as it is formed, taking away its acid characteristics, and 

 producing a nitrate. If no base is present, the nitric acid 

 accumulates in the soil ; and as the organisms cannot 

 continue their action in the presence of any acid, nitrifica- 

 tion soon ceases. Almost invariably it is carbonate of lime 

 which acts in this way nitrate of lime being formed ; but 

 other bases, such as potash, are also capable of combining 

 with the acid. 



The requirements for nitrification may, therefore, be 

 summarized as 



i. The presence of the proper organisms. 



2. Nitrogen, either in the form of ammonia or as 

 organic matter. 



3. Organic matter. 



4. Oxygen. 



5. Moisture. 



6. A proper degree of warmth. 



7. Carbonate of lime or some other base, 



CONDITIONS PREVENTING NITRIFICATION. 

 The conditions adverse to nitrification which are most 

 frequently met with are 



i. Absence of oxygen. 



2. Absence or deficiency of carbonate of lime. 



3. Presence of acid in the soil. 



The absence of oxygen from the soil may be due to the 

 naturally close texture of the latter, as in the case of heavy 

 clays, to imperfect cultivation in the case of arable land, 

 or to the fact of the soil being water-logged, either from its 

 impervious nature or from want of drainage. Whatever 

 the cause, nitrification will cease ; and under these cir- 

 cumstances an opposite action denitrification may take 

 place, in which, under the influence of micro-organisms 

 which are only active in the absence of oxygen, the nitrates 

 present in the soil are destroyed and the nitrogen lost to 

 the air in a gaseous form. 



The absence of carbonate of lime, though not common, 

 is sometimes met with. The obvious remedy is the addi- 

 tion of some form of lime to the land ; lime, chalk, marl, and 

 shell sand being all suitable for the purpose. 



The presence of acid in the soil is usually connected with 

 one or both of the two foregoing points. When plant refuse 

 or other organic matter decays, various acids are formed 

 (carbonic, humic, &c.). If there is a plentiful supply of 

 carbonate of lime, most of these acids will be neutralized 

 that is, will have their acid characteristics removed by 

 combining with it, just as we have already seen in the 

 case of nitric acid. Even if carbonate of lime is deficient, 

 these acids may still be washed out of the soil if the 

 drainage is good. On the other hand, if the acids accu- 

 mulate, the land becomes sour and nitrification ceases. 



B 



