Objects to be Accomplished by Plowing. 137 



and fifty-six, we shall know the number of oat crops that can be 

 taken. 



When this number of crops has been taken from the soil, it is 

 evident that its capacity for producing wheat or oats is exhausted- 

 and can never be restored until the missing mineral is replaced 

 from some foreign source. 



In practice, however, thei soil becomes incapable of yielding 

 either wheat or oats long before the supply of mineral matter is 

 exhausted. A wheat plant, for instance, may absolutely starve 

 for want of ammonia in a soil which an exact chemical analysis 

 shows to be superabundant in ammonia, or it may perish for lack 

 of phosphate of lime in a soil replete with that substance; or it 

 may be unable to procure a due supply of silicic acid from a soil 

 consisting of pure sand. 



Every farmer knows that if, after it appears to be exhausted of 

 its mineral matter, the soil is allowed to rest, exposed to the action 

 of frosts, rains, dews and sun-light, it will, after the lapse of a 

 certain time, recover its fertility; the phosphate of lime, silicic 

 acid and carbonate of potash, in which it seemed utterly deficient, 

 have now been restored to it by the operation of its own internal 

 processes, and not supplied to it from without. 



Again, it is found that, on many kinds of laud, very small crops 

 are obtained at the first plowing; but that, at every successive 

 plowing, the crop increases. It would seem from this that plants 

 increased in magnitude just in proportion to the diminution of the 

 supply of food. 



An attentive examination of these apparent anomalies will lead 

 us to a clear understanding of the causes of fertility in a soil. 



First — Soils may contain a superabundance of mineral matters, 

 though they are so unevenly distributed that a larger part of the 

 soil is so deficient in them that it may be absolutely barren. 

 Thus, if we measure oif ten square feet at one corner of an acre, 

 and cover it thickly with lime, the opposite corner of the acre 

 receiA^es no benefit from the application; there is too much lime 

 in one plat and none in the other. If the arable surfaces of the 

 two soils are mixed together, both will be benefited and both 

 will be more fertile. Or we may suppose that a soil deficient in 

 phosphate of lime has a bone buried in it. After a certain time 

 it will be found that some of the phosphate of lime and gelatine, 

 with its included nitrogen, is dissolved, and the particles of earth 

 in contact with the bone are saturated with the solution, and 



