106 FAEM-YAKD MANURE 



generally less suited for the development of roots in them. But to 

 believe that such manures as nitrate of soda, sulphate of ammonia, 

 potash salts, or superphosphates owe their efficiency solely to their 

 physical action on the soil, seems absurd. If it were so, it is difficult to 

 see why other saline compounds, not containing the essential elements 

 of plant food nitrogen, potassium, or phosphoric acid should not 

 produce the same effect, for it is highly probable that precisely similar 

 physical action upon the soil would be produced by other salts. 



Nor does the more recent attitude assumed by the supporters of 

 this American theory, viz., that the beneficial effect of manures is to be 

 attributed to their destructive action on the toxic substances produced 

 in the soils, either by the growth of the plants themselves or by 

 bacterial action, seem, as yet, to be supported by sufficiently strong 

 evidence to justify its general adoption. 



It is, however, evident that the effects of the application of a 

 manure to soil are complicated and cannot be adequately explained by 

 merely considering its influence in increasing the store of plant food. 

 This last influence, nevertheless, must be of importance, and if the 

 store of available plant food, rather than that of total plant food be 

 considered, it is probably the greatest factor involved. 



The special manures, on account of their more definite character 

 and simpler chemical constitution might, perhaps, with advantage, be 

 considered first, but the more extended use and greater importance in 

 farming practice, of the more complex, general manures justify their 

 being given the preference. In this chapter, therefore, an account of 

 the more important general manures will be given, to be followed, in 

 the next chapter, by a description of the special manures. 



GENERAL MANURES. 



Farm-yard Manure. This has long been the most popular 

 manure used on the farm. It would seem that, inasmuch as it con- 

 tains the remains of the vegetable substances used as food and litter on 

 the farm, it should be a most suitable means of restoring to the land 

 the ingredients removed from it in the crops. A little thought, how- 

 ever, will show that it cannot completely restore such losses. Some of 

 the crops are sold ; often these are particularly rich in nitrogen, potash 

 and phosphates, and of those eaten by the animals of the farm, only a 

 portion is voided as excrement the animal has to build up its body 

 out of the materials supplied in its food. This consumes large quan- 

 tities of plant food, particularly of nitrogen and phosphates. Then, too, 

 the production and sale of milk removes large quantities of manurial 

 ingredients from the soil, and though butter contains little other than 

 carbonaceous material, cheese is highly nitrogenous. 



It will be advisable to consider briefly the chemical nature of the 

 raw materials which go to form the average farm-yard manure. The 

 ingredients of this substance may be first divided into two groups : 



1. Animal excrements, both solid and liquid. 



2. Litter and waste food materials. 



The excreta of animals consist of the undigested parts of the food 

 consumed by them, together with excretions consisting of effete matters 



