198 THE CHEMISTRY OF GARDEN CROPS 



however, is of an evanescent character, and rapidly flies away in the 

 form of carbonate of ammonia ; hence, a heap of farmyard manure, 

 left for several years, loses much of its value as manure, and guano 

 should be kept in bulk as long as possible, and protected from the 

 atmosphere, or its ammonia will disappear. One difficulty experienced 

 by chemists and others in preparing artificial manures is that of 

 'fixing' the needful ammonia, so that it may be kept from com- 

 bining with the atmosphere, and at the same time be always in a 

 state in which it can be appropriated by the plant. We cannot 

 supply plants with nitrogen directly, but in all good manures there is 

 a certain proportion of it in combination, and in many instances the 

 percentage of nitrogen is made the test of the value of a manure. 



The importance of humus the black earthy substance resulting 

 from the decay of vegetation in a soil is that it contains in an 

 assimilable form many of the ingredients essential to plant life. 

 Humus is also highly charged with carbonic acid, which decomposes 

 the crude minerals in the soil and renders them available as plant 

 food. When vegetable refuse is burned, the nitrogen one of the 

 costliest manures is dissipated and lost. But by burying the refuse 

 the soil gets back a proportion of the organic nitrogen it surrendered 

 and something over in the way of soluble phosphatic and potassic 

 salts ; and as this organic nitrogen assumes the form of nitric acid, 

 it is assimilated by the growing plant to the great benefit of whatever 

 crop may occupy the ground. 



The practical conclusion is that in the treatment of the soil a 

 skilful gardener will endeavour to promote its fertility by affording 

 the natural influences of rain, frost and sun full opportunity of libe- 

 rating the constituents that are locked up in the staple ; by restoring 

 in the form of refuse as much as possible of what the soil has parted 

 with in vegetation ; and by the addition of such fertilising agents as 

 are adapted to rectify the natural deficiencies of the soil. Thus 

 instead of following a process of exhaustion, the resources of the 

 garden may be annually augmented. 



