PHYSIOGRAPHIC FACTORS 



173 



Several electrometric methods have been adapted to the meas- 

 urement of soil moisture. All require calibration in terms of the 

 wilting percentage of the soil involved but thereafter permit rapid 

 determinations at short or long intervals and direct translation of 

 measurements into available water. The method that seems to be 

 most in favor at present is the measurement of resistance between 

 two electrodes imbedded in gypsum blocks and buried in the 

 soil. 29 The resistance varies inversely with amount of soil water 

 and also with soil temperature. Other methods measure dielectric 

 constant, or electrical or thermal capacity of the soil, values that 

 vary with changes in soil moisture. 



Two physical measurements, making use of (1) tensiometers 207 

 and (2) soil point cones, 158 have been used successfully. 



A tensiometer measures the tension existing between the soil 

 and the soil water. It consists of a porous cup set in the soil and 

 connected to a manometer by a tube of small diameter. Water in 

 the instrument makes connection through the porous cup with the 

 soil water, from which equilibrium tension is transmitted to the 

 mercury of the manometer. Since the height of the mercury col- 

 umn indicates the tension in the soil, the manometer can be cali- 

 brated for a wide range of soil moisture values, and readings can 

 be taken at any time and translated directly into values for avail- 



TABLE 9— Percentage composition of oxygen and carbon dioxide in soil 

 air extracted at different depths in a silty loam soiK 30 ). Note that the percent- 

 age of 2 decreases and of CO2 increases with depth both winter and sum- 

 mer but that subsoil aeration is far better when the soil is dry in summer than 

 when it is wet in winter. 



