How do mineral manures sometimes improve the mechanical texture of the soil?] Silica (or sand), it will be recollected, exists in all soils; but, in its pure state, is not capable of being dissolved, and therefore cannot be used by plants. The alkalies (as has been stated), have the power of combining with this silica, making compounds, which are called _silicates_. These are readily dissolved by water, and are available in vegetable growth. Now, if a soil is deficient in these soluble silicates, it is well known that grain, etc., grown on it, not being able to obtain the material which gives them strength, will fall down or _lodge_; but, if such measures be taken, as will render the sand soluble, the straw will be strong and healthy. Alkalies used for this purpose, come under the head of those manures which develope the natural resources of the soil. Again, much of the mineral matter in the soil is combined within particles, and is therefore out of the reach of roots. Lime, among other thing, has the effect of causing these particles to crumble and expose their constituents to the demand of roots. Therefore, lime has for one of its offices the development of the fertilizing ingredients of the soil. 3d. Those manures which improve the mechanical condition of the soil. The alkalies, in combining with sand, commence their action on the surfaces of the particles, and roughen them--_rust_ them as it were. This roughening of particles of the soil prevents them from moving among each other as easily as they do when they are smooth, and thus keeps the soil from being compacted by heavy rains, as it is liable to be in its natural condition. In this way, the mechanical texture of the soil is improved. It has just been said that _lime_ causes the pulverization of the particles of the soil; and thus, by making it finer, improves its mechanical condition. Some mineral manures, as plaster and salt, have the power of absorbing moisture from the atmosphere; and this is a mechanical improvement to dry soils. [Name some mineral manures which absorb ammonia?] 4th. Those mineral manures which have the power of absorbing ammonia. _Plaster_, _chloride of lime_, _alumina_ (_clay_), etc., are large absorbents of ammonia, whether arising from the fermentation of animal manures or washed down from the atmosphere by rains. The ammonia thus absorbed is of course very important in the vegetation of crops. Having now explained the reasons why mineral manures are necessary, and the manner in which they produce their effects, we will proceed to examine the various deficiencies of soils and the character of many kinds of this class of fertilizers. CHAPTER IX. DEFICIENCIES OF SOILS, MEANS OF RESTORATION, ETC.