_Magnesian Limestone._ Leading scientists making tests of limestone for normal soils, use magnesian limestone freely. They recommend its use to farmers wherever there is advantage in point of price. The advice is safe that the limestone of a given fineness should be chosen whose total percentage of carbonates of calcium and magnesium is the highest. The example of these scientists, buying pulverized limestone for agricultural colleges and experiment farms, and for their own farms, should loosen the curious hold that the early warnings of a laboratory experimenter took upon public imagination. The farmer should buy limestone on a basis of ability to correct soil acidity, and make each dollar do the most possible toward that end. Most limestones contain some percentage of magnesium, and in the case of a pure dolomite over 45% carbonate is present in combination with calcium carbonate. A stone rich in magnesium slakes less readily than one high in calcium, and therefore is preferred by manufacturers shipping pulverized burnt lime to reach its destination before slaking. CHAPTER XV WHAT SHALL ONE BUY? _Relative Values._ The relative strengths of the various materials containing lime may be known and yet doubt continue respecting the choice to be made. The conflicting claims of dealers, and inaccurate deductions from a single test made by some individual, aid the confusion. If there were always the single purpose of correcting soil acidity, and if there were the same ease of application in case of all the materials, the choice would present much less difficulty. Notwithstanding this, most land now has a lime requirement, or will have one as leaching, crop removal and chemical change within the soil continue, and the puzzle is no worse than a score of others that present themselves continuously in farming. _Destroying Acids._ The cost of liming to improve the physical condition of land is prohibitive for most farms remote from supplies of stone that can be burned and put upon the land at a low price per ton. Where stone is at hand, and soils are intractable, lime burned on the farm should be used. Some slight benefit to a stiff soil may be obtained from the light application that is deemed practicable where all forms are costly, but this benefit is not usually marked in case of an application of a ton or less of burned lime. It is a safe statement that most buyers of lime in some form or other will profit chiefly through the correction of soil acidity and promotion of bacterial life. This renders the situation more simple as any carbonate, hydrate or oxide of lime will accomplish these purposes. _Composition._ The first consideration is the actual content of calcium and magnesium. A guaranteed analysis is the only safe basis of purchase. The unstable nature of fresh burned and hydrated forms makes an exact statement of percentages impossible for goods not wholly fresh, but at least the purity of the original limestone can be judged.