The Role of Secondary Nutrients in Crops

(1) Calcium: Calcium is an essential component of the plasma membrane and helps prevent the exudation of cell sap; it is indispensable for cell wall formation; calcium deficiency impairs cell division and the formation of new cells; calcium acts as an activator for certain enzymes, such as amylase; it neutralizes acidity and detoxifies; calcium plays a role in plant disease resistance—for example, through the formation of calcium oxalate—and is crucial for regulating cellular osmosis. In the early stages of calcium deficiency, terminal buds and young leaves turn pale green; subsequently, the leaf tips develop a characteristic hooked shape, followed by necrosis. Calcium is a poorly mobile element that is not easily reused; therefore, deficiency symptoms first appear on the upper young stems and leaves. For example, when Chinese cabbage is deficient in calcium, the inner leaves turn brown. Fertilizers containing calcium include lime, gypsum, calcium nitrate, calcium ammonium nitrate, and superphosphate. Lime is a commonly used calcium-containing fertilizer on acidic soils. When the soil pH is between 5.0 and 6.0, the recommended application rate per hectare is 1,100–1,800 kilograms for clay soils, 700–1,100 kilograms for loamy soils, and 400–800 kilograms for sandy soils; apply slightly more when the soil is highly acidic and slightly less when it is less acidic. Gypsum is a calcium-containing fertilizer commonly used on alkaline soils, with an application rate of 1,500 kilograms per hectare or approximately 2,000 kilograms of phosphogypsum. Calcium nitrate, calcium chloride, and calcium hydroxide can be used for foliar spraying; the concentration varies depending on the crop. For fruit trees and vegetables, the spraying concentration of calcium nitrate is 0.5%–1.0%. (2) Magnesium: Magnesium is a component of chlorophyll; magnesium deficiency hinders chlorophyll synthesis in crops. It acts as an activator for many enzymes involved in carbohydrate metabolism, facilitates the transport of phosphates within the plant, and participates in fat metabolism while promoting the synthesis of vitamins A and C. It plays a crucial role in photosynthesis. The most obvious symptom of magnesium deficiency is pale green leaves, characterized by yellowing of the leaf tissue starting from the lower leaves, while the veins remain green—this is the primary distinction from nitrogen deficiency. Severe magnesium deficiency can cause premature leaf senescence and defoliation. Magnesium-containing fertilizers include magnesium sulfate, magnesium hydroxide, Epsom salt, magnesium chloride, magnesium nitrate, magnesium oxide, and calcium-magnesium phosphate fertilizers. Light-textured soils. Acidic soils, highly leached soils, and fields where large amounts of phosphate fertilizer have been applied are prone to magnesium deficiency. The application rate of magnesium fertilizer varies depending on the soil and crop, but generally ranges from 15 to 25 kilograms of pure magnesium per hectare. Magnesium sulfate and magnesium nitrate can be applied via foliar spraying; for vegetables, the concentration of magnesium sulfate is 0.5%–1.5%, and that of magnesium nitrate is 0.5%–10%. (3) Sulfur: Three amino acids contain sulfur; therefore, sulfur is a component of proteins, and sulfur deficiency hinders protein formation. Sulfur is also present in certain enzymes, such as lipase and urease. Sulfur enhances nitrogen fixation efficiency in legumes; it participates in redox processes within plants; and it influences chlorophyll formation to some extent. Sulfur is not highly mobile; deficiency typically manifests as chlorosis in young leaves, with new leaves exhibiting uniform loss of green color, turning yellowish-white and prone to shedding. Sulfur deficiency is rarely encountered in agriculture because soil contains sufficient sulfur to meet plant needs. Sulfur-containing fertilizers include gypsum, sulfur, magnesium sulfate, ammonium sulfate, potassium sulfate, and superphosphate. Cereals and legumes may exhibit sulfur deficiency when available sulfur in the soil falls below 12 ppm. Crops sensitive to sulfur include cruciferous and leguminous crops, as well as onions, garlic, and Chinese chives. The recommended application rates per hectare are 150–300 kilograms of gypsum and 30 kilograms of sulfur. With the development of agricultural production and the continuous increase in crop yields, sulfur—which is not easily mobile—typically manifests as chlorosis in young leaves when deficient; new leaves exhibit uniform chlorosis, turning yellowish-white and prone to premature shedding. Sulfur deficiency is rarely encountered in agriculture because there is sufficient sulfur in the soil to meet plant needs. However, crop demand for secondary nutrients is increasing, and symptoms of secondary nutrient deficiency have already appeared in some crops in certain regions. Therefore, applying fertilizers containing secondary nutrients such as calcium, magnesium, and sulfur is crucial for increasing crop yields and improving the quality of agricultural products.

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