20 GAKDEX MANAGEMENT. 



50. All cultivated soils contain the above ingredients. "When burnt, except in 

 the case of chalk, they assume a red colour, which is due to the presence of 

 iron-stone. Out of this stpparent sameness arise the greatest varieties of soils, 

 from their proportionate admixture, and, especially, from the proportions of 

 organic matter with which these chemical constituents are mixed. 



51. Of this organic matter rich black garden soils often contain from 20 to 

 24 per cent, by weight, and in peaty soils it often amounts to from 60 to 70 

 per cent. ; in good garden land it may range from 5 to 12 ; and in good agri- 

 cultural soils, seldom more than 6 per cent. This organic matter, so essential 

 to soils, is chiefly of vegetable origin, — the roots and stems of former crops, 

 with a mixture of animal refuse. Decomposed under the influence of air, heat, 

 and water, they produce the brownish or blackish powdery substance known 

 to the chemist and scientific gardener as humus. Humus exists in marly 

 soils in which lime to the extent of 5 per cent, is present, and loamy soils in 

 which clay to the extent of from 25 to 50 per cent, is present. 



52. Having thus briefly sketched the history of the formation of soils, and 

 their chemical constituents, let us take an equally rapid glance at the pro- 

 cesses by which these proportions can be ascertained. 



53. The practical analysis of the soil, washed and purified as described at 

 par. 44, will be as follows : — Any lime existing in the solution can readily be 

 precipitated by the application of sulphuric acid, by which carbonic acid is 

 liberated, and the soluble heavy body — sulphate of lime or gypsum — remains. 

 Magnesia, which exists in all soils, and sometimes to an injurious extent, is 

 precipitated when treated with hj^drochloric acid ; carbonic acid is thus evolved, 

 and, by the addition of sulphuric acid, sulphate of magnesia, or Epsom 

 salts, ai-e precipitated, the amount being determined by washing, drying, and 

 weighing the resulting sulphate. Silica and clay will probably be the chief 

 remaining ingredients in the solution : when dried, they are exposed to a red 

 heat in a platinum or porcelain crucible, noting the loss sustained in drying.* 

 When ignited and cooled, a portion is weighed out accurately, triturated in 

 a mortar of agate or "Wedgwood stone, with about four times its weight of 

 pure cai'bonate of soda, the whole mixture being transferred to a crucible of 

 platinum, and exposed to a red heat for fifteen minutes. "When cold, put the 

 cnicible and its contents into a porcelain evaporating-basin ; add water and 

 hj'drochloric acid, and leave the whole at rest for some time ; — the contents 

 will gradually loosen and become dissolved. The solution will probably exhibit 

 floating gelatinous particles : this is silica in combination with water, or 

 hydrate of silica. When evaporated to dryness, but at a low temperature, a 

 little hydrochloric acid is to be added, heat applied for a shoi-t time, and the 

 whole filtered ; the siHca is retained on the filtering-paper, the alumina 

 has passed through in solution. After evaporation, wash the filter copiously 

 with hot water, until a single drop of the water, caught on a slip of glass, 

 no longer leaves a residuum when heat is applied. Hartshorn is now applied 



• Dr. Scoffern's " Haudy-book of the Chemistry of Soils." Bell & Daldy. 



