673 



Oxygen is necessary t<> the development of these micro < 

 If the air is excluded they are certain to die. They can only 

 develop in a soil that has a good supply of air. This will show the 

 necessity ol a good tillage. The better the soil is tilled the more freely 

 will the air have access to it. Moisture in the soil is necessary, 

 and absence of light. Xitrification takes place best at <>u K.. 

 and diminishes from that down to 40" K. and up to i }o" K. Certain 

 substances are poisonous to these micro-organisms ; common salt, 

 for instance. 



There are other bacteria that act in quite the reverse way to the 

 nitrifying micro-organisms by decomposing the nitrates in the soil. 

 These bacteria are only active when air is excluded from the soil, 

 or when the air is limited in the soil and it contains large quantities 

 of organic matter which uses all the oxygen for its oxidation. 

 These conditions will be found in water-logged, peaty, heavy and 

 badly tilled soils. 



There is a third class of bacteria, which are only active in the 

 presence of leguminous plants. They iix themselves on the roots 

 of these plants, and by their action, combined with that of the 

 plant itself, are able to convert the nitrogen of the atmosphere into 

 a state in which the plant is able to assimilate it. The conditions 

 necessary are much the same as for nitrification. No other plants 

 have this power except those of the leguminous order. It is for 

 this reason that leguminous plants enrich the soil with nitrogen. 



Nitrification usually takes place near the surface ; at least, very 

 little takes place at a depth below 18 inches. The depth to which 

 it takes place will greatly depend on the openness of the soil. 

 Roots assist greatly in allowing nitrification to take place at a greater 

 depth by allowing free access of air to the depth they go down. 

 Drainage has the same effect by opening channels for the passage 

 of the air to a greater depth and removing any excess of water 

 from the soil, which would otherwise prevent the free passage of 

 air through it. 



Ammonia is the easiest of all the nitrogenous compounds to con- 

 vert into nitric acid through the agency of these micro-organisms. 

 The earthworms, when present in great numbers, play a very import- 

 ant part in the amelioration of a soil. Feeding on the organic 

 matter of the soil, in order to secure a sufficiency of food they have 

 to pass a very large quantity of earth through the intestinal canal, 

 which they deposit at the mouth of their burrows as a fine earth or 

 casting. These castings contain a very large quantity of organic 

 matter and other plant food readily available for the plants, so that 

 the surface of the soil is enriched by fertilisers brought from a depth 

 by the earthworms. The organic and mineral matter in the castings 

 are thoroughly mixed together in such a state that each of the 

 elements of plant food are easily obtained by th'i plants. By 

 means of the burrows which the earthworms make in the soil, water 

 can penetrate to a much lower depth, and air can penetrate more 



