This section is from the book "Commercial Gardening Vol1", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
There are few substances beyond those already mentioned used as manures, simply because there is very little to be obtained from them, or because the foods they yield are generally present in superabundance in the soil.
Magnesium Salts are sometimes applied as a potato manure, as magnesium carbonate or magnesium sulphate (otherwise Epsom salts). Magnesium occurs in the ash of all plants (see Tables at p. 109), and is returned to the soil in farmyard and other natural manures.
Iron is also one of the essentials of plant life, but there are usually large available supplies in the soil. Without a trace of iron it would be impossible for the chlorophyll or green colouring matter of leaves to develop, no matter how perfect other conditions might be. Recently sulphate of iron at the rate of 8 oz. to a square rod has been used where iron has been considered deficient owing to the yellowish colour of the leaves. When sickly looking yellowish-leaved plants will not respond to a complete fertilizer, or to a nitrogenous manure, the soil is then probably deficient in available iron. Very often, however, the yellowish appearance of leaves is due to sour and sodden soil, or to the absence of lime.
Salt, or Chloride of Sodium, is sometimes used as a special manure for Asparagus and Sea Kale and other plants. In weak doses it seems to be beneficial, and is said to liberate potash. From 1 to 2 lb. to the square rod may be used. Kainit, however, may be a safer manure to use in quantity.