271 



vated plants is concerned. If now, for various reasons, as, for example, 

 specific root secretions, certain species of bacteria, which are attracted to 

 any definite plant variety and incited to great increase, carry over various 

 nutritive substances, primarily, nitrogen, in a form unfavorable for the culti- 

 vated plants, it can happen that chemically the supply of nutritive substances 

 may be sufficient, perhaps even abundant, and yet the product may fall off. 

 We then face the phenomena of soil exhaustion or "fatigue." Hiltner men- 

 tions experiments in reference to this. He perceived definite indications of 

 soil exhaustion in the third generation of peas, which during a period of 

 three years were grown seven times in pots in the same soil, but differently 

 fertilized. "The plants became sick, were easily susceptible to attack, turned 

 yellow prematurely and gave poor seeds." In the later generations, the dis- 

 eased conditions were overcome in this experiment. "The roots of the pea 

 plants were now noticeably browned, but were perfectly white and healthy 

 inside, and it could be proved that a regular bacteriorhiza was present, which, 

 formed by well-adjusted, beneficial bacteria, prevented the further penetra- 

 tion of the injurious organisms."^ 



In regard to the exhaustion of the grape, Behrens (loc. cit., p. no) cites 

 the observations of A. Koch, according to which it couldbe produced by an 

 accumulation of injurious micro-organisms. After sterilizing the diseased 

 soil (not the healthy soil), the growth of the vines improved. 



If such a change in the composition of the bacterial flora takes place in 

 a direction injurious to cultivation, it explains the increase of soil exhaustion 

 due to the repeated growth of the same plant on any given piece of land, 

 with short intermissions. And this accumulation of destructive elements is 

 of importance not only for the bacteria, but also for other vegetable and 

 animal enemies which can cause soil exhaustion. 



Among the bacteria which accumulate in the soil with repeated culti- 

 vation of the Leguminoseae, Hiltner found that the pectin fermenting organ- 

 isms became active. He found that in soil greatly exhausted by peas, per- 

 fectly healthy pea seed rotted especially because of these bacteria known as 

 acid formers. 



Another variation in the normal work of soil bacteria is the turning the 

 fertiliser to peat. In heavy soils, often after some years, the fertilizer has 

 been found pretty much undecomposed. In the same way green manure 

 turned under too deep, turns to peat. As a result of the limited supply of 

 air, the formation of raw humus is completed. The end and aim of working 

 the soil, however, is the production of a suitable humus covering, for by 

 the humus we obtain an equalization of the extremes of heat and cold, mois- 

 ture and drought and the suitable nutritive soil which alone makes the exis- 

 tence of most bacteria possible. If this is present, field soil can develop its 

 actual life, which, to a certain degree, is measurable by the production of car- 

 bon dioxid. How the bacteria co-operate in this, is shown by some statements 



1 Bodenpflege und Pflanzenbau. Arb. d. D. Landwirtsch.-Ges. Part 98, p. 74. 



