INVESTIGATIONS ON THE PRODUCTION OF PLANT FOOD. 193 



past occupied a great amount of attention from agricultural chemists. 

 From time immemorial practical men have felt that rain had a fertil- 

 izing effect. Medieval writers attributed it to some aerial spirit or 

 celestial nitre washed down. Liebig, more precise, put it down to 

 ammonia. As a result ot Liebig's support a vast number of analyses 

 have been made of rain from all parts of the world, but all agree in 

 showing that there is not enough ammonia present to make any 

 practical difference. 



To what, then, are we to attribute this marked effect of rain ? In 

 soil investigations the direct attack is often least effective ; it is 

 usually necessary to work round the problem and see it from another 

 point of view. In this case help came from a rather unexpected 

 quarter. During the course of other soil investigations it was found 

 that soil particles possess two atmospheres : the free atmosphere 

 in the soil pore spaces, and another atmosphere dissolved in the soil 

 water or soil colloids. 



Table showing Composition and Volume of Soil Air and Atmosphere. 



Soil Air. 





Per cent. 



C0. 2 





N 2 



Free air ..... 

 Dissolved air .... 



98-0 



20-3 



79'3 

 20 



The Atmosphere. 



003 



20-97 



79-0 



The free atmosphere is very much like our own, except that it 

 contains more carbon dioxide ; it is eminently suitable for aerobic 

 organisms. The dissolved atmosphere, however, is entirely different ; 

 it has not been fully investigated, but it is known to be almost 

 devoid of oxygen and to consist mainly of carbon dioxide and 

 nitrogen. The fact that it exists in such close proximity to the free 

 atmosphere shows that the oxygen is used up more rapidly than it is 

 renewed, and this means that the plant roots and micro-organisms 

 which are immersed in the soil water are perpetually in need of more 

 oxygen. So far as we know there is no process in the soil that will 

 hurry up this renewal of dissolved oxygen, and plants and micro-organ- 

 isms alike are perpetually restricted by the lack of it. 



Now rain is a saturated solution of oxygen, and when it falls on the 

 soil it not only supplies the needful water but also renews the stock 

 of dissolved oxygen, and thus gives the micro-organisms and the 

 plant roots a new lease of activity. 



