114 JOURNAL OF FORESTRY 



in the light soil a very much smaller difference in moisture content is 

 required for a given difference in force. 



One of the most interesting parts of the paper is the comparison 

 betwen the wilting coefficient and moisture intake by Xanthium seeds 

 for nine different soils. These soils ranged from coarse sand with 

 a wilting coefficient of only .73 to clay loam with a wilting coefficient of 

 16.34. Except for discrepancies in the case of the coarse sands, the 

 results are remarkably uniform. The average intake, excluding the 

 two coarse sands, is approximately 49 per cent, which agrees with the 

 intake from the Oswego silt loam and the fine sand at their wilting 

 coefficients. "This probably means," says Shull, "that the wilting 

 coefficient represents a fairly definite water-holding power for the 

 soil particle, regardless of its size." Comparing the results for all the 

 soils with the osmotic pressure in atmospheres for Xanthium seeds 

 (see table quoted above), it is found that the "back-pull" of the soil, 

 or the force with which it withholds water from seeds and plant roots, 

 at the wilting coefficient is from 3 to 4 atmo'spheres. This Shull con- 

 siders surprisingly low. 



In addition to the Xanthium seeds, Shull has tried to use seeds 

 which do not have semi-permeable seed coats. Since osmotic solu- 

 tions are useless for these seeds, he used a vapor pressure method, 

 which consists essentially in measuring the vapor pressure equilibrium 

 of air-dry seeds suspended over sulphuric acid of varying strength, 

 and calculating the internal pressure of the seed from the vapor 

 pressure of the solution over which it is found to be in equilibrium. 

 The calculations are near enough to the osmotic determinations to be 

 of considerable interest. But, owing to the insufficiency of our knowl- 

 edge of concentrated solutions and of the exact relations of colloids 

 to water vapor, these calculations give only a rough estimate of the 

 internal forces of the seeds and consequently were not used in measur- 

 ing the water-holding power of the soil. 



This piece of research has shown that the force with which the 

 soil holds water at the moisture equivalent is equal to about 1 atmos- 

 phere. The force at the wilting coefficient is from 3 to 4 atmospheres, 

 and when the water is reduced below the wilting coefficient this force 

 increases rapidly until at the air-dry condition it is about 1,000 

 atmospheres. 



The next step is to discover the relations between the roots of the 

 plant and the soil moisture. Shull's work throws more Hght on this 

 problem than any investigation since Briggs' and Shantz's publica- 



