The following is an article written by T W Walker, Emeritus Professor of Soil Science Lincoln University, Canterbury
Magnesium (Mg) is an element essential for the growth and health of planet and animals. Some soils are so deficient in Mg either naturally or because of man’s treatments that Mg must be applied if plant and animal growth is to be improved or maintained.
Is is perhaps fortunate that the deficiency usually has to be very severe before plants respond markedly to applications of Mg. To some extent this is because Mg is very mobile in plants and if young growing leaves run short of Mg it moves out of older leaves to keep the plant growing. It’s like the “old man” helping his impecunious son with just enough money to keep the ailing family business going; the problem becomes acute when the “old man” runs out of money.
The astonishing response of clovers in pasture to phosphorous (P), sulphur (S), potassium (K), and molybdenum (Mo) when one or more is deficient is because clovers are stimulated to grow and fix more nitrogen (N). We do have soils in New Zealand where Mg must be applied to stimulate clover growth but they are few and far between and it is rare to get startling responses in pasture growth when Mg is applied.
Mg deficiency occurs commonly in animals even where pasture growth is not stimulated by Mg applications. In this respect Mg is rather like selenium (Se). As far as we know plants do not require Se but animals do. Se prills applied to a pasture will help animals without increasing pasture growth. Perhaps a more apt comparison would be with cobalt (Co). It has been know for over 60 years that animals need Co but no-one has yet found soils in New Zealand where clovers respond to Co, not even where animals benefit greatly from Co application to soil.
What New Zealand grassland farmers must appreciate is that Mg, like Co and Se, may be necessary on many soils for the sake of animal health rather than to improve pasture growth. Just as Co and Se can be got into the animal in drenches and licks, so can Mg. I am not arguing for one or the other, except to say that it makes good sense to me correct animal deficiencies through the soil and the planet. If my diet were deficient in protein and carbohydrate I would rather correct it by daily increments than a great blow out every few months.
Causes of Magnesium Deficiency
Mg deficiency in soils may be inherited, acquired or induced. Some parent materials (the stuff on which soils form) are low in Mg – containing minerals and the soils will automatically be low in Mg – the deficiency is said to be inherited. Where soils, even those rich in Mg initially, have been forming in high rainfall area for many thousands of years, minerals containing Mg, K, P and other elements will have weathered (decomposed) and much of them lost in the drainage water. The minerals weather faster when temperatures are higher and Mg will disappear faster in warm, wet climates. Old soils in such climates may be depleted in most elements essential for planet growth – the deficiency is said to be acquired during the processes of soil formation.
Where soils are farmed intensively, be it in dairying or horticulture, Mg deficiency can be induced easily because of the failure to recognise the need to apply Mg. In the initial development of pastures on many soils lime is applied which supplies calcium (Ca) and raises the pH. Superphosphate is applied which supplies P, S, and Ca. If necessary, muriate of potash (KCI) is applied to supply K. These treatments usually stimulate clover growth and so add large amounts of N to the system. Thus all major nutrient deficiencies (S, P, K, Ca and N) are corrected except for Mg. The Mg content of the herbage will be reduced automatically unless available Mg levels in the soil are naturally high, just as the Se and Co levels in the herbage may be lowered because of the greatly increased production of herbage.
Even when soils are relatively rich in Mg applications of lime and particularly K can lower the Mg content of plants very significantly. At a given level of N uptake, plants take up a certain total level of the bases Ca, Mg, K and Na (sodium). Excessive K uptake seriously lower uptake of Mg and often Ca. The problem is intensified when available Mg levels in the soil are low.
Losses of Mg in drainage water may be increased by farming activities. When a fertiliser such as muriate of potash (KCI) is used correct K deficiency, the chloride fraction is virtually all lost in the drainage water but it cannot be lost on its own. When KCI is applied to the soil it dissolves in the soil water but most of the K becomes immediately attached to the fine clay and humus particles, displacing Ca and Mg which go into solution with the chloride. If Ca, Mg, and K are held in the proportion of 80:15:5 on the fin soil particles, then this is the approximate proportion of these elements we eventually find in the soil water balancing the chloride. So when the chloride is washed out in the soil water it will not be balance with the K that was put on with it, but mainly with Ca and Mg from soil reserves. While the Ca reserve may be made good by liming or the use of superphosphate, available Mg can only be restored if Mg minerals weather fast enough or Mg is applied. Any nitrate lost in drainage water will also be accompanied by Ca, Mg and K in approximately the proportions present in the so-called exchangeable (available) form. In soils with little capacity for retaining sulphate, any sulphate lost in the drainage water will also be accompanied by the bases Ca, Mg and K.
So little phosphate is lost in drainage water that it is not responsible for any significant loss of bases. Another important source of loss of bases is when the soil had been limed to a high pH so that the soil water contains bicarbonate. This will also be balance with bases, mainly Ca, but also some of the others. Mg will also be lost from soils where crops such as hay are removed and fed off elsewhere, where dung and urine are returned unevenly or not at all and in milk and other animal products.
