One of these valuable properties not possessed by limestone, is said to be that of acting as a fungicide and insecticide. As a rule, fungi prefer acid reaction in the substances in which they grow, so that the strongly alkaline properties of lime may make a limed soil unsuitable for their growth.[239]
282. Analysis of Lime.—Lime, which is prepared for use as a fertilizer, is rarely submitted to a chemical examination. It is easy to see, however, that such an examination is of some importance. If the real value of a sample be dependent on the content of lime, the actual quantity present as determined by analysis, must fix the value for agricultural purposes. The more important things to be determined are the quantities of lime, and of slaked lime, of undecomposed calcium carbonate, and of insoluble matter. It will be also of interest to determine the respective quantities of lime present as oxid, hydrate, and carbonate. If any question be raised in the case of slaked lime in respect of its origin, it can usually be answered by an examination of the unburned or unslaked residues. In perfectly slaked lime containing no débris, the analyst will be unable to discover whether it has been made from limestone, marble or shells. The lime used for agricultural purposes should be reasonably free of magnesia, and should not be air-slaked before transportation to the field. In dry air-slaking, a considerable quantity of carbonate may be formed.
283. The Process.—(1) Insoluble and Soluble Constituents.—A representative sample of the lime having been secured, it is reduced to a powder and passed through a half millimeter mesh sieve or ground to a fine powder in an agate mortar. Digest two grams of this sample with an excess of hydrochloric acid, for two hours with frequent stirring; filter, wash the residue with hot distilled water until chlorin is all removed, and dry to constant weight. The lime, magnesia, silica, and other constituents of the filtrate, are determined by the usual processes of mineral analysis.[240]
(2) State of Combination of the Lime.—In a lime containing only small quantities of magnesia the lime carbonate may be determined by estimating the carbon dioxid by any one of the reliable processes in use.[241] In every case sufficient acid must be employed to combine with all the bases present. Tartaric or hydrochloric acid may be used. From the volume or weight of the carbon dioxid obtained the quantity of calcium carbonate may be calculated. Since magnesium carbonate is more easily decomposed by heat than the corresponding calcium compound, any residual carbonate in a well-burned sample is probably lime. The total percentage of lime in the sample is to be determined in the usual way by precipitation as oxalate and weighing as carbonate or oxid. The lime existing as oxid can be determined by exposing a weighed sample in an atmosphere of aqueous vapor until all the lime is slaked. After drying at 100° the increase in weight is determined and the calcium oxid calculated from the formula, CaO + H₂O = CaO₂H₂.
If now the total lime be represented by a; the lime combined as carbonate by b; and that present as oxid by c; the quantity x existing as hydrate may be calculated from the equation
x = a - (b + c).
| Example: | Let the total lime be | 88 | per | cent. |
| CaO as carbonate, | 2 | “ | “ | |
| CaO as oxid, | 78 | “ | “ |
Then the CaO as hydrate = 88 - (2 + 78) = 8 per cent. The total lime as oxid and hydroxid may also be separated from that present as carbonate by solution in sugar.[242] One gram of calcium oxid is completely soluble in 150 cubic centimeters of a ten per cent sucrose solution. Magnesia, iron and alumina do not interfere with the determinations.
284. Gypsum or Land Plaster.—This substance is highly prized as a top dressing for grass and for admixture with stall manure for the purpose of fixing ammonia. Its value in both cases depends upon its percentage of hydrated calcium sulfate. The quantity of gypsum mined in the United States in 1893 was a little over 250,000 tons. Of this amount only about 50,000 tons were used as fertilizer.[243] In the same time there were imported into the United States, in round numbers, 170,000 tons. If the same proportionate part of this were used for fertilizing purposes, it may be said that the annual consumption of land plaster in the United States at the present time for agricultural uses is about 75,000 tons.
Gypsum, being a very soft mineral, is easily ground and should be in the state of a fine powder when used for fertilizing purposes. It is soluble in about 500 parts of rain water, so that when applied as a top dressing it is carried into the soil by rain. Its favorable action is both as a plant food and mechanically in modifying, in an advantageous way, the physical constituents of the soil. It is also valuable for composting and for use in stables by reason of its power of fixing ammonia by the formation of lime carbonate and ammonium sulfate: