26o ARTIFICIAL MANURES [chap. 



employed as cattle food, but sometimes they contain 

 substances injurious to stock, like the rape seed we 

 have just mentioned and the cake which is derived 

 from pressing castor-oil seeds, or they have become 

 damaged in some way and are only utilisable as manure. 

 Such materials contain, as a rule, about 5 per cent, of 

 nitrogen and comparatively small quantities of phos- 

 phoric acid and potash, but when cheap they form very 

 valuable manures, especially on light soils. Many 

 other industrial residues get occasionally employed as 

 fertilisers, in fact, anything of animal origin, like wool 

 and silk, fur, hair, etc., contains nitrogen, and is thereby 

 valuable as a fertiliser. Residues from the textile 

 factories dealing with wool and silk, and fur or feathers, 

 are sold in the United Kingdom under the general term 

 of shoddies ; they are extremely variable in composition, 

 ranging from 3 to 13 per cent, of nitrogen, and they are 

 always slow in their action, partly because the material it- 

 self is not readily attacked by bacteria, and partly because 

 of the difficulty of getting the material finely divided 

 and disseminated through the soil. But when they can 

 be bought cheaply, such residues form valuable fertilisers 

 for perennial crops. It should be kept in mind that 

 vegetable fabrics like cotton, linen, and jute contain no 

 nitrogen, so that their residues are valueless as manure. 

 In dealing with fertilisers it is necessary that the 

 farmer should bear in mind the very different action of 

 the three constituents upon the plant, for although all 

 three substances — nitrogen, phosphoric acid, and potash 

 — are equally necessary to the growth of the plant, as 

 we have seen when considering water cultures, yet they 

 possess very different functions in its development. 

 Nitrogen is mainly concerned with the vegetative 

 development of the plant, and increases the tendency to 

 form leaf and stem ; thus if a plant is given an excess 



