_Condition of the Plant-food Ingredients in the Soil._—But even the abundant _presence_ of the plant-food ingredients, as shown by analysis, will not avail, unless at least an adequate portion of the same exists in a form or forms accessible to plants. Of course this condition would seem to be best fulfilled by the ingredients in question being in the _water-soluble_ condition. But in the first place, plants are quite sensitive to an over-supply of soluble mineral salts, as is evidenced by the injurious effects produced by the latter in saline and alkali lands. Furthermore, substances in that form would be very liable to be washed or leached out of the soil by heavy rains or irrigation, and would be lost in the country drainage. It is therefore clearly desirable that only a relatively small proportion of the useful soil-ingredients should be in the water-soluble or physically absorbed condition, but that a larger supply should be present in forms not so easily soluble, yet accessible to the solvent action which the acids of the soil and of the roots of plants are capable of exercising. This _virtually_ available supply we may designate as the _reserve food-store_. Finally, there is practically in all soils a certain proportion of the _soil-minerals in their original form_, as they existed in the rock-materials from which the soil was formed. These minerals being usually in a more or less finely divided or pulverulent condition, they are attacked much more rapidly by the chemically-acting “weathering” agencies, viz., water, oxygen, carbonic and humus acids, than when in solid masses; and thus, transformation of the inert rock-powder into the other two classes of mineral soil-ingredients progresses in naturally fertile soils with sufficient rapidity to produce, in a single season, sensible and practically important results, known as the effects of _fallowing_. _The Reserve._—The nature of these processes has been discussed in chapters 1 to 4; and it will be remembered that two of their most prominent results are the _formation of clay_, and of _zeolitic-compounds_, the latter being, as heretofore stated (pp. 36 ff) hydrous silicates of earths and alkalies, easily decomposable by acids, and also capable of exchanging part or the whole of such basic ingredients with solutions of others that may enter the soil. These zeolitic compounds therefore fulfil two important functions in the premises, viz.: a ready yielding-up of part of their ingredients to acid solvents, and a tendency to fix, by exchange, a portion or the whole of the soluble compounds that may be set free in, or brought upon the land. The first-mentioned property is of direct avail in that the soil-humus forms, and the roots of plants exude, acid solvents on their surface, and can thus draw upon the reserve store of food; the second tells in the direction of preventing the waste of water-soluble manurial ingredients supplied to, or formed in the soil. (See above, chapter 3, page 38).