CONCLUSION. 



The experiments with which the preceding pages have had to deal 

 have shown that collodion membranes are suitable for the making of 

 osmotic cells to be used in the study of the water-supplying power of 

 soils. The thistle-tube osmometers here used have served their purpose 

 well, though many improving modifications might be suggested, and 

 the instrument, even in its present form, promises to be very useful 

 in the study of the neglected problems of the dynamic water-relation 

 between plant and soil. 



Our expermients have had to do with only one soil, aside from 

 various water contents and degrees of packing, this soil being rather 

 Hght (3 parts of sand and 1 part of clay loam, by dry volume). There 

 seems to be no reason for doubt that the method here used may be 

 found adapted to similar studies with other soils. 



Packing is here shown to be of prime importance in determining 

 the water-supplying power of the soil, and the lack of suitable methods 

 for obtaining strictly comparable packing of a number of soil samples 

 is one of the main obstacles to a rapid investigation of the field thus 

 opened. There is reason to hope, however, that the requisite methods 

 may soon be devised. In default of a better one, the mechanical 

 method of packing employed by Cameron and Gallagher may be 

 found advantageous, at least in the early stages of this study. 



The importance of the degree of packing here emphasized brings 

 again to the front the old question regarding the most satisfactory 

 basis upon which to calculate soil-moisture content, in ecological and 

 agricultural inquiries. It is once more made clear that it is the per- 

 centage of contained soil moisture on the basis of actual soil volume, 

 and not at all this percentage calculated on dry weight of the soil, 

 which plays an important part in determining the efficiency of the 

 soil as a source of water supply to growing plants. This point has 

 received emphasis from many writers on soil physics, but few seem 

 yet to have sought to take cognizance of it in actual studies. Weight 

 percentages should soon vanish from serious consideration in regard 

 to soil-moisture relations. 



After all, however, it is not the soil-moisture content (even when 

 measured on the basis of actual volume) nor the data of physical 

 analysis, that are directly effective as the subterranean water-relation 

 controlling plant activities. The effective condition is the complex soil 

 property, water-supplying power, which is itself a function of the nature, 

 size, and arrangement of the solid particles and of the water content 

 per unit volume. While the analysis of this water-supplying power 

 into its component elements is very diflEicult, the whole maze of prob- 

 lems entailed by such analysis may be avoided by studying directly the 



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