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MINOR PLANT FOOD ELEMENTS - Part 2 |
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Magnesium
Magnesium is the key element of chlorophyll, so essential in Nature's sun-worked food factory. It is present in most soils in adequate quantities, but deficiency is common in our light coastal soils. Heavy clays of the inland areas contain ample available magnesium. Deficiency is marked by yellowing between veins of old leaves and is frequently accompanied by iron deficiency. It can be corrected by adding magnesium nitrate, magnesium sulphate (Epsom salts), or, if the soil is acid as a pH of 8.0, or, in other words, is ten times as alkaline. Roses do best in soils where the pH value lies between 5.5 and 7.0 -slightly acid. very acid, dolomite. Magnesium sulphate is much cheaper than the nitrate, but less desirable in large quantities or repeated use because of its sulphur content. The nitrate is doubly useful because of its nitrate radical. Green vegetable matter has a fairly high magnesium content. Compost and most other organic manures would provide some organic magnesium compounds, and they are always to be preferred to inorganic salts if available in adequate quantities. Phosphorus Phosphorus is not needed by roses in as great quantities as is commonly believed, but is the most commonly deficient plant food element in Australian soils, especially where available iron is abundant. The brilliant autumn foliage of deciduous trees and the blueness of hydrangea blooms in our mountain districts are related to this proportionate association of these two elements. In these same localities very few roses are grown. They do not seem to do as well there as in most other parts, but this is probably due more to climatic conditions than to soil chemistry. Phosphorus stimulates root-growth and is needed by plants before leaf-growth is apparent. The most readily available inorganic form of phosphorus is superphosphate, which contains about twenty-two per cent water-soluble phosphate. More of it is soluble in weak acids, particularly citric acid, which is excreted by roots. If the soluble phosphate combines with metallic bases, such as iron or aluminium, the resultant compound is insoluble and the phosphorus is lost to plants. Superphosphate should be used sparingly and never more freely than one and a half to two ounces to the square yard. Ammonium phosphate ("Floraphos") is another inorganic salt of phosphorus. It is less dangerous than superphosphate and more forcing, due to its nitrogen. It can be used to hasten the maturing of blooms. Rock phosphate gives up its phosphorus very slowly under the action of acid root excretions. Bone-meal is rich in phosphorus and, in the soil, it and other elements are rendered available slowly by bacterial agencies. The effect of these two fertilizers is seldom seen for at least two years, but it then lasts for many years. It is safe to use them at any time of the year. They may be dug deep into the rooting zone of the roses, and well mixed with the soil a month or so before planting and subsequently worked into the surface soil from time to time. Bone-dust is often used in combination with blood manure as blood-and-bone manure, the most obvious action of which is due to its nitrogen, but there are many other elements present, including large amounts of phosphorus. The same applies to urine and bird-droppings. Phosphoric acid added carefully to liquid manures is another way of feeding plants with phosphorus. Iron Chlorophyll metabolism needs iron, which varies in amount greatly in Australian soils from almost complete absence in some coastal sandy soils to excess in some mountainous parts. It is often present in great quantities, but in forms that are insoluble in water, soil products, or root excretions. Iron deficiency in plants is evident by pale young leaves and poorly coloured flowers. It can best be corrected by adding large quantities of organic matter, especially if it has been grown on soil rich in available iron. Chemically it can be added as ferric nitrate or ferrous sulphate, preferably the former, wherein both the iron element and the nitrate radical are helpful. It is the more expensive salt. Iron filings are usually easy to obtain, and a very light scattering of them over rose beds every few years will provide a steady supply of available iron to the roses as the filings oxidize and slowly break down. Manganese, zinc, copper, boron, and other elements are found as mere traces in plant ash, but when any one of them is lacking in a soil, the plants are just as handicapped as when starving for a major element. Thousands of square miles of Victoria, South Australia, and scattered coastal districts of Australia have been found to be barren waste land wholly because of deficiency of zinc, copper, and cobalt. At a cost of less than five shillings an acre those deficiencies have been corrected and the land has been converted into fertile, lush pastures. Areas of South Australia are deficient in manganese and copper. Small quantities of zinc have greatly increased some Australian wheat crops, especially in the Wimmera district of Victoria. Areas of the United States of America have been helped by minute doses of copper. Boron is very deficient in some otherwise very rich soils in New South Wales (for example, in the Orange district), and in New Zealand. Mysterious diseases amongst plants and livestock in many parts of the world have been found to be due to a deficiency of one or more of the minor plant food elements. The remedy has been simple once the cause was discovered. Manganese and boron are the only two minor elements that may be of practical interest to rose-growers. Manganese is best added, chemically, as an extremely light dressing of manganese dioxide, or potassium permanganate (Condy's crystals) in very pale-pink solution or added to liquid manures. Boron is most easily applied as sodium biborate (borax), but must be used in very minute quantities, under scientific supervision. |
