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deposit of mud and sand was left on the surface, which formed an an- 

 nual layer of. fresh material, did not include the process of fallowing or 

 resting lands, because constant fertility was maintained by the annual 

 top dressing which was left by receding waters. 



The practice of resting and fallowing soils, or that of changing the 

 crops more or less systematically, has always been tound to be advan- 

 tageous, although the reasons for its necessity have not been satisfac- 

 torily explained. 



Various theories have been offered by physiologists explanatory of 

 the principles upon which the benefits of rotative cropping depends. 

 Modern chemistry has shown that plants require certain mineral sub- 

 stances for their support, and that although the same primary elements 

 may be found in all, yet they are found to be in very difterent propor- 

 tions in different kinds, some showing a mere trace of a substance which 

 may abound in others. These mineral matters being obtained from the 

 soil, it follows that if they are not present in sufficient quantities, or do 

 not exist in a sufficiently soluble state so as to be taken up by the roots, 

 the plant which demands them for its normal growth must suffer in 

 consequence of such deficiency 5 and in regard to specific inorganic 

 substances, it is evident that the plant which requires a large percent- 

 age of such would fail to succeed where another plant requiring only 

 a trace of the substance would maintain a healthy growth. Some 

 plants require much potash or soda, some much lime, others a large 

 proportion of silica. A rotation which would allow these plants to fol- 

 low each other in succesion, or cause one crop which requires only a 

 small quantity of any particular inorganic substance to succeed another 

 which requires that substance in large amount, would consequently be 

 beneficial. 



Taking these facts as a basis, the theory is propounded that the ne- 

 cessity for a rotation of crops is caused by the exhaustion of certain 

 inorganic substances which, if supplied in due quantities, would insure 

 the successful growth of the same plant on the same soil for an indefi 

 nite period. But in the absence of the knowledge indispensably nec- 

 essary for an accurate estimate of the exact quantities required, a sys- 

 tem of rotation is not only advisable but it is the only resource of the 

 cultivator until science determines the exact specific relations ^hich 

 exist between the plant and the soil from whence it receives its food. 



The deductions naturally following the above explanation regarding 

 plant food led to the supposition that chemical analysis would indicate 

 with a degree of certainty the exact line of practice to be followed in 

 regard to rotation of crops, or perhaps obviate the necessity for any 

 change except that of convenience. This was to be effected by analyz- 

 ing the soil and the plant to be grown in it, so that the ingredients 

 removed by the latter could be replaced, and thus the fertility of the 

 soil indefinitely maintained. But at present there are no indications 

 that such accurate knowledge is forthcoming, neither the analyses of 



