In the filtrate from the above separation the excess of sodium carbonate is removed by the addition of hydrochloric to slight acidity, and the sulfuric acid estimated as described in the preceding paragraph.
Pure gypsum has a composition represented by the following formula: CaSO₄·H₂O.
It contains:
| Sulfur dioxid | 46.51 | per | cent. |
| Lime | 32.56 | “ | “ |
| Water | 20.93 | “ | “ |
A commercial sample of ordinary gypsum should have about the following composition:[244]
| CaSO₄·H₂O | 88.15 | per | cent. |
| CaCO₃ | 3.50 | “ | “ |
| Fe₂O₃ and Al₂O₃ | 1.50 | “ | “ |
| Insoluble | 2.80 | “ | “ |
| Organic matter | 0.50 | “ | “ |
| Water and undetermined | 3.55 | “ | “ |
287. Common Salt.—Common salt is highly esteemed in many quarters as a top dressing for lawns and meadows, and also for cultivated crops. Its action is chiefly of a mechanical and katalytic nature, since it does not form a very large percentage of the mineral food of plants. On account of its affinity for moisture it is also said to have some value as a condenser and carrier of water in times of drouth. On account of its great cheapness, selling often for less than ten dollars a ton, its use in moderate quantity entails no great expense. Its ability, however, to pay for its own use in the increased harvest is of a doubtful character when it is applied at a cost of more than a few dollars per acre. In the chemical examination of a sample of common salt which is to be used as a fertilizer, a complete analysis is rarely necessary. When desired it can be conducted according to the usual methods of mineral analysis. For practical purposes the moisture, insoluble matter, magnesia and chlorin should be determined and the quantity of sodium chlorid calculated from the latter number. Traces of iodin or bromin which may be present are of no consequence.
The moisture is determined by drying two grams of the well-mixed and finely-powdered sample to constant weight at 100°. The chlorin is obtained by precipitation of an aliquot part of a solution of the salt by set silver nitrate, using potassium chromate as indicator.
In the determination of insoluble matter it should not be forgotten that a little gypsum may be present, and this should be dissolved by rubbing to a finer powder and by repeated digestion in water. The magnesia and lime are separated and determined in the usual manner. If the quantity of gypsum present be sufficient to warrant it the sulfuric acid may be separated and weighed in the manner already described. Common salt when present in the soil in proportions greater than one-tenth per cent is injurious to vegetation.
288. Green Vitriol.—When iron is used as a fertilizer it is usually applied as ferrous sulfate. The value of iron in a soil is incontestable and by reason of the fact that fertile soils are always well aerated the iron present in the arable layer is found in the ferric state. When green vitriol is applied to the soil it undergoes gradual oxidation and appears finally in a more highly oxidized form. Iron acts directly on the plant in promoting the development of the chlorophyll cells, and is also found in almost all parts of the vegetable organism. A too great quantity of ferrous sulfate is destructive of plant growth in which respect it resembles common salt. It should therefore be applied with due regard to the dangers which might arise from an excessive quantity. It is not likely, however, that when applied in a finely powdered state at the rate of from two to four hundred pounds per acre it would ever prove poisonous to vegetation.