CHAPTER VI 



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FARM MANURE 



For a soil to possess fertility, that is, to be able to properly 

 ;support the growth of plants, certain conditions are necessary. 

 The following may be mentioned as being perhaps the most im- 

 portant. 



(1) Its mechanical or physical condition must be suitable. 



(2) It must contain sufficient plant food in a form which is 

 readily available to the crop. 



(3) It must not contain any appreciable quantity of poisonous 

 or injurious substances. 



(4) It must not contain injurious insects, fungi or other or- 

 ganisms which are destructive to crops. 



(5) The temperature, sunshine, rainfall and other climatic 

 conditions must be suitable. 



Of these the second and -third and to some extent the first, are 

 matters in which chemistry may be of service. 



Every crop removed from the soil robs it of materials which 

 have been used in building up the plant's tissues. Soil which 

 annually bears a crop must in time become exhausted of its store 

 of plant food and unfitted to bear further crops. Often one con- 

 stituent of plant food becomes exhausted first and in many cases 

 restoration of this constituent would renew the fertility for some 

 time longer. Substances which are added to the soil in order 

 to replace the ingredients which have been removed by previous 

 crops are called manures. 



All constituents of plants present in a soil, except the carbon, 

 are diminished by the growth of crops upon it, but the substances 

 which usually first become deficient are combined nitrogen, and 

 available phosphorus, calcium, potassium and possibly sulphur. 

 Consequently manures are valued according to the quantities 

 of those ingredients present in them, although in many cases the 



