25 
Thus the table presents ten groups of plants to be employed in a rotation, which are variously 
exhausting of saline matters and exhausting or ameliorating as respects azote. 
VII, On the Chemical Principles of Rotation. 
In purchasing a farm we do not merely aim to gain possession of a superficies on which plants 
may be set: the object is to obtain such a natural compost of mineral and organic matters as may 
afford nutriment to plants, in the most perfect manner and for the longest time. If, in a particular 
locality, there be but one or two remunerating crops our object is to secure a soil which will best 
feed these. The affinities of plants have been already set forth, whereby a judgment may be formed 
of the fitness of the farm. In converting the minerals of the earth into crops we must adopt such a 
system as not to exhaust it too rapidly in one respect, without drawing any resources from another 
part. If in a situation where every crop is marketable, we adopt a series which takes from the earth 
only phosphoric acid, we do ourselves injustice by turning to no advantage the purchase of azote, 
lime, potash and sulphur in the soil. 
By a well digested succession of crops we economize each body of the soil, converting it into 
money without loss or improvidence. As we have paid for every kind of plant-food in the earth 
we incur a loss by allowing any part to remain unappropriated. Instead of cultivating one crop and 
going abroad for manure in a year or two, by cautious economy, we obtain that manure at home. 
The soil presents us with a magazine of saline matters or plant-food, for the most part in an insolu¬ 
ble condition. Annually, the dews of evening and showers act on the insoluble materials, and 
dissolve a portion : by tillage and judicious management more is rendered available. If the crops do 
not appropriate all the parts rendered soluble, some percolate into the soil and are wasted. The expo¬ 
sure of the earth, and most crops, rapidly deprive it of organic matter, and it becomes unfit for the 
growth of many, and diminished in fertility for all plants. By carelessness much waste is thus 
brought about, and this likewise occurs if manures be used. Farm-yard manure is but a condensed 
fertile soil, it merely wants the sand and clay which are for the most part mechanical components of 
lands, and contains the saline and azotized matters constituting the plant-food. Guano represents the 
soil also, but the fertilizing ingredients are here extremely condensed. 
When a good farm, or stable manure, or guano is purchased, we obtain azotized matter, phos¬ 
phoric acid, salts of lime, of potash, of soda and compound, of sulphur, all of which will be lost to 
the surface tillage, by volatilization, or percolation in solution, if neglected; or all of which may be 
reaped in harvests if judiciously managed. This end is to be accomplished only by a suitable rota¬ 
tion, which is therefore, as we have heretofore asserted, an economical expedient only. 
7 
