Magnesium fertilizer application strategy in modern agriculture

Magnesium deficiency in the arable layer is becoming more acute every year. The reason lies not only in the natural leaching of the element by precipitation and its removal with the harvest but also in the quality of modern mineral fertilizers. Over the last half-century, the average NPK content in fertilizers has increased from 22.1% in 1950 to 55.3% in 2000, partly due to the purification of raw materials from magnesium "impurities." As a result, in light soils, the balance of this element is consistently negative, provoking magnesium starvation in plants.

• Solution application rate (pome and stone fruits) — 1000 l/ha
• Solution application rate (vegetables) — 200 l/ha
• Fertigation — 3–5 l / 1000 m²

Crop magnesium requirements and leaching risks

The magnesium requirement of plants depends directly on the crop type and planned yield. On light soils, it ranges from 65 to 270 kg/ha per crop rotation. Nutrient removal from the soil increases sharply if the share of grain legumes, potatoes, vegetables, and lupine increases in the cropping structure.

Magnesium ions in the soil solution are highly hydrated, which is why the soil retains them weakly. Annual losses of the element due to leaching by precipitation range from 10–20 kg/ha, with these figures being higher in light soils and wet years. The application of potassium chloride triggers magnesium leaching most severely. When using potassium sulfate and single superphosphate, losses are lower, and replacing single superphosphate with double superphosphate (which contains no gypsum) minimizes magnesium leaching.

Deficiency diagnosis and magnesium availability factors

Magnesium fertilizers are applied when the content of the element in plants is insufficient or low. Exceeding the optimal nutrition level is pointless: it will not lead to a yield increase but will result in excessive growth of vegetative mass to the detriment of product quality. With a significant excess of magnesium, crops show signs of toxicity.

The primary visual sign of magnesium starvation is patchy necrosis, which makes leaves variegated. The veins remain green, while the areas between them fade. Because magnesium is mobile, the plant redistributes it from older lower leaves to the upper growth points, which is why the deficiency always starts from the bottom. Prolonged starvation disrupts metabolism and causes tissue death.

Depending on the crop, the color of the damaged leaf areas between the veins varies:

• potato — pale green;
• cereals, tomatoes, citrus — light yellow;
• apple — yellow-brown;
• plum, peach, berry crops — orange, red, or purple;
• cotton — crimson;
• corn — purple-reddish spots alternating with whitish chlorotic and necrotic stripes.

Cereals remove little magnesium but suffer from its deficiency more than industrial and vegetable crops. Their fibrous root system is shallow and unable to reach the element from the subsoil horizon. The problem is most acute in light soils, where magnesium reserves are minimal.

Magnesium uptake is strongly influenced by the composition of the soil solution. Excess potassium, ammonium, sodium, and calcium block its entry into the roots, with monovalent cations acting as stronger antagonists. Sodium interferes with magnesium absorption in sodium-loving crops — beets, cabbage, barley, lupine, and flax. Acidic soils with a high concentration of aluminum and manganese also block magnesium uptake, while nitrate nitrogen, on the contrary, facilitates its absorption.

Liming accelerates the nitrification of ammonium nitrogen and converts mobile forms of aluminum into an inactive state. This improves plant nutrition, but on soils with a deficiency of available magnesium, the use of calcium fertilizers does not fully solve the problem.

• Average yield increase on responsive soils — 20–25%
• Magnesium content in manure — 0.08%
• Magnesium content in liquid manure — 0.05%

The magnesium requirement is calculated based on the target yield, the level of element deficiency in the soil, its acidity, and also taking into account the rates of potassium and ammonium nitrogen fertilizers. The laboratory marker for the requirement is the volume of magnesium displaced by 0.025 N CaCl2 (this indicator correlates well with the accumulation of the element in plant tissues) or a 1 N KCl extract.

On sandy, loamy sand soils, and red soils, magnesium application is mandatory. When calculating rates, the requirements of specific crops are taken into account.

On acidic soils without the application of manure, soluble magnesium fertilizers are applied annually. On neutral soils, the element is leached weakly, so it is provided only for magnesium-loving crops, necessarily taking into account the input from organic matter. During chemical land reclamation of acidic soils with magnesium-containing limestones, the supply of the element is sufficient for one or two rotations of crop rotation.

Raw material sources and classification of magnesium fertilizers

Russia holds vast reserves of magnesium-containing rocks. Three groups of rocks are used as raw materials for fertilizer production and direct application:

• magnesium silicates — dunite, serpentinite, olivinite;
• carbonate rocks — dolomite, magnesite;
• raw natural salts — kainite, carnallite, langbeinite, polyhalite, kieserite.

According to their solubility, magnesium fertilizers are divided into three groups:

• Water-soluble (raw salts and their processed products) — epsomite (magnesium sulfate), kainite, carnallite.
• Citrate-soluble — fused magnesium phosphate. Nutrients from this fertilizer are well absorbed by plants.
• Water-insoluble (finely ground natural materials and rocks) — dolomite flour, magnesite, vermiculite, serpentinite. They are used for liming acidic soils, as magnesium is released upon interaction with the acidic soil solution.

Magnesium fertilizers can be simple or complex. Complex fertilizers contain two or more nutrients (nitrogen-magnesium, phosphorus-magnesium, potassium-magnesium, boron-magnesium, lime-magnesium). A three-component fertilizer — magnesium ammonium phosphate — is also produced.