IMPORTANCE FOR PLANT LIFE

Magnesium plays a role in many metabolic functions in plants (such as carbohydrate and protein formation) as well as in transport mechanisms. Its most critical role is being located at the center of the chlorophyll molecule in leaves and its direct connection with photosynthesis. Magnesium deficiency leads to chlorosis (yellowing) and reduces yield. In forage crops, magnesium deficiency can also negatively affect animal nutrition.

ABSORPTION MECHANISMS

The amount of magnesium that diffuses from minerals into the soil solution is relatively limited. If the clay–humus complex is already saturated with calcium or potassium, Mg plays a more secondary role within this complex. Roots expend energy to absorb magnesium; in other words, Mg uptake occurs largely through active transport rather than passive movement.

INTERACTIONS AND SPECIAL FEATURES

The most decisive factor in uptake imbalances is the K₂O/MgO ratio. This ratio should be maintained at approximately 2:1. If it rises above 3:1, corrective measures should definitely be taken.

Note: If the ratio is expressed as K/Mg, the optimum range is approximately 0.8 to 1.2.

SOIL MAGNESIUM & CYCLE

Magnesium in soil can have different origins: igneous, marine sedimentary, and organic sources. Once it is dissolved in the soil solution, magnesium becomes susceptible to leaching, which contributes to annual Mg losses. Magnesium can also influence the soil’s pH.

CYCLE DIAGRAM

1. The recycling of nutrients contained in manure, plant residues, and organic by-products from human activities is an important fertilizer source for magnesium as well.

2. Magnesium is found as magnesium carbonate in dolomite, a hard limestone extracted from quarries. It is also obtained from underground mines as oxide, hydroxide, or as magnesium sulfate (kieserite).

3. Dissolved Mg²⁺ in soil originates from magnesium released from the clay–humus exchange complex.

4. The movement of dissolved magnesium to deeper layers with excess soil water (leaching) must be considered when preparing fertilization plans.

5. Surface runoff and erosion can also carry magnesium bound to solid particles out of the field.

6. Plant roots can take up magnesium only in the form of dissolved Mg²⁺ in the soil solution.

7. Magnesium is removed from the field with harvest, primarily as food and feed products.

INDICATOR

In soil tests, magnesium is determined as exchangeable magnesium measured with extraction methods that are similar across laboratories. The optimum magnesium level is evaluated in relation to the cation exchange capacity (CEC), and the target is for about 6% of the CEC to be occupied by Mg²⁺. Therefore, knowing the CEC value is also necessary to accurately determine the soil’s capacity to supply magnesium.

SENSITIVITY TABLE & SYMPTOMS

Magnesium deficiency primarily affects older leaves. Typically, interveinal chlorotic (yellowed) patches appear. If the deficiency persists, leaf tips become necrotic and dry out. In general, the chlorophyll content and the number of chloroplasts in the plant decrease; this reduces photosynthetic capacity and negatively affects yield.

EXCESS & REQUIREMENT

Magnesium excess may occur, especially when fertilization has historically been imbalanced between magnesium and potassium. If soil tests show that the K₂O/MgO ratio has fallen below 1, it is recommended to emphasize potassium fertilization. This helps restore the magnesium–potassium balance.

ORIGIN

Magnesium is very abundant in the Earth’s mantle layer beneath the crust. Peridotite, considered one of the most common rocks in the solar system, is a rock that contains more than 40% silicates and more than 40% magnesium.

Magnesium is also found in marine sedimentary soils: in carbonate form (as dolomite) or in sulfate form (as kieserite). The magnesium used in fertilizer production is mainly obtained from these two sources.

KEY FACTORS

MAGNESIUM CONTENT IN SOIL

Adequate magnesium content in soil should be approximately 120–200 ppm expressed as MgO. Values below 120 ppm indicate that the soil is poor in magnesium; below 80 ppm is considered extremely insufficient.

ANTAGONISM

A K₂O/MgO ratio above 2 reduces magnesium availability to the plant. If the ratio rises above 3, there is a serious risk of blockage and Mg uptake can be significantly restricted.

TEXTURE

Sandy soils increase the risk of magnesium leaching. Because their cation exchange capacity is low, Mg²⁺ can be carried more easily into deeper layers of the soil profile.

CLIMATE

After winter, especially following rainy periods, magnesium availability may be limited—partly due to leaching and partly due to changes in soil temperature and biological activity.

pH

When pH rises above 7.5, the risk of magnesium deficiency increases due to saturation of the clay–humus complex. On the other hand, when pH drops below 5.5, Mg availability also decreases. In other words, magnesium uptake can face problems in both highly acidic and overly alkaline conditions.