388 Decomposition of Orgaiuc Matter in Soil 



activity sets in; bacterial numbers, COg, and nitrates all increase, the 

 curves agree so closely with those for temperature that we are justified 

 in regarding the temperature rise as the determining factor. The 

 increased activity is not always equally sustained, nor does it always 

 quite coincide with the rise in temperature ; occasionally it follows later. 



But this agreement soon ceases, and after a short period the activity 

 begins to fall off notwithstanding the continuance of the favourable 

 temperature. This is not a result of the sustained higher temperature 

 because it is not obtained in laboratory experiments where soils are 

 kept at different constant temperatures, all other conditions being 

 alike^. The result seems to be due to lack of moisture, because the 

 curves now begin to resemble the moisture curves. But the moisture 

 curve does not fit very well, and, therefore, it is not the only factor 

 concerned. The rainfall curve fits better. We conclude then, that 

 under favourable temperature conditions, rainfall becomes the domi- 

 nating factor, but that rainfall does something else besides supplying 

 water, and search was therefore made for this new factor. 



The beneficial effect of showers of rain has long been known, and 

 agriculturists have always felt that something more than a moisture 

 effect is concerned. Liebig considered that ammonia was present in 

 sufficient quantities to affect the plant growth, but the many analyses 

 which have been made dispose of this view. As we pointed out in our 

 previous paper^ our observations suggest that the dissolved oxygen of 

 the rain is the important factor, renewing the oxygen in the dissolved 

 atmosphere of the soil, and thus giving the organisms a new lease of 

 activity. This view is borne out by the circumstance that the effect 

 is most evident on dunged soils and on cropped soils where considerable 

 oxygen is used up ; on the poor unmanured soil the effect does not 

 show ; indeed it is rather reversed at some periods. 



The three factors, temperature, moisture, and the third, which we 

 beheve to be the supply of dissolved oxygen, fit the nitrate and COg 

 curves tolerably well over a large part of the year, and thus suffice to 

 account for most of the phenomena. But there still remain two periods 

 which are not thus fitted, and where, therefore, some other factor must 

 be operating ; a period of depression coming after the spring rise, and 

 a period of autumn activity coming after the summer sluggishness. 

 We must therefore see what further factors may be involved. 



^ See E. J. Russell and H. B. Hutchinson, this Journal, 1912, 5, 157 el seq. 

 ' This Journal, 1915, 7, 24, and this vol. p. 331. E. H. Richards in these laboratories 

 has since shown that rain is nearly a saturated solution of oxygen. 



