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the carboniferous period, plant life flourished most luxuriantly, 

 and the gigantic conifers and other growths of that day cleansed 

 the atmosphere and rendered higher life possible by decompos- 

 ing the deleterious carbonic acid, returning the life-bearing 

 oxygen, and storing the carbon for man's future use in the vast 

 coal measures of the world. 



Nitrogen also, in this normal condition of vegetation, is sup- 

 plied by the atmosphere, and is always taken up in the form of 

 ammonia, which exists in the air, and is generally collected by 

 rain and dew, which also furnish the remaining elements of 

 plant food, viz : oxygen and hydrogen. 



Now all these substances which constitute the plant food, 

 both those supplied by the soil, and those supplied by the 

 atmosphere, must not only be present, -but present in the 

 relative quantity required by the plant for it to grow ; no one 

 of them can can be absent or even present in too small quan- 

 tity without the plant failing to grow and mature ; hence, 

 normal vegetation depends upon the available amount of that 

 constituent/^' the plant food which is present in least quantity. 



In addition to these elements of plant food, there must be a 

 matrix in which the food is, and which may at times supply 

 some of the elements of the food, sustaining it also in its proper 

 position, and protecting it from vicissitudes of climate. This is 

 the vegetable mould or humus mixed with clay and sand. 



Let us now review the normal or natural system of vegeta 

 tion in its totality. A tree springs up, assimulates all its ash- 

 food from the soil, and all its air-food from the atmosphere ; it 

 arrives at maturity and dies; as soon as dead, decay, which is 

 the same thing as slow combustion, sets in, and our analysis 

 proceeds; all the materials which were derived from the at- 

 mosphere being returned to it, except humus, while the soil 

 receives the ash-food back again. 



In this system the exhaustion of ash-food from the soil is not 

 possible; on the contrary, by the action of the atmosphere on 

 the soil more ash-food is rendered available, so that the amount 

 is constantly increasing. 



The second system, whereby the natural yield is maintained, 

 presents some very different characteristics. 



This system only obtains where the produce of the soil is 

 used for the wants of man and animals. Here there is some- 

 thing carried away, so that the materials of the soil are con- 

 stantly decreasing; thus, in a wheat crop, making fifteen bush- 

 els per acre, there is carried off annually from the soil by the 

 grain eight Ibs. of phosphoric acid and seven Ibs. of potash ; 

 and should the straw be also exported, there will be an addi- 

 tional drain of six pounds phosphoric acid and ten Ibs. potash. 

 Total, 14 and 18 pounds. 



This excessive loss of mineral material, if recurring yearly, 

 can by no means be restored by the slow method of atmospheric 



