CHAPTER IV. 



I^dPROVEMENT OF SOIL BY MANUBTNQ. 



The growth of crops does not necessarily decrease 

 the fertility of soils, if the product is not removed but 

 is allowed to decay on the land. In most cases soils 

 improve so long as they are not cultivated by man. 

 The growth of plants through ages has been a chief 

 cause of the fertility of many soils. The mineral 

 matter drawn up from the sub- soil by the roots of 

 plants remains near the surface when the plants have 

 decayed. The supply of carbon and nitrogen in the 

 surface soil is also increased. In most cases the phys- 

 ical properties of soils which have long produced nat- 

 ural crops, whether of trees, grass or weeds, are such 

 as well to fit the soils to continue producing them 

 in abundance. On hill-sides the best of the soil may 

 be washed away. There may be no accumulation of 

 plant food on coarse, porous, sandy soils. But gen- 

 erally either prairie or forest soils are fertile in a 

 marked degree, when first brought into cultivation. 



Cropping Beclnce)^ Fertility. — The work of the 

 farmer tends to reduce the fertility of the soil. The 

 crops grown are removed from the soil ; or, if con- 

 sumed by animals, the animal product is removed. 

 In either case plant food has been removed from the 

 soil. There may still remain an abundant supply. At 

 the famous Experiment Station, at Rothamsted, Eng- 

 land, after wheat had been grown on the same ground 

 for about fifty years, without the application of plant 

 food, the crop sometimes nearly equaled the average 



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