Chapter 15. Magnesium


Magnesium is a constituent of chlorophyll. It is also active in the metabolism of phosphorus. A deficiency rarely affects yield but can reduce the nutritional quality of crops.

Cation exchange is the only means of holding magnesium against losses in the soil.

Fertilizing for magnesium in soils naturally low in magnesium requires an inorganic amendment. It is especially difficult if both calcium and potassium are high.

Table 24. Fertilizers For Supplying Magnesium lists the magnesium content of typical fertilizers.

Magnesium In The Plant

Magnesium puts the Green in green plants. It is the only metal which is a constituent of chlorophyll. Chlorophyll is identical to the hemoglobin in blood, except that chlorophyll contains magnesium instead of iron. It is not too excessive to claim that a lack of magnesium produces anemic plants.

Only about 20% of the magnesium in plants, however, is in chlorophyll. The rest functions as a regulator for various metabolic processes. Magnesium is necessary in every operation involving phosphorus; an apparent phosphorus deficiency can sometimes be tempered with magnesium fertilizer. In addition, magnesium influences nitrogen metabolism and is important in the assimilation of carbon dioxide during photosynthesis.

Magnesium and sulfur are the most neglected of the major nutrients, sulfur no doubt because until recently fertilizers contained enough to satisfy plant requirements. In the case of magnesium, nothing short of a gross deficiency seems to affect yields, unless phosphorus is also low.

This masks, however, the effect of magnesium on the nutritional value of crops. Like sulfur, some amino acids contain magnesium; a deficiency will result in an insufficient supply of true proteins requiring those amino acids and an enlarged pool of free amino acids. The missing proteins reduce the quality of produce for both animal and human consumption.

An antagonistic relationship exists among calcium, magnesium and potassium: all three are cations, and the total absorption of cations by plant roots is limited. Plants, however, have a built-in preference for potassium, the soil supply of which is usually adequate to excessive; and calcium is the predominant component of lime. Magnesium is rarely prominent in a soil amendment, and it often ends up short.

There are no characteristic symptoms of a mild magnesium deficiency - a moderate deficiency may result in a yellowing of leaves between the leaf veins - perhaps only an awareness that the plant is not functioning or producing well. Owing to the reduced assimilation of carbon dioxide, growth is stunted, and ripe fruit lacks sweetness. A deficiency retards phosphorus metabolism and protein production.

Magnesium In The Soil

Magnesium behaves much like calcium in the soil. Both are easily leached in humid areas. Conservation of either depends upon the cation exchange properties of the soil.

The age of the soil and weather conditions influence the cation exchange capacity and the presence of magnesium. Owing to the particular clays in many of the young unweathered western soils, the exchange capacity is usually high. In addition, these soils are also high in magnesium. Not all soils in the west are so blessed, but many of them are natural cation reserves and very well filled.

Older, weathered soils in the humid areas of the east and south, however, are less favored. Except for some soils (in Pennsylvania for example), old soils are especially leached of magnesium, and the clays are poor at contributing to the cation exchange capacity. Organic matter is the predominant influence in determining the exchange capacity. Moreover leaching has left these soils acid, and so the exchange reservoir is filled mainly with non-nutritive acid ions.

Magnesium Balance

The following discussion is relevant only where magnesium is low in the soil. Cation balance is not critical where magnesium is moderate to high, unless it begins to approach levels of the order of 70% of the cation reservoir. And where that extreme situation does exist, I have no help to offer.

Almost all soils in humid areas must be limed periodically. The question then arises, what kind of lime is appropriate. It is reasonable to suppose that a balance should exist among the nutrient cations (calcium, magnesium and potassium).

We do know that excessive potassium can lead to a magnesium deficiency and sometimes a calcium deficiency. An excess of calcium has been responsible for deficiencies in both magnesium and potassium. Experiments have led to the conclusion that, for many crops, the soil should contain at least as many magnesium ions as potassium ions1.

Recently, one criterion for cation balance has been adopted by several soil testing laboratories. According to this criterion, 60-70% of the soil reservoir should be filled with calcium, 10-15% with magnesium, 2-5% with potassium and the remainder with acid ions. Within the last few years, however, the hypothesis of cation balance has been challenged, and experiments have shown that yields are substantially independent of these or any similar guidelines based upon the percentage of ions in the cation reservoir. A controversy still exists on the issue.

One issue is that experiments used to test this criterion are set up so that all other nutrients are well supplied. This masks the relationship between phosphorus and magnesium, because magnesium has less importance if phosphorus is high.

Furthermore, tests based on yield alone is an additional bias against magnesium, which is more important in determining the quality of a harvest rather than its quantity2.

The concept of an appropriate distribution of the nutrients which make up the cation reservoir does have two uses. One is to determine the amount of lime required to raise the pH to a desired point, and the other is to set a minimum level for magnesium. Tentatively, the following may be a useful guide: In terms of lbs/acre, the soil should contain at least one tenth as much magnesium as calcium, and at least 60% as much magnesium as potassium 3.

A guideline for setting minimum levels of potassium should take into account the need to balance nitrogen but not so high as to overwhelm magnesium or calcium. The proper nitrogen/potassium balance is determined by the crop requirements; tables 3. Estimated Fertilizer Requirements - Field Crops - 5. Average Nutrient Requirements For Vegetables may be useful for the purpose.

In practice, a conflict between balancing nitrogen and not overpowering magnesium should occur only with a depleted, weathered soil possessing a low organic content; such a soil has a low cation exchange capacity and little ability to store magnesium. In order to preserve the proper magnesium/potassium balance in that case, potassium and consequently nitrogen should be limited; which of course affects the yield. However, table 2. Effects Of A Nutrient Disorder On Crop Quality has examples where heavy fertilization with potassium fertilizers without taking into account the necessity of balancing magnesium and calcium affects the appearance of fruit and vegetable crops.

