CHAP, xiii.] ESSENTIAL CONSTITUENTS 249 



of value to the plant — the single manures, as we may 

 call them — and the compound manures which contain 

 two or more fertilising ingredients. We may again 

 divide these materials into nitrogenous, phosphoric, and 

 potassic manures, according to the elements which 

 predominate in them. The next most important factor 

 to be taken into account is the actual percentage of 

 fertilising material which the manure contains. With 

 certain exceptions to be dealt with later, i lb. of 

 nitrogen possesses the same value in whatever fertiliser 

 it may be contained, so that we must compare all 

 nitrogenous manures on the basis of the amount only 

 of nitrogen they contain. Thus, if sulphate of ammonia 

 contains 20 per cent, of nitrogen, and soot only 4 per 

 cent, then sulphate of ammonia is five times as valuable 

 as the soot, if we leave out of account certain other 

 considerations which may give an extra value to one 

 or other of these substances. In the case of sulphate of 

 ammonia, only the 20 per cent, of nitrogen possesses any 

 value to the soil or plant, the remaining four-fifths, 

 however indispensable to the constitution of the manure, 

 must be regarded as surplusage. It should not, however, 

 be considered that there is any necessary waste, though 

 the manure does only contain a proportion of the 

 pure fertilising constituent, for these bodies must be 

 combined to be of any use. Pure nitrogen gas, for 

 example, as we have already stated, exists in air in 

 enormous quantities, but is of no value to the plant 

 until it has been brought into combination. Pure 

 phosphoric acid, again, though it does exist and would 

 act as a fertiliser, is a very scarce material, of interest 

 only in the laboratory. Thus the surplusage which 

 may be present in a manure, in addition to its percentage 

 of nitrogen or phosphoric acid, must be regarded as the 

 vehicle necessary to carry these valuable constituents. 



