THE SOIL A^'D THE PRODUCE. 191 



have produced the same crops as before. In the crops 

 yielded by them in the manured state, the constituents of 

 the soil and those of the manure had a certain definite 

 sliare : if tlie fields had not been manured, the crops would 

 have been smaller. Now if we attribute the increased pro- 

 duce during the course of the rotation to the supply of 

 farm-yard manm*e, and suppose that the constituents of the 

 farm-yard manure have been again removed in the crops, 

 which is not true in all cases, then the field, at the end of 

 the rotation, is in the same state in which it was at the 

 commencement, before it had been manured. Accord- 

 ingly, we may assume, without great risk of error, that 

 the produce of different crops, which a plot of ground 

 will yield in a new rotation without manuring, will be in 

 proportion to the store of nutritive substances, ready for 

 assimilation, which it contains in its natural state. Hence 

 from the unequal products pelded by the two fields in 

 that state, we may, with an approximation to truth, infer 

 certain inequahties in the amount of food or in the con- 

 dition of the fields. 



Of course, mferences of tliis kind are admissable only 

 within very narrow hmits ; for when we compare two 

 fields which he in the same or in different districts, we 

 must. remember that in each case various factors operate 

 upon the products, making these unequal, even though 

 the nature of the soil be otherwise identical. 



If, for instance, two fields, both unmanured, are planted 

 with one and the same cereal, it is by no means a matter 

 of indifference, as regards the produce of corn and straw, 

 what crop has preceded the cereal. If tlie last crop in 

 the preceding rotation was clover on the one, oats on tlie 

 other field, the results will vary, even though the con- 

 dition of the soil in both was originally identical; and 

 the produce reaped, in that case, indicates merely the 



