102 DISEASE IN PLANTS. 



true that the presence of such ingredients in excess 

 or in combinations unsuited to the plants also leads 

 to disaster, as also does the presence of minerals 

 or other compounds which poison the root-hairs 

 e.g. products of decomposition, soluble salts of 

 copper and other poisons. That these matters 

 are bound up with the whole question of manuring 

 and of proper soil-analyses will be evident. 



Another essential factor is the nature and quan- 

 tity of organic materials in the soil, whether leaf- 

 mould and decomposing vegetable remains, stable 

 manures, or other animal matters, all of which affect 

 different species very differently, and produce very 

 different results in different soils. It is necessary 

 to apprehend in this connection what has been 

 stated above : that soil is not a mere dead 

 structureless medium, and that the root-hairs of 

 ordinary plants cannot deal with large quantities 

 of putrefying organic matter : that a good soil 

 must abound in useful bacteria and fungi to 

 render such substances available and in very 

 various ways and that it must be open and 

 aerated, of proper temperature and suitably sup- 

 plied with water, and so forth, or disaster will 

 result. Here, again, then we are brought into 

 close contact with all that is known of fermenta- 

 tion, nitrification, and the various biological 

 changes going on in soil, and the application of 

 such knowledge to the practice of manuring and 

 tillage in all its forms. 



In view of the above remarks, the danger of 

 " over-feeding," in this sense, has a real meaning 



