34 SYMBIOSIS 



agricultural discovery subsequent to Liebig. For it turned out 

 that the soil presented all the problems of " population," though 

 but of " soil population." In this " soil population " there 

 obtains a wonderful " division of labour," and nothing is left to 

 accident. The higher plant, as has been said, indispensably 

 needs apart from other inorganic substances nitrates. How 

 are these provided ? How also are those organic substances 

 which are often so amply furnished to the plant by the soil after 

 it has been enriched by manure, to be re-converted into 

 inorganic matter so as to constitute the ideal food for the health 

 and the toil of the higher plant ? The reconversion, we are told, 

 is neither chemical nor physical. It is " biological." It repre- 

 sents labour performed by that important part of the " soil 

 community " which has long been entirely overlooked, but has 

 recently come into great prominence : the bacteria, the number 

 of which is enormous, running into millions per gram. " How 

 do these organisms live ? They must have food ; and they must 

 have energy ? " 



They thrive in part on the spare-capital of the higher plant 

 population with whom they stand in a symbiotic exchange 

 relation so far as their food and well-being are concerned. Nor 

 is the soil an^nert medium, but it plays a great part in the business 

 of crop-production. The recognition that the plant is a living 

 thing and that the type of soil is an important factor in crop 

 production, Dr. Russell tells us, has restored perspective and 

 broadened our conception of the factors necessary for plant growth. 



It has several times happened in the history of agricultural chemistry 

 (says Dr. Russell) that the new illuminating idea wanted to revivify the 

 subject in a stagnant period has come in from some outside technical 

 problem that had to be solved. 



So it was here. The growth of the towns and of stricter 

 ideas on public health had brought into prominence the need for 

 better sewage purification, and it was imperative that the 

 problem should be dealt with somehow or other. 



Schloessing and Miintz found that satisfactory purification 

 involved the conversion of ammonia into nitrate, and by a brilliant 

 investigation they found that this process was neither chemical 

 nor physical, but biological. Their work was extended to the 

 soil with remarkable results. It was seen that the soil was not 

 a mere inert mass, but that it was teeming with life and pulsating 

 with change. What I would specially urge in this connection 



