From this, we see that we may pursue either one of two courses. After we know the exact composition of the soil--which we can learn only from correct analysis--we may manure it with a view either to making it fertile for all kinds of plants or only for one particular plant. For instance, we may find that a soil contains a very little phosphoric acid, and no potash. If we wish to raise potatoes on such a soil, we have only to apply potash (if the soil is good in other particulars), which is largely required by this plant, though it needs but little phosphoric acid; while, if we wish to make it fertile for wheat, and all other plants, we must apply more phosphoric acid as well as potash. As a universal rule, it may be stated that to render a soil fertile for any particular plant, we must supply it (unless it already contains them) with those matters which are necessary to _make_ the ash of that plant; and, if we would render it capable of producing _all_ kinds of plants, it must be furnished with the materials required in the formation of _all kinds of vegetable ashes_. It is not absolutely necessary to have the soil analyzed before it can be cultivated with success, but it is the _cheapest_ way. [How is the fertility of the soil to be maintained, if the crops are _sold_? What rule is given for general treatment? Give an instance of matters in the soil that are to be rendered available by mineral manures?] We might proceed from an analysis of the plant required (which will be found in Section V.), and apply to the soil in the form of manure every thing that is necessary for the formation of the ash of that plant. This would give a good crop on _any_ soil that was in the proper _mechanical_ condition, and contained enough organic matter; but a moment's reflection will show that, if the soil contained a large amount of potash, or of phosphate of lime, it would not be necessary to make an application of more of these ingredients--at an expense of perhaps three times the cost of an analysis. It is true that, if the crop is _sold_, and it is desired to maintain the fertility of the soil, the full amount of the ash must be applied, either before or after the crop is grown; but, in the ordinary use of crops for feeding purposes, a large part of the ash will exist in the excrements of the animals; so that the judicious farmer will be able to manure his land with more economy than if he had to apply to each crop the whole amount and variety required for its ash. The best rule for practical manuring is probably to _strengthen the soil in its weaker points, and prevent the stronger ones from becoming weaker_. In this way, the soil may be raised to the highest state of fertility, and be fully maintained in its productive powers. 2d. Those manures which render available matter already contained in the soil. [How may silica be developed? How does lime affect soils containing coarse particles?