TO BE ASCERTAINED BY EXPERIENCE. 87 



land. After cultivating one grain crop for a very 

 lengthened period in a rotation, it will be found of 

 advantage to make an occasional change to some 

 other. The land appears to grow tired of a crop after 

 a time, and to do better with another even of the same 

 class. There are some districts in Scotland, where 

 clover was for more than a century grown once in five 

 years, their rotations in those districts extending over 

 that space of time; now they can only get it once in 

 ten years, or every other rotation, and that not so good 

 as formerly : they call such land clover-sick. Instances 

 of this character show very strongly the value of rota- 

 tion in cropping, and establish by facts the theoretical 

 view that has been taken of the advantages likely to 

 result from such a system of cultivation. As we come 

 to know more of the composition of our various crops, 

 of the soils, and of manures, we may expect to attain 

 greater exactness in our calculations of the amount 

 taken off during any single year, or during an entire 

 rotation. 



In each district, the farmer, by careful observation 

 and study, can after a time mark out the system of 

 cropping and of manuring best adapted to his particu- 

 lar soil and locality. 



1. If he knows the character of the rock from which 

 his soil was originally formed, his task is comparative- 

 ly easy; for from the known composition of the rock, 

 he can come very near that of the soil. 



2. If he has no knowledge of this kind, he can still 

 hope to arrive at good results, by deductions from the 

 known character of the # crops that have been chiefly 

 cultivated upon his farm. He can tell what are the 

 substances that have been most probably exhausted by 

 these crops, and experiment accordingly with manures 

 in which those are the chief constituents. 



3. A still more satisfactory way, would be to procure 

 good analyses of soils by really competent persons 



