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CHAPTER VII. 



EFFECTS WHICH THE GROWTH OF PLANTS PRODUCES UPON THE 

 SOIL, AND PLANS ADOPTED FOR MAINTAINING ITS FERTILITY. 



136. Having in the preceding chapters described the 

 conditions which are necessary for the existence of plants, 

 and also the sources from which the materials that we discover 

 in their structure are derived, we may now with greater 

 advantage proceed to the consideration of the eftects which 

 the gi'owth of those cultivated by the farmer has been ob- 

 served to produce upon the soil, and also of the various plans 

 which have been adopted for the purpose of maintaining its 

 fertility or of increasing its natural capabilities. 



Observation teaches us that nature, without the aid of man, 

 provides food in sufficient abundance for the growth of the 

 lierbs and trees with which she clothes the surface of the 

 uncultivated soil ; and in the infancy of society the wild roots, 

 herbage, and fruits of the field and forest were sufficient for 

 the support of the wandering tribes that were thinly scattered 

 over the earth. But as population increased, and men asso- 

 ciated together in numbers in cities and villages, the necessity 

 of producing, in the vicinity of their habitations, a larger 

 supply of vegetable food than the soil spontaneously afforded, 

 laid the foundation of agriculture. For centuries the know- 

 ledge which in these countries was possessed of the cultivation 

 of plants was extremely limited, generally only a few vegeta- 

 bles occupied the attention of the farmer, and it is known, that 

 many of the plants which are now highly esteemed for food, 

 were at one time useless weeds; thus, the potato in its wild 

 state in South America, travellers tell us, would scarcely in 

 the produce of an acre afford food sufficient to sustain an 

 Irish family for one day (Darwin) : the peach of our gardens 

 is said in its original state among the mountains of Hindostau 

 to bear a bitter and poisonous fruit, and the now valuable 

 tuniip, when uncultivated, is found to yield a bulb not much 

 larger than a pig-nut. The highly nutritious grain crops also 

 that we now grow upon our farms, in their natural condition 

 contained bat a triffing amount of nutritious matter, but by 

 h2 



