Plants need to absorb a large number of nutrients for normal growth. The lack of essential nutrients will limit the growth potential of plants. In higher plants, except for the three elements carbon, hydrogen, and oxygen, which mainly come from water and carbon dioxide, other essential nutrients are obtained from the soil in the form of inorganic ions, called mineral nutrients.

Most of the nutrients in soil are originally derived from the weathering of soil-forming minerals and then participate in the earth's bio-chemical cycles. Based on the "mineral nutrition theory" of plants, in modern agricultural production, increasing the supply of mineral nutrients will increase crop yields to varying degrees. However, in high-yield agricultural systems, where most of the crop's biomass leaves the cultivated area, the unidirectional migration of nutrients from the soil to the plant makes it particularly important to replenish soil nutrients through fertilization.

In the economic crop areas of the south, the application amounts of macronutrients such as nitrogen, phosphorus and potassium are generally very sufficient. Physiological disorders caused by the lack of nutrients are mostly caused by the lack of medium and trace elements. During field promotion in southern economical farming areas, it was found that plant magnesium deficiency symptoms are relatively common in fruit trees and some fast-growing crops. The acidic red-yellow soil and sandy soil in the south have low magnesium content, and heavy rainfall causes serious leaching losses, resulting in low soil available magnesium content. In addition, the land in the south has a high multiple cropping index and a large demand for nutrients. Unbalanced nutrient inputs will also affect the effectiveness of magnesium. Therefore, understanding the nutrient requirements of plants and the physiological processes of nutrient absorption is very necessary for reasonable nutrient management.

Plants have a high demand for magnesium

Plants have a large demand for calcium and magnesium other than nitrogen, phosphorus and potassium, but the magnesium contained in soil or irrigation water is usually sufficient for growth. However, in high-yield agricultural systems, if fertilization management is still carried out according to the original definition of medium elements, magnesium deficiency symptoms may easily occur on some soils, which is not conducive to the formation of yields.
 

Different crops have different sensitivity to magnesium. Crops such as citrus and potatoes are very sensitive to magnesium. When there is insufficient magnesium in the soil, magnesium deficiency symptoms are easily observed on the leaves. For crops that are sensitive to magnesium, such as soybeans, grapes and fruit trees, the application of magnesium fertilizers will increase yield and quality to varying degrees. Crops such as rice and wheat are not sensitive to magnesium. In fact, the critical concentration of magnesium in plants varies greatly depending on plant species, varieties, organs and development stages. These are all factors that need to be considered when managing magnesium nutrition.

Magnesium deficiency symptoms and causes

Magnesium is easily mobile in plants, and magnesium deficiency in plants first manifests itself in the middle and lower old leaves. In dicotyledonous plants, interveinal chlorosis appears, gradually changing from light green to yellow or white, with brown or purple spots of varying sizes appearing, but the veins remain green. In severe cases, premature aging and leaf shedding may occur. . Graminous plants show dark green spots at the base of the leaves and light yellow in the rest. When there is severe magnesium deficiency, the leaves fade with stripes and necrotic spots appear on the leaf tips.

Magnesium deficiency symptoms in crops tend to occur when fruits or storage organs are enlarged. Magnesium will be transferred to the fruit during the ripening process. Old leaves and leaves near the fruit will turn yellow first, and the symptoms will be obvious. When seeds germinate and seedlings grow, magnesium is transported to the required parts. Therefore, magnesium deficiency symptoms are generally less likely to occur in the early stages of plant development. Magnesium deficiency mostly occurs in the middle and late stages of plant growth. If it occurs in the later stage, the output will not be greatly affected. If it occurs in the early stage, both output and quality will be seriously affected.

The lack of magnesium in plants is usually caused by two reasons: First, the magnesium content in the soil is low and cannot meet the nutrient needs of plants. Second, the magnesium content in the soil is not low, but due to the interaction of other nutrient molecules in the soil, the absorption of magnesium by the roots is low.

How to supplement plant magnesium

To supplement magnesium, targeted measures should be taken based on the reasons for magnesium deficiency in plants. For situations where the soil is deficient in magnesium or when crops require a large amount of magnesium, an appropriate amount of magnesium fertilizer must be supplemented; for magnesium deficiency caused by soil nutrient imbalance, in addition to a reasonable supplement of magnesium fertilizer, the input of other nutrients must also be controlled.

Apply magnesium fertilizer

Magnesium fertilizers can be divided into water-soluble, slightly soluble and sparingly soluble magnesium fertilizers according to their solubility. Water-soluble magnesium fertilizers dissolve quickly and are easily absorbed by plants, such as magnesium sulfate, potassium magnesium sulfate, magnesium chloride, etc. Slightly soluble magnesium fertilizers have slow effects and long lasting effects, such as calcium magnesium phosphate fertilizers, dolomite powder, magnesium ammonium phosphate, etc. Insoluble magnesium fertilizers, such as serpentine, magnesite, etc., are raw materials for processing magnesium fertilizers and magnesium salts, and are generally applied directly without being used as fertilizers.

When choosing the type of magnesium fertilizer, you must consider the growth characteristics of the crop, the soil conditions, and the convenience of field management. For long-term crops or use as base fertilizer, you can choose slightly soluble long-acting magnesium fertilizers, such as calcium magnesium phosphate fertilizer or magnesium ammonium phosphate, etc. Calcium magnesium phosphate fertilizer is alkaline and has a certain effect on regulating soil acidity. For short-term crops or as top dressing, magnesium sulfate, magnesium chloride, etc. can be selected. For plots with irrigation facilities, it can be applied in conjunction with an irrigation system. To relieve the symptoms of magnesium deficiency in plants in the short term, magnesium fertilizer can also be supplemented through root topdressing (foliar fertilizer). Magnesium sulfate is a common foliar magnesium fertilizer. Commercially available foliar magnesium fertilizers also include some organic chelate/complex products, such as sugar alcohol magnesium, LSA-magnesium (lignosulfonate complex magnesium), etc. Foliar absorption is better.

The amount of magnesium fertilizer applied should be determined based on the fertilizer requirement of the crop, the nutritional status of the soil, nutrient utilization efficiency and stage nutrient requirements. In terms of production, it is generally sufficient to apply 1-1.5kg Mg per acre of base fertilizer. If symptoms of magnesium deficiency occur, approximately 5-10kg of magnesium sulfate per acre can be applied and the effect will be observed.