CHEMISTRY OF MANURES 175 



former are principally provided by the dung of animals 

 and birds, sewage from towns, meat, fish and vegetable 

 refuse, blood, fat and other slaughterhouse waste, horns 

 and hoofs, etc., bones, fossils, hair, shoddy, seaweed and 

 so forth. The inorganic (or artificials) are mostly chemical 

 by-products, the number of which increases from j^ear 

 to year, and they all come under the main headings, 

 nitrates, phosphates, sulphates, potash. These comprise 

 together all the foods that plants need out of the soil. 



One cannot in a short chapter Kke this deal with the 

 composition of all these manures, but I have endeavoured 

 to show that the chemistry of the plant and the chemistry 

 of manures are two very intimate things, and if we succeed 

 with our crops it is because the right food and only that 

 is available at the right time. In our little study of the 

 mechanism of a plant we saw that all the soluble salts 

 and other food must pass up the central artery. Now 

 the discrimination is not done by the roots. It is true 

 they wander about to find sustenance, but they simply 

 have to absorb whatever they find themselves in. In 

 other words, all sorts and conditions may come in at the 

 box ofiice, but the choice lies with the authorities up 

 the " winding stair." If a green crop wants nitrate, it 

 is no use for the Chairman of Superphosphates Ltd. to 

 come bursting in with an urgent message. The answer 

 simply is, " Can't see you, sit down and wait for five or 

 six months, and then we'll talk it over." 



It might be supposed that we could grow a crop as 

 many times as we liked on the same spot by just giving 

 it what it wanted in the way of manure, but whilst this 

 is not impossible, it is found best in practice to follow 

 the rotation principle. The following Uttle tables show 

 how in general this may be worked out, and although the 

 question of rotation is generally considered a matter for 

 the farmer only, I have always thought it could be 

 advantageously appHed by small allotment-holders to a 

 greater extent than was done during the war, especi- 



