_Acidity, Neutrality, Alkalinity._—A test never to be omitted is that of the reaction of the soil on litmus or other test paper, to ascertain its acid, neutral or alkaline reaction. Should the latter occur quickly (by the prompt blueing of red litmus paper), “black alkali” would be indicated; but a blueing after 20 to 30 minutes means merely that a sufficiency of lime carbonate is present. An acid reaction (the reddening of blue litmus paper) of course indicates a “sour” soil (see chap. 8, page 122). _Chemical Analysis of Soils._—When the observations mentioned above give no very decisive results or inferences as to the soil’s chemical character, the more elaborate processes of qualitative and quantitative chemical analysis may be called in. It would seem at first sight that these ought to yield very definite results to guide the cultivator; yet such is by no means always the case. Both the previous history of the land, and the method of analysis, influence materially the practical utility of the results of chemical soil analysis. The cause of this uncertainty becomes obvious when we consider the three groups of ingredients outlined above, viz., the insoluble or unavailable, wholly _undecomposed rock minerals_; the “_reserve_,” consisting of compounds not soluble in water but soluble in or decomposable by weak acids; and the _water-soluble portion_, either actually dissolved in the water held by the soil, or held by the soil itself in (physical) absorption. While the latter portion is directly and immediately available to plants, the amounts thus held are usually quite small, and (outside of alkali lands) would rarely suffice for the needs of a crop during a growing season.[110] This demand must be materially supplemented by what can be made available from the soil minerals and the “reserve” by weathering, conjoined with the direct action of the acids secreted from the plant’s root-hairs upon the soil particles to which they are attached. It is obvious that the greater or less _abundance_ of the plant-food in the soil-material upon which these processes may be brought to bear, must essentially influence the adequacy of the plant-food thus supplied. Moreover, the greater or less extent to which these sources may have been drawn upon previously in the course of cultivation, will similarly influence that adequacy, on account of the diminution of the readily available supply. [110] The investigations of King (On the Influence of Soil Management upon the Water Soluble Salts in Soils and the Yield of Crops, Madison, 1903) show that from some soils at least, a sufficiency of plant-food ingredients for a season’s crop may be dissolved by distilled water alone, if the soil be repeatedly leached and dried at 110°. Whether such a supply can be expected under field conditions, remains to be tested.