Pasture species also vary in composition both among themselves and with stage of growth. Perhaps the most important variation is caused by the difference between grasses and clovers. Clovers tend to have higher levels of Ca and Mg than grasses which are correspondingly richer in K. The higher incidence of magnesaemia in winter and early spring may arise because of grass dominance at these times as well as lower intake of herbage. The excessive use of fertiliser N by stimulating grass growth at these times may exacerbate the problem.
What to do about it
I am often asked if farmers should return to the soil all nutrients lost in produce sold off the land, or in drainage water, or by any other means. In the long run this must surely be the case but levels of some essential nutrients in a soil may be so high that deficiencies will not arise for hundreds of years. An illustration is afforded by the presence of calcium carbonate in some soils formed from limestone or which have been over limed. Such soils may have pH levels as high as 8.5. Ca would be removed from these soils in produce or in the drainage water but no-one in his right mind would dream of liming such a soil until the pH dropped to about 6 or less, which could take very many years. To use lime would be like banking money at no interest during a period of inflation with the added chance that it could do positive harm by intensifying some trace element deficiency such as iron, manganese, boron, copper, zinc or cobalt.
Soils which are rich in available Mg with large reserves of Mg – minerals are well known in New Zealand and there is no point in adding Mg to such soils until levels decline to low values which could take generations. Hypomagnesaemia in animals may still occur on such soils and in these cases direct Mg supplementation of the diet would be called for.
Soils where available Mg and mineral reserves are low are also known and these are soils where dolomite or some other source of Mg should be applied when the incidence of hypomagnesaemia is a problem.
I always think is is a great pity that the only outcrop of dolomite limestone in New Zealand is at Collingwood, miles from nowhere. Dolomite is a form of limestone used throughout the world as a liming material. It outcrops across much of England and is used widely for liming. It is a chemical mixture of calcium carbonate (the major ingredient in ordinary limestone) and magnesium carbonate. Collingwood dolomite is a very pure limestone containing about 60% calcium carbonate and 40% magnesium carbonate and has at least the same liming value as pure calcium carbonate. If the dolomite outcropped over most of New Zealand it would be the cheapest material to use to raise soil pH and to cure Mg deficiency at the same time.
Most farmers know that we lime acid soils to raise the pH and correct soil acidity. We need to get the pH to about 5.8 for ryegrass – white clover pastures and nearer to 6.5 for lucerne and some horticultural crops. What is less appreciated is that although we apply 400kg of Ca in every tonne of pure calcium carbonate, the Ca is rarely needed to correct Ca deficiency except on the most acid soils.
The reason why white clover fails to grow well as soils become more acid is firstly because Mo becomes less available, and Mo is vital for the N – fixation process: then the bacteria that do the N-fixing begin to die out and then aluminium, manganese and other metals begin to dissolve more readily and become toxic to the plants.
Long before Ca deficiency begins to affect clover growth on most soils, the clovers will have been seriously affected by one or more of the other factors.
Thus the beneficial effects of liming usually result from raising soil pH and the consequent effects on chemical, biological and often physical properties of soils. Dolomite will do this just as readily as ordinary limestone and a tonne supplies about 240kg of Ca and 100kg of Mg.
Where soils are acid and hypomagnesaemia a problem, dolomite should be considered as a cure for both. Where dolomite is as cheap as ordinary limestone there is no question that dolomite is to be preferred. With increasing distance from the quarry, dolomite becomes increasing expensive relative to local limestone’s.
On quite acid soils where initial dressings of 5 tonnes/ha limestone is recommended, both ordinary limestone and dolomite could be considered in the ratio say of 3:2 or 4:1. In the first case (assuming the ordinary limestone to be 100% pure, which is rarely the case) the total application of 5 tonnes would supply 1680kg of Ca and 200kg of Mg and in the second case 1840kg of Ca and 100kg of Mg. Thereafter a tonne of dolomite every four years should be adequate to maintain the Mg status as well as the pH of the soil. Depending on the incidence of hypomagnesaemia and the results of soil tests it may be enough to alternate the use of dolomite and ordinary limestone every four years or so, using more ordinary limestone if necessary to maintain soil pH.
The cost of transporting dolomite to Kerikeri may make a dairy-farmer think twice about using it whereas a grower of kiwifruit would not give it a second thought. Alternatives such as serpentine rock, serpentine super, calcined magnesite and Epsom Salts are available for applying to soil or herbage, hay or silage, or in water or licks and some may offer cheaper methods of preventing hypomagnesaemia. Individual farmers will have to make their own calculations, but in my view, dolomite is the ideal material to use on acid soils low in Mg: unfortunately transport costs may make it a costlier alternative to some other materials. However remember that ground serpentine rock is much less effective than dolomite as a source of Mg and almost twice as much is needed to supply the same amount of available Mg; it has no liming value either.
T W Walker.
1984, revised 1997.