These manures have four kinds of action when applied to the soil.
1st. They furnish food for the inorganic part of plants.
2d. They prepare matters already in the soil, for assimilation by roots.
3d. They improve the mechanical condition of the soil.
4th. They absorb ammonia.
Some of the mineral manures produce in the soil only one of these effects, and others are efficient in two or all of them.
The principles to be considered in the use of mineral manures are essentially given in the first two sections of this book. It may be well, however, to repeat them briefly in this connection, and to give the reasons why any of these manures are needed, from which we may learn what rules are to be observed in their application.
Relate what you know of the properties of vegetable ashes?
How does this relate to the fertility of the soil?
According to what two rules may we apply mineral manures?