BACTERIA AND ENZYMES IN AGRICULTURE 259 



physical character of the soil is, however, improved by its 

 presence. The resistant portions of fibre and straw tend to 

 make the soil more porous, and the humus which it contains 

 increases the power of the soil to retain water and ammonia 

 salts, and also improves the texture of the soil. 



After the manure is placed on the field the various amino 

 compounds will suffer further decomposition, yielding eventu- 

 ally ammonia. It is a matter of some uncertainty whether 

 ammonia is immediately available for plant food ; at any 

 rate, there is no doubt that nitrates are more readily 

 taken up by a plant, and, therefore, a prolonged retention of 

 nitrogen compounds in the soil, and their slow conversion into 

 ammonia, and finally into nitrate, is an advantage. As a 

 matter of fact, nitrification of the ammonia generally takes 

 place before it has been removed from the soil by the plant. 

 Moreover, the weight of the dry matter of the plant increases 

 per unit of nitrogen, supplied as nitrate. 



The conditions of nitrification of ammonia, whether 

 supplied as stable manure, or in the various forms of artificial 

 manure, especially sulphate of ammonia, have been con- 

 sidered from the laboratory point of view in Chapter XIII ; 

 the importance of the presence of humus has just been noted. 

 It will be further clear, upon consideration, that nitrification 

 will not take place satisfactorily in soils which have become 

 sour or acid ; an addition of lime is necessary in such cases. 

 The beneficial effect which unquestionably attends, in many 

 cases, the use of pressed sludge, which contains only small 

 quantities of nitrogen relatively speaking, is due to the* 

 presence of lime, which has been added to facilitate the opera- 

 tion of pressing. The physical effect of the admixture of such 

 material with the soil is of importance, and the presence of a 

 certain proportion of matter of the nature of humus is also 

 beneficial. 



The Loss and Recovery of Nitrogen in the Soil. It is 



32 



