Magnesium in the Soil

Magnesium is a component of several primary and secondary minerals in the soil, which are essentially insoluble, for agricultural considerations. These materials are the original sources of the soluble or available forms of Mg. Magnesium is also present in relatively soluble forms, and is found in ionic form (Mg++) adhered to the soil colloidal complex. The ionic form is considered to be available to crops.


Magnesium is essential for many plant functions. Some of them are:

  • Photosynthesis: Mg is the central element of the chlorophyll molecule.
  • Carrier of Phosphorus in the plant
  • Magnesium is both an enzyme activator and a constituent of many enzymes
  • Sugar synthesis
  • Starch translocation
  • Plant oil and fat formation
  • Nutrient uptake control
  • Increase Iron utilization
  • Aid nitrogen fixation in legume nodules

Factors Affecting Availability

  • Soil Mg content: Soils inherently low or high in Mg containing minerals
  • Soil pH: Low soil pH decreases Mg availability, and high soil pH increases availability
  • Soil Mg:Mn ratio: High available Mn can directly reduce Mg uptake. This may be independent of the acid conditions normally associated with excess available Mn in the soil.
  • Soil CEC: Low CEC soils hold less Mg, while high CEC soils can hold abundant Mg. However, if a high CEC soil does not happen to have strong levels of Mg, it will tend to release less of the Mg that it holds to the crop.
  • Cation competition: Soil with high levels of K or Ca will typically provide less Mg to the crop
  • High cation applications: High application rates of other cations, especially K, can reduce the uptake of Mg. This is most common on grasses, and corn seems to be the most sensitive grass.
  • Low soil temperatures


  • Other cations: Being a major cation, Mg availability is related to the soil CEC, and it is in competition with other major cations such as calcium (Ca++), potassium (K+), sodium (Na+), ammonium (NH4+), iron (Fe++), and aluminum (Al+++). It appears that potassium is a stronger competitor with Mg than it is sometimes considered to be. We have frequently seen that whenever the soil K level is higher than desired, or when the soil K:Mg ratio (in lb./acre) is above 1.5:1 (or the soil Mg:K ratio is less than 0.67), plant Mg levels are reduced. This effect occurs sooner and more severe in grasses, especially corn, than in other crops. It may seem inconsistant to list a specific numerical K:Mg ratio, when we earlier stated that specific numerical ratios are not valid. However, we are simply stating that Mg problems are more frequent or severe when the soil K:Mg ratio exceeds 1.5. We are not claiming that there is an ideal K:Mg ratio.
  • Phosphorus: Phosphorus uptake is often enhanced when applied with Mg fertilizers. However, mixing some liquid or suspension sources of P and Mg can lead to a reaction can result in the formation of a large amount of precipitated material, to the point of near solidification of the mixture.
  • Sulfur: Sulfur leaching is often increased where supplemental Magnesium is applied

Deficiency Symptoms

The classic deficiency symptom is interveinal chlorosis of the lower/older leaves. However, the first symptom is generally a more pale green color that may be more pronounced in the lower/older leaves. In some plants, the leaf margins will curve upward or turn a red-brown to purple in color. Full season symptoms include preharvest leaf drop, weakened stalks, and long branched roots. Conifers will exhibit yellowing of the older needles, and in the new growth the lower needles will go yellow before the tip needles.


Magnesium toxicity’s are rare. Crops grown on heavy Montmorillonite clay soils that have been poorly fertilized with potassium may exhibit excesses of magnesium in their tissue. But, before the tissue level approaches toxicity, potassium deficiency will occur. Higher tissue levels of magnesium are usually found in the older leaves on the plant and may be associated with diseased or damaged leaves.

Grass Tetany

This is a magnesium deficiency in ruminants. It occurs when livestock are fed a diet of forages low in Mg. Some of the factors that reduce Mg uptake in forages are:

  • Weak soil Mg levels
  • Acid soils
  • High soil K:Mg ratios (lb./acre). Ratios greater than 1.5:1 are a serious risk in most forages, but corn silage or green chop would be a concern on a soil with a K:Mg ratio of 1:1, or higher, especially if other negative factors exist.

Some other factors that seem to increase the incidence of grass tetany are:

  • Long periods of cool cloudy weather in the spring
  • High nitrate N (NO3-N) in the forage. This can be related to first point relating to cool, cloudy weather.
  • Poor drainage
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