Magnesium is a part of chlorophyll, therefore its role in the plant is exceptional. Magnesium is of great importance for respiratory metabolism; it catalyzes a number of reactions of formation of energy-rich phosphate bonds and their transfer.
Since phosphate bonds enriched with energy are actively involved in various syntheses, as for magnesium, without these processes are doomed to the impossibility of independent implementation. Magnesium improves the mobility of phosphorus and contributes to its greater absorption by plants, increases the use of iron in plants. Magnesium accelerates the ripening of seeds and affects their quality, affects the formation of inflorescences and fruits, early maturity and uniformity of maturity, helps to improve the quality of the crop.
Fig. 1. The value of magnesium in human life
The causes of magnesium starvation of crops can be the following factors:
- Inadequate available magnesium in soil:
on sod-podzolic and gray forest sandy and loamy soils – less than 80 mg / kg of exchange MgO;
on sod-podzolic and sulfur forest loamy soils – less than 120 mg / kg of exchange MgO;
on peat-bog soils – almost everywhere.
- The neutralization of soil acidity by purely calcium forms of lime (CaCO3) leads to a violation of the optimal ratio of Ca to Mg in the soil solution and the absorbing complex, which negatively affects the availability of magnesium by plants. There is evidence that even on soils with a high content of metabolic magnesium with a Ca to Mg ratio of more than 8 for field crops, and more than 10 for perennial plants, plants use a lack of available magnesium, which also necessitates the application of magnesium with fertilizers.
- The use of ammonium nitrate, ammonia water and liquid ammonia in crops at doses of more than 120-180 kg / ha of active ingredient (AI) causes the antagonism of NH4 + and Mg2 + ions.
- The use of potash fertilizers in doses of more than 140-180 kg / ha causes antagonism between K + and Mg2 + ions.
- A significant increase in losses of soil magnesium with leaching from the habitable layer with infiltration waters at high levels of nitrogen and potassium fertilizers.
- Magnesium deficiency in plant nutrition causes a violation of their assimilation of nitrogen, potassium and calcium, which negatively affects the size and quality of the crop.
- Magnesium starvation occurs when the magnesium content in plants is below a certain level, called the “lower limit” of its normal content, which is established by the method of plant diagnostics in phases of development that are individual for each culture.
Table 2. External signs of magnesium starvation
| Culture | Development phase | Signs of starvation |
| Winter crops | Tillering | On calcareous soils, the upper leaves are pale green banded, the lower leaves are dark red, they are folded into a tube, with a strong deficiency of magnesium, they dry out and die. After overwintering, the plants die. On calcareous soils, the lower leaves are yellowish-green in color with stripes. |
| Winter and spring cereals | Exit to the tube | The leaves are yellow-green, curled inwards. The maturation of bread is delayed for 10-12 days. |
| Peas and other legumes | Tillering – flowering | Typical signs of chlorosis are observed on the lower leaves: the color turns from pale green to yellowish, brown spots appear on the edges, gradually capturing the entire intervein space to the center of the leaf. The number of nodules on the roots is sharply reduced. |
| Potato | On no dung background – in the budding phase, on manure – with tuberization | Leaves acquire a yellow-green and yellow color, become brittle, then become covered with brown spots, dry, fall off. With a strong long-term deficit, the tops dry out prematurely. |
| Corn | Phase 4-6 leaves | The lower leaves brighten, a distinct bandedness appears, green stripes alternate with yellow. On acidic soils, some leaves have a red-violet color, later brown spots appear on them. |
| Lupine | At the beginning of flowering | The lower leaves turn yellow, slightly curl. The formation of nodules on the roots is reduced. In yellow lupine, the edges of the leaves turn yellow. The blue lupine is red-violet. Leaves fall ahead of time. |
| Mangel | Phase 6-8 Leaves | The lower leaves, starting from the top, turn pale, then yellow; the veins remain green, then brown spots appear in the interstitial space, which spread over the entire leaf. Leaf petioles are brittle. Dead tissue has a dark brown color, they crumble. |
| White cabbage | During the growing season | The color of the lower leaves brightens, from the upper edge (between the veins) to the center of the leaf, then they become cream, the veins remain green. With severe starvation, the color changes to red-violet with alternating green in large veins – “marbling” of the leaf. |
| Cauliflower | Bloom | The signs are the same as that of white cabbage. |
| Tomatoes | Budding – fruit formation | Typical signs of chlorosis develop on the lower leaves, which then cover the upper leaves. petioles become juicy and brittle. Redness of the fruit slows down, they fall. |
| Cucumbers | Flowering and during intensive fruiting | The leaves turn yellow, near the veins bright green, become juicy and brittle, then the dying tissues acquire a brown color. Leaves fall, and signs appear on the upper tiers. Small fruits are formed. |
| Apple tree | During vegetation | Typical signs of chlorosis appear on the lower leaves, they fall prematurely. |
| Cherry | Also | Chlorosis begins in the middle of the leaf. Dead tissue forms spots of brown-brown color, and the leaves fall prematurely. |