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MINOR PLANT FOOD ELEMENTS - Part 1

It is seldom realized that plants contain more sulphur than phosphorus; it is very important in chlorophyll formation. In­adequacy of sulphur in gardens is unknown, for it abounds in dung, blood-and-bone manure, vegetable matter, sulphates, superphosphate (as excess sulphuric acid), flowers of sulphur, and in many sprays, such as Bordeaux mixture, lime-sulphur, iron sulphide, and the various colloidal and wettable sulphur preparations. Some of it oxidizes and escapes as sulphur dioxide. Commonly, in putrefactive processes, some sulphur com­bines with hydrogen to form sulphuretted hydrogen. Any sul­phur added by way of spray, dust, or sulphate manure is usually in excess of plant food requirements. Ample is available from organic matter in steady supplies; putrefaction quickly liber­ates large quantities. Excessive sulphur in soil is one of the worst calamities that can befall a garden. It means death to many millions of soil micro-organisms and will almost certainly banish all earthworms.

Calcium

Calcium exists in all plant tissues, particularly the older parts. In soils it is most common as limestone, impure calcium car­bonate. This is freely acted on by even dilute solutions of weak organic acids, which abound in soils containing much organic matter. Carbon dioxide and a calcium salt are formed; the gas is liberated; the salt is usually very soluble and apt to leach away quickly. This chemical action reduces soil acidity and helps the bacteria that assist in nitrification. This faster meta­bolism will call for greater supplies of manures. Lime should never be used unless the soil is found by actual test to be unduly acid. A pH value of less than 5.5 would indicate a need for light liming.* Heavy liming can easily be harmful.

* The gauge of acidity (or hydrogen ion concentration) is called the pH scale. A pH of 7.0 is neutral. Less than 7.0 it is acid, and more than 7.0, alkaline. The pH value denotes the intensity of acidity and not the quantity of acid present. A pH of 5.0 is ten times as acid as a pH of 6.0, and a pH of 9.0 is only one-tenth as Bone-meal contains large quantities of calcium as well as other foods, especially phosphorus, but when calcium is applied in this form it is not as effective in reducing soil acidity or improving the physical condition of the soil as when lime or dolomite is used. Dolomite consists of sixty per cent calcium carbonate and forty per cent magnesium carbonate; it is an excellent corrective for excess acidity. Gypsujn is calcium sulphate; it does not increase or decrease soil acidity but helps to improve the physical condition of soils and is slowly available as a plant food.

In heavy clay, calcium causes a coagulation or flocculation of the fine soil particles and so renders the soil more friable, better aerated, and more easily drained. This good effect lasts for many years.

Calcium stimulates root-growth and deficiency appears to bear some relation to die-back. It helps to make potash and phosphorus more readily available and soluble phosphates less liable to be lost or changed into insoluble unassimilable forms. It renders iron less available-Calcium oxide, or quicklime, is made from limestone. When moistened with water it swells, heats, and breaks down to a fine powder, calcium hydrate, limil, or slaked lime. On ex­posure to air it slowly absorbs carbon dioxide and is reconverted into calcium carbonate.