22 REPORT OF THE AGRICULTURAL RESEARCH INSTITUTE 



oxygen could hardly be accounted for by that which was 

 present in the soil. Other experiments showed that this 

 soil utilizes under such conditions even much more oxygen 

 than that required for the nitrate formation. On the other 

 hand analyses of the gases in 1914 showed that despite the 

 large amount of oxygen required, the percentage of this 

 element in the soil was nearly normal. It became evident 

 therefore that a sufficient supply of oxygen must have been 

 diffusing into the soil from the atmosphere. 



The process of diffusion of gases through soils has 

 been generally believed to be a very slow one. Direct 

 experimental evidence on the subject is limited to that of 

 E. Buckingham * who concluded that there was a direct 

 relation between the " porosity " of a soil and the rate at 

 which gases could diffuse through it. The " porosity " he 

 defined as that fraction of the whole volume of soil which 

 is occupied by gases. He defined any particular portion of 

 soil as unity and the " porosity " is therefore less than 

 unity. Soils commonly include as gas some 20 to 40 per 

 cent, of their volume, and the " porosity " in such cases 

 varies from 0-2 to 0-4. Buckingham considered that the 

 equation : 



D = K S 2 



was sufficiently accurate to define the quantity of a gas 

 diffusing through a known column, (or depth) of soil, under 

 known gradient; and that this quantity is less than would 

 diffuse if no soil were present by the square of the porosity. 

 The results of such experimental work should, if possible, 

 be checked by independent evidence, but the difficulty is to 

 provide such independent evidence. 



Buckingham's equation was applied to several of the 

 cases which have formed the subject of the work on soil 

 gases at Pusa, and in some respects at least it is supported. 

 For example where green manure was applied during the 

 monsoon, the calculation showed that 0-0658 c. dm. of C0 2 

 was leaving the soil per sq. dm. per day during the first 10 



1 Contributions to our knowledge of the aeration of soils by E. Buckingham ; 

 U. S. Deft. Agri. Bur. soils, Bull. no. 25, Washington, 1904. 



