XII.] ORIGIN OF MANURES 225 



and the phosphoric acid than animals which are stationary 

 in weight, while animals in the last stages of fattening 

 hardly retain anything at all. It is because the animal 

 thus keeps back so little of the plant food which was, in 

 the first place, taken from the soil for the production 

 of the vegetable feeding stuff, that the excrements of 

 animals has always been regarded as the most valuable 

 of fertilisers since any settled agriculture began. In 

 fact, it is only comparatively recently that any other 

 fertiliser has been known, for though various industrial 

 residues, such as woollen rags, clippings of hoofs and 

 horns, bones, and malt dust have been utilised as 

 manures for the last two or three hundred years, their 

 efficacy was very limited, and the business of crop- 

 raising centred round the proper use of farmyard 

 manure. The great range of artificial manures now 

 available, which are either derived from industrial pro- 

 cesses or represent the accumulated fertility of some 

 other country, have all come into use since about 1840. 



Farmyard manure consists essentially of the excreta 

 of the various animals, horses, cattle, and pigs kept in 

 the farmyards, mixed with the litter — straw or other- 

 wise — which is used to absorb the urine and keep the 

 animals clean. Very slight consideration shows us, 

 however, that in the farmyard manure that goes on the 

 land we are dealing with a very different product from 

 the fresh mixture of straw and excreta. The mixture, 

 in fact, undergoes great changes during the time it is 

 under the feet of the animals, and these changes are 

 mainly brought about by bacteria. The first change 

 taking place while the manure is being " made," as a 

 farmer would say, is continued to a greater or less 

 extent when the manure is afterwards removed from the 

 yards or the boxes and made up into heaps. At first 

 the straw of the manure shows little alteration, but 



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