22 



PAPERS PUBLISHED. 



SOIL PROBLEMS. 



I. " The Atmosphere of the Soil : its Composition and the Causes 

 of Variation^ E. J. Russell and A. Appleyard. Journal 

 of Agricultural Science, 1915. 7, 1-48. 



The free air in the pores of the soil to a depth of six inches is very 

 similar in composition to the atmospheric air, but it differs in two 

 respects : 



(a) It contains more carbon dioxide and correspondingly less 

 oxygen, the average in 100 volumes being 0.25 volumes of carbon 

 dioxide and 20.6 of oxygen against 0.03 volumes of carbon dioxide and 

 20.96 of oxygen in atmospheric air. 



(b) It shows greater fluctuation in composition. 



Usually the sum of the carbon dioxide and oxygen is only slightly 

 less than in atmospheric air, but at periods when nitrates rapidly increase 

 there is a perceptible falling off of oxygen, and a still greater one in 

 waterlogged soils. 



Besides this free air there is another atmosphere dissolved in the 

 water and colloids of the soil. This consists mainly of carbon dioxide 

 and nitrogen and contains practically no oxygen. 



The fluctuations in composition of the free soil air are mainly due 

 to fluctuations in the rate of biochemical change in the soil, the curves 

 being similar to those showing the amount of nitrate and the bacterial 

 counts as far as they were taken. The rate of biochemical activity 

 attains a maximum value in late spring and again in autumn, and 

 minimum values in summer and winter. In autumn the bacteria 

 increase first, then the carbon dioxide rises, and finally the nitrate 

 increases. 



From November to May the curves closely follow those for the soil 

 temperature which thus appears to be the dominating factor ; from 

 May to November they follow the rainfall and to a less extent the soil 

 moisture curves. The difference between rainfall and soil moisture 

 indicates that rainfall does something more than add water to the 

 soil. It is shown that the dissolved oxygen brought in is probably a 

 factor of considerable importance in renewing the dissolved soil atmos- 

 phere and facilitating biochemical change. 



Grass land usually contains more carbon dioxide and less oxygen 

 than arable land, but we cannot attribute the difference wholly to the 

 crop owing to the large differences in soil composition and conditions. 

 It is difficult to ascertain the precise effect of a crop, but as the soil 

 differences are eliminated so the differences in composition of the soil 

 air becomes less and less. No evidence could be obtained that the 

 growing crop markedly increases the amount of carbon dioxide in the 

 soil air ; if it gives rise to any great evolution of carbon dioxide in 

 the soil it apparently exercises a correspondingly depressing effect on 

 the activities of soil bacteria. This result agrees with one obtained 

 earlier in reference to the nitrates in the soil. 



Such weather conditions as barometric pressure, wind-velocity, 

 variations in temperature from the mean, smnll m in fall, etc., seem to 

 have but little effect on the soil atmosphen 



