This is well known as the source of plaster of Paris. As a manure it is rarely used by itself, but it is largely applied in the form of superphosphate. It is thought that the presence of gypsum in the soil not only increases the solubility of the potash, but also prevents the loss of nitrogen (in the form of ammonia) from stable manure. Some authorities doubt this; but in any case gypsum would scarcely pay for special application. It is favoured for light sandy or gravelly soils, from 2 to 3 cwt. per acre being considered a reasonable dressing.