PRACTICAL RULES SU6GESTE1/ BY THEORY. 493 



all that the wheat crop needs — the failing vegetable and other matters of 

 the surface being increased and renewed by the enriching roots of the 

 preceding clover. And if now, turnips refuse again to give a fair re- 

 turn, it is because you have not added to the soil a fresh supply of that 

 manure without which they cannot thrive. Add the manure, and the 

 same rotation of crops may again ensue. 



We have already had frequent occasion, in studying the inorganic 

 constituents of plants, to observe that different species require very un- 

 like proportions of the several kinds of inorganic food which they derive 

 from the soil. Some require a large proportion of one kind, some of 

 another kind. If a soil, therefore, abound especially in one of these va- 

 rieties of inorganic food, one kind of plant will especially flourish upon 

 it — while, if it be greatly deficient in another substance, a second plant 

 will remarkably languish upon it. If if abound in both substances, then 

 either crop will succeed which we may choose to sow, or they may be 

 alternately cultivated with a fair return from each. 



Upon this principle the true general explanation of the benefit of a 

 rotation of crops appears to depend. There may be special cases in 

 which peculiar qualities of soil or climate may intervene and give rise 

 to appearances, or cause results to wliich this principle does not apply, 

 but for the general practice it seems to afford a satisfactory explanation. 



It may be said that this explanation seems to ira ply that the same 

 kind of crop maybe reaped from the same soil for an indefinite number of 

 years, by simply adding to it what the crop carries off'. This is certain- 

 ly implied in the principle — and iftve knew exactly ivhat to add for each 

 crop, we might possibly attain this result, except in cases where the soil 

 undergoes some gradual chemical alteration within itself, which it may 

 require a change of treatment to counteract. At all events it does not 

 seem impossible to obtain crop after crop of the same kind — and we may 

 hope hereafter not only to be able to etfect this, but to do it in a sutR- 

 ciently economical manner. 



Two practical rules are suggested by the fact that different plants require 

 different substances to abound in a soil in which they shall be capable of 

 flourishing. 



1°. To grow alternately as many different classes or families of plants 

 as possible — repeating each class at the greatest convenient distance of time. 



In this country we grow chiefly root crops,— corn plants ripened for 

 seed, — leguminous plants sometimes for seed (peas and beans), and 

 sometimes for hay or fodder (clover and tares), — and grasses, and these 

 in alternate years. Every four, five, or six years, therefore, the culture 

 of the same class of plants comes round again, nnd a demand is made 

 upon the soil for the same kinds of food in the same proportion. 



In otlier countries — tobacco — flax — rape, poppy or madia, cultivated for 

 their oily seeds — or beet for its sugar, can be cultivated with profit, and 

 being interposed among the other crops, they make the return of each 

 class of plants more distant. A perfect rotation would include all those 

 classes of plants which the soil, climate, and otlier circumstances allow 

 to be cultivated with a profit. 



2°. A second rule is to repeat the same species of plant at the greatest 

 convenient distance of time. In corn crops there is not much choice, 

 since in a four years' course two corn crops, out of the three (barley, 

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