One reason why the relationship among calcium, magnesium and potassium can be so loose is that, within a wide range of values, excessive magnesium is not a concern. Some soils in California have enough magnesium to fill 40% of the cation reserve and yet produce high yields. To be sure, soils with 70% magnesium can not grow crops, but this still leaves room for variation.

Magnesium Fertilizers

Table 24. Fertilizers For Supplying Magnesium lists the magnesium content of typical organic materials and of the principal fertilizers.

Most organic residues have a small but significant amount of magnesium. About 20 - 30 lbs of magnesium/acre can be supplied by fresh poultry manure spread at a rate of 5 tons/acre and the other manures at 10 tons/acre, or a hay mulch made from bales split up into one-inch layers4. This quantity is enough to supply most crops with sufficient magnesium, although some of the magnesium is likely to be lost by leaching. Compost is an excellent source of magnesium, but not enough information is available to indicate typical amounts.

These residues, however, would add a much greater amount of potassium than magnesium. For example, table 24. Fertilizers For Supplying Magnesium suggests an application of 10 tons/acre of non-poultry manure for about 25 lbs of magnesium. However, table 9. Nutrient Content of Manure shows that nonpoultry manure may contain about 10 lbs of potash/ton; so a rate of 10 tons/acre will add 100 lbs of potash, or four times as much potassium as magnesium. Similarly, a hay mulch will add more than ten times as much potassium as magnesium.

Soybean meal (and probably cottonseed meal and seedcake residues) is a good source of magnesium, but it is likely to contain about four times as much potassium as magnesium. Only poultry manure seems to have a reasonable balance, supplying a bit less than twice as much potassium as magnesium.

Most organic residues are better sources of magnesium than of calcium, but they are not an ideal magnesium fertilizer. If a soil has a low ratio of magnesium to potassium, most organic residues will not improve the ratio, and they may make it worse.

The two most common inorganic fertilizers for magnesium are dolomitic limestone and sulfate of potash magnesia. Dolomitic limestone is the cheapest of inorganic magnesium fertilizers and is the logical choice for acid soils. But sulfate of potash magnesia is useful if potassium is also low.

Often, however, owing perhaps to a mistake in fertilizer usage, a soil may be high in both calcium and potassium, in which case neither of these amendments is appropriate. The two alternatives are epsom salts and magnesia, both soluble. Neither one is satisfactory, for reasons to be given presently, and they are both customarily used in small quantities, perhaps enough for temporary relief of a magnesium deficiency, but not enough to raise the soil reserve. They are best used only in an emergency or after a test trial to determine their effectiveness.

Epsom salts are a natural mineral, although they are also synthesized. Magnesia is usually made by heating magnesite, a naturally occurring magnesium carbonate, to drive off the carbon dioxide, a process similar to that used in the production of burned lime from calcitic limestone. Magnesia is a common constituent of commercial fertilizers fortified with magnesium.

Epsom salts are expensive and impractical to spread in large amounts; quantities of the order of 150-200 lbs/acre are common, but this supplies only a small amount of magnesium. An alternative is to apply epsom salts as a foliar spray several times during the season, at a rate of about 10 - 15 lbs/100 gallons of water, saturating the plants [51].

Magnesia is cheaper to add in larger quantities, but it will raise the pH. Spreading magnesia might be feasible in small amounts where a slight pH rise is tolerable. The maximum permissable rate of application varies with individual soils, because it depends upon the permissable rise in the pH and the cation exchange capacity. The liming value of magnesia is calculated in appendix C. Acid and Basic Fertilizers - magnesia . Another disadvantage to magnesia is that it is a dehydrating agent and might affect soil life.

In summary, supplying sufficient magnesium while maintaining a good balance with calcium and potassium is difficult in magnesium-deficient soils with a low organic content. It requires sufficient planning.

1 that is, the total number in solution and in the cation exchange reservoir    [return to text]

2 This argument depends on a definition of quality to include not only the appearance of produce but also the nutritional value.    [return to text]

3 These suggestions are based upon the following reasoning: The conversion factor between the pounds of a nutrient and the number of atoms, or ions, varies with the nutrient. So a magnesium/calcium ratio of 10/100 in terms of lbs/acre is equivalent to a ratio of 10/60 in terms of ions/acre. This idealized 10/60 ratio is an extrapolation from the earlier criterion (10% magnesium and 60% calcium). Similarly, a magnesium/potassium ratio of 60/100 in terms of lbs/acre is equivalent to a ratio of 1 in terms of ions/acre. As noted earlier, the relation between magnesium and potassium does have an experimental basis for some crops.

The term ions/acre is not a standard unit of measure. The customary measure in soil science is milliequivalents/100 grams of soil, abbreviated as meq/100 g (although this unit is now being replaced in some technical journals). Exchangeable cations and the cation exchange capacity are reported in terms of this unit. The conversion between lbs/acre and meq/100 g is given by the formula, (lbs/A) = F * (meq/100 g), where F = 20 * (FW)/(V) (FW is the formula weight, and V is the valence of the ion). The idealized magnesium/calcium ratio stated above is the same in either case, but a magnesium/potassium ratio of 1/1 in terms of ions/acre is 2/1 in terms of meq/100 g.    [return to text]

4 Assuming a bale size 11 by 18 by 30 inches weighing 35 lbs, we would find that one bale split up into one-inch layers will occupy a space of about 56 sq. ft. Thus 774 bales would be required to cover one acre, and the total weight of the bales would be about 13-1/2 tons. If each ton supplies two lbs of magnesium, the mulch would add about 27 lbs of magnesium/acre.    [return to text]

© 2013 Robert Parnes

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