54 THE WATER-SUPPLYING POWER OF THE SOIL 



either soil physicists or plant physiologists. The oiih^ published 

 measurements of this power, so far as we are aware, are a few given by 

 Livingston^ in his studies on the soil of Tumamoc Hill, at Tucson, 

 Ai'izona. This author allowed evaporation to proceed from prepared 

 soil surfaces and considered that when no superficial dry layer formed, 

 the soil was able to transmit moisture to the surface at least as fast as 

 it was lost through evaporation. The dishes of soil were weighed at 

 intervals and the rate of water loss was calculated for unit area of 

 the general soil surface. The soil employed was a clay, having a water- 

 retaining power of 52 per cent of its dry volume, unpacked, or 48 per 

 cent of its dry weight. Li\dngston reports that this soil, when its 

 water content amounted to 30 per cent of its volume, possessed a 

 water-supplying power of at least 0.0077 gram per hour per square 

 centimeter of surface. With a water content of 20 per cent of its 

 volume the same soil could not transmit moisture to the surface at a 

 rate as great as 0.0055 gram per hour per square centimeter. It thus 

 appears that by varying and controlling the evaporating power of the 

 air it should be possible to determine with some accuracy the water- 

 supplying power of different soils. But Livingston's procedure is of 

 little value when field conditions are to be studied, for the tests involve 

 the handling of the soil and the packing of it into dishes. 



What is needed, before physiological ecology can proceed quanti- 

 tatively to the study of soil-moisture relations, is some method by which 

 the water-supplying power of the soil may be approximated without 

 any disturbance of the soil itself or of plants rooted therein. This need 

 has been emphasized by the writer last mentioned, but the requisite 

 method has not yet been forthcoming. Obviously, the prime desider- 

 atum in this connection is some sort of water-absorbing surface, the 

 absorbing power of which does not seriously decrease as absorption 

 progresses, this surface being capable of being placed in the soil of field 

 or pot at any desired depth. In connection with such a surface there 

 must, of course, be some device by means of which its rate of absorption 

 may be determined from time to time. Such a surface might be buried 

 in the soil and left there for an indefinite period, thus allowing the 

 adjacent soil to assume its natural volume and condition of aggregation. 

 Thus the practically insuperable problem of artificial packing might be 

 avoided. 



The experiment of Whitney and Cameron^ and those of Livingston 

 (1906) with cane-sugar osmometers in the soil suggested to the 

 present writers the possibility of employing, as the required standard 

 water-absorbing surface, an osmotic membrane backed by a suitable 

 solution. This membrane, it seemed, might be appressed to the soil 



^Livingston, B. E., The relation of desert plants to soil moisture and to evaporation. Carnegie 



Inst. Wash. Pub. 50. Washington. 1906. Pages 30-7. 

 'Whitney, M., and F. K. Cameron, The chemistry of the soil as related to crop production. 



U. S. Dept. Agric, Bur. Soils Bull. 22. 1903. 



