coefficient. — Vegetation responds to an 

 inadequate water supply by wilting. This means that, although the 

 plants still may be taking up water, the amount of moisture drawn 

 from the soil is not sufficient to cover the loss by transpiration. The 

 water which remains in the soil at permanent wilting marks the critical 

 point in the water supply that has been termed the "wilting coefficient" 

 by Briggs and Shantz (1912). With the same soil structure the wilting 

 coefficient is approximately the same for practically all plants. As 

 might be expected, however, it changes with the degree of division of 

 the soil particles and varies directly with the water capacity of the soil. 

 In densely packed fine-grained soils the wilting point is reached at a 

 higher water content than in loose sandy soils which retain water 

 poorly. In dune sand the wilting coefficient often lies below 1 per 

 cent; in clay loam, however, it reaches 17 to 20 per cent. 



In determining the v/ilting coefficient it is often difficult to tell when 

 a plant is permanently wilted. Briggs and Shantz define the condition 

 of permanent wilting thus: "Without the addition of water to the soil 

 the leaves are unable to recover turgidity even in a saturated 

 atmosphere." 



