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IV. The Precepts of Practical Writers. 
The points advanced by practical writers as Thaer, Low, Stephens and Ream, as the 
principles of rotations are of considerable moment, especially in the field, but are no more than 
surmises for the most part. They may be resolved into the three following assertions and precepts: 
1. That each plant requires a particular food and should therefore be repeated at as long 
intervals as possible. 
2. That seed crops being peculiarly exhausting are to be interchanged with green or forage 
crops and roots. 
3. That plants which require hoe tillage, being cleaning crops, should follow those which are 
sown broadcast and encourage weeds. 
In these positions we recognize the imperfect observations of farmers; each one is true within 
certain limits, and excepting the last, which is only a practical expedient, it is impossible through 
them to reach any general principle. That each plant requires a particular food is an assertion 
merely which, so far from carrying conviction, is altogether denied by some practical men and, 
whether true or false, is beyond the means of these writers to prove. The second assertion, that 
seed crops are exhausting, is sustained by experience ; but in what way they are exhausting is not 
stated, and without this information the assertion is of little value. As we have remarked, the third 
position is a practical expedient only, because both seed and forage plants may be hoed crops, as 
corn, beans, cotton — tobacco, turnips, cabbages. 
Hence the precepts of practical writers resolve themselves into the two points, that the same 
and allied species should be cultivated at as long intervals as expedient and that seed plants are to 
be as seldom introduced as possible. Both these positions are of practical value, but they do not 
merely labor under the defect of conveying no precise information, but may be used in forming 
schemes of rotation of no economy whatever. Thus the following plan is perfectly conformable 
with these precepts, but very objectionable. 
Manure, corn, tobacco, oats with clover, wheat, beans. 
or, as in the rotation for clay lands, by Mr. Rham, 
Manure, roots, oats with clover, beans, wheat. 
In the first a seed crop is followed by a foliage crop, but both of these are exhausting; in 
the second, beans are succeeded by wheat, both exhausting, but — and this is the imperfection of 
such arbitrary precepts — the exhaustion in every case is not of the same kind or degree. We 
are informed that certain crops are exhausting, but not of what; they impoverish the earth, yet we 
