358 A TEXTBOOK OF THEORETICAL BOTANY 



Many of these substances are formed by soil organisms, and it is probable 

 that they play an important part in the bacteriology of the soil. Indeed 

 it is possible that competition between higher plants in nature is not 

 unconnected with the operation of similar excretions. 



Bacteria in the Soil 



The soil is the natural home of myriads of Bacteria, and some of the 

 chemical processes which they carry out are responsible for maintaining 

 soil fertility and are therefore of the greatest importance to us. 



It was proved by Boussingault in 1858 that plants are unable to absorb 

 Nitrogen directly from the atmosphere, of which it composes about four- 

 fifths, but that it is absorbed in the form of nitrates from the soil. Nitrates, 

 being very soluble, are easily washed out of the soil by rain and so lost, 

 passing eventually into the sea. This means that fresh supplies of nitrates 

 must be continuously produced if the soil is not to become barren. 



These nitrates are obtained in two distinct w^ays. Firstly, by the decom- 

 position of the proteins of dead organisms to yield ammonia. Very many 

 different kinds of saprophytic Bacteria and Fungi take part in this operation. 

 Secondly, by the fixation of Nitrogen from the air, which also produces 

 ammonia. This is the work of two Bacteria, both endowed with this peculiar 



power. One is a rounded, coc- 

 cus-like organism called Azoto- 

 bacter chroococcum (Fig. 345), the 

 other is a spore-bearing anaerobe, 

 Clostridium pastorianum (Fig. 

 li| w m 0- -<d|fc 346), which finds suitable con- 



ditions in the soil humus. 



The result of both processes 

 is to produce ammonia. The 

 ammonia is oxidized by a third 

 bacterium, Nitrosomonas euro- 

 paea, to nitrite, while this in 



^, „ turn is oxidized to nitrate by 



Fig. 345. — Azotobacter chroococcum. Cells . j ; • ah 



stained to show the mucilaginous capsule. Nitrobacter wmogradskyi. All 



these processes are exothermic 

 and the organisms live on the energy got from them, as explained 

 below. 



There is an opposing process of de-nitrification also at work, however, 

 due to a number of Bacteria which have the power of reducing nitrates, 

 releasing elemental Nitrogen. In healthy soils the process is comparatively 

 slight, but in the sea it operates on a gigantic scale, returning Nitrogen to the 

 atmosphere. 



The supply of nitrates thus depends on a chain of bacterial reactions 

 and is subject to complex fluctuations as the result of environmental con- 

 ditions, especially aeration and acidity. In acid soils, such as peats, it is 



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