256 SOILS AND MANUKES 



impurity ; (2) Any non-volatile substance which remains 

 when the substance is heated to redness is also impurity. 

 If the proportion of insoluble or non-volatile matter 

 appears to be unusually large, it is to be regarded with 

 suspicion. The only reliable test, however, is the quan- 

 titative determination of the ammonia. This test can 

 only be carried out in a properly equipped laboratory, 

 but it should never be neglected, as it is an insurance, 

 not only against intentional fraud, but also against the 

 accidental presence of any undue quantity of impurity. 



Manurial value. The manurial effects of sulphate of 

 ammonia depend upon its concentrated character, solu- 

 bility, and capacity for nitrification. 



With the exception of ammonium chloride, which is 

 rarely or never used, and the recently introduced nitrolim, 

 it is the most highly concentrated of all nitrogenous 

 manures, i.e., it contains the largest proportion of nitro- 

 gen. Other substances, e.g., dried blood, meat meals, 

 etc., may contain as much as 16 per cent, of nitrogen 

 they usually contain less but sulphate of ammonia con- 

 tains about 20 per cent. Small quantities may therefore 

 be expected to produce relatively large effects, and ex- 

 perience shows that such is the case. 



When applied to the soil it rapidly undergoes nitrifi- 

 cation. The ammonia is thereby converted into nitric 

 acid, which is the most suitable form of nitrogen for the 

 nourishment of plants. The presence of a certain amount 

 of lime in the soil is, therefore, essential for its proper 

 action. The lime is required both to effect the liberation 

 of the ammonia from the sulphuric acid, and to maintain 

 the conditions in the soil under which alone the nitrifying 

 organisms can act. 



On soils which are deficient in lime, sulphate of 

 ammonia is said to be positively injurious. On the other 

 hand, if the proportion of lime is excessive, as in the case 



