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exhausts the soil doubly of azotized matter and phosphoric acid, and cannot be succeeded by corn 

 except in tlae richest soils. Hemp raised for fibre may be cultivated many years in a soil containing 

 much humus but the seed crops are rapidly exhausting. 



Hence we have crops which exhaust the soil of azotized matters — crops which remove an 

 excess of phosphoric acid — and grasses and clovers, cut before bearing seeds, which exhaust the soil 

 of neither of these essential bodies but on the other hand enrich it in organic matters. Many culti- 

 vated plants, as corn, wheat, cotton, hemp, flax, cabbages, etc. raised for seed, exhaust in both respects 

 and are therefore peculiarly expensive crops. With this amount of information, based on experience 

 and several hundred analyses, we have the means of rendering intelligible the precepts of practical 

 writers on the succession of crops. 



Precept first resolves itself into the principle, that plants exhaust the soil unequally in respect to 

 azotized matters and must therefore be so adjusted that the most exhausting should recur as seldom 

 as possible. 



Precept second. Seed crops, which exhaust the soil of phosphoric acid, are to be interchanged 

 with herbage plants, which do not remove as nmch of this important substance. 



These directions have now assumed a definite form and are an explicit guide to the well 

 informed farmer ; he at once perceives that there are, over and above the precepts of expediency as to 

 hoed or cleaning crops and deep rooted crops, classes of plants which differ remarkably from each 

 other in their action on his fields. 1. Seed crops which exhaust the soil of azote. 2. Seed crops 

 which do not exhaust the soil of azote. 3. Exhausting forage and root crops. 4. Crops which 

 neither exhaust the soil of humus nor phosphates, but renovate the azote. With this amount of 

 knowledge he can shape a fair system of rotation, whatever may be his crops — he can introduce 

 indigo, cotton, tobacco, corn, bene, oil plants and many others which are not found in the arbitrary 

 tables given by Low, Thaer, and Stephens or falsely placed by Buel and Armsrtong. But 

 if we recur to our definition of the object of arotation — the production of the greatest profit in 

 crops, with the least exhaustion of the soil or manure — we find that there is yet something wanting 

 in the principles of rotation. In the fourth class above, we have plants Avhich neither exhaust the 

 soil of azote nor phosphoric acid ; it now becomes necessary to know in what respect they do exhaust 

 it, so as to satisfy the economical condition of impoverishing the soil in the least degree. 



VI. Plants Exercise a Natural Affinity for Specific Saline Matters. 



It has been already shown that the seeds of cultivated plants cannot be matured without phos- 

 phoric acid, and we find upon examination that this is not the only instance in which a distinct 



