How to choose diammonium and compound fertilizer reasonably
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June 29 07:41:05, 2025
Ms. Tian from Donggang City, Liaoning Province asked: A small plot of rice was planted in Datian for breeding purposes. Only 12 kg of diammonium phosphate per mu was applied, but the seedlings turned yellow. What could be the reason, and what should she do?
A: The yellowing of the seedlings is likely due to a lack of nutrients, especially nitrogen deficiency. In the nursery stage, it's essential to use fertile soil and apply more organic fertilizer. The amount of chemical fertilizer used here might not be sufficient. Ms. Tian applied enough fertilizer, but it lacked nitrogen and potassium. Also, no organic fertilizer was used, which could lead to a deficiency in trace elements. It is recommended that she apply 10 kg of urea per acre, or a mix of ammonium sulfate, ammonium bicarbonate, and 5–10 kg of potassium sulfate. If symptoms like leaf curl or stunting appear, applying zinc sulfate or a multi-micro fertilizer according to instructions would help.
Mr. Wu from Jiansanjiang Farm, Heilongjiang, asked: Previously, newly planted soybeans were fertilized with diammonium phosphate, but now it doesn’t seem to work. Is there a problem with the fertilizer?
A: It’s not the fertilizer that’s at fault, but rather its improper use. Newly developed land tends to have higher fertility, so diammonium phosphate was sufficient before. However, after years of cultivation, the soil fertility has declined, making light applications of diammonium insufficient in both nitrogen and potassium. It’s still acceptable to continue using diammonium, but ensure proper application of nitrogen and potassium fertilizers and avoid over-application.
Ms. Li from Nanyang City, Henan Province asked: When purchasing compound fertilizer, solubility is important, but people often overlook the correct nutrient ratio. What are the differences in solubility between different types of fertilizers?
A: Generally, chemical fertilizers are highly soluble in water, which makes them effective quickly. Among them, nitrogen fertilizers are the most soluble, followed by potassium, while phosphate fertilizers vary in solubility—some are water-soluble, some only weakly soluble, and others poorly soluble. The solubility of compound fertilizers depends on their composition and processing methods. Fertilizers with higher nitrogen and potassium content tend to dissolve more easily.
Ms. Guo from Wuhu City, Anhui Province asked: When transplanting greenhouse melons, high-tower granulated compound fertilizer (NPK 30:5:5) was used, but the lower leaves got burned. Could this be fake fertilizer?
A: No, it’s not fake. Leaf burn is usually caused by improper application. High-tower granulated compound fertilizer contains a lot of urea, which dissolves quickly and can damage young plants if not properly mixed into the soil. It should be kept away from the roots or thoroughly mixed with the soil. Also, make sure to ventilate the greenhouse to prevent ammonia buildup, which can cause leaf burn.
Mr. Lan from Dezhou City, Shandong Province asked: Greenhouse cucumbers use general-purpose compound fertilizer, but it doesn’t dissolve and the seedlings don’t grow well. Why?
A: It’s not the right type of fertilizer. A high-nitrogen, high-potassium compound fertilizer is more suitable. These fertilizers are more water-soluble and better suited for the nutrient needs of cucumbers during the fruiting stage. They should be dissolved in water and applied in furrows, then covered with soil for optimal absorption.
Mr. Zhao from Weishi County, Henan Province asked: How to grow solanaceous vegetables in winter?
A: For winter seedlings of solanaceous crops, it's important to select well-draining bed soil, apply sufficient base fertilizer, and ensure good insulation. Healthy seedlings are key to preventing diseases like blight and rickets. To control soil-borne diseases, you can apply 3–4 ml of carbendazim per 3 liters of water, covering 1 square meter of bed soil. For damping-off, use 64% mancozeb at 120–170 grams per 50 kg of water. Aphids can be controlled with pyrethroid insecticides or acetamiprid. Overwatering often causes yellowing, so monitor moisture levels carefully.
Mr. Zhang from Huainan County, Anhui Province asked: Which is better between urea and ammonium-based nitrogen fertilizers?
A: The effectiveness of each depends on how they are used. Urea has a high concentration and takes 3–5 days to convert into ammonium nitrogen under microbial action. Before conversion, it’s not easily absorbed and may leach out. So, it should not be over-applied and water should not be added immediately after application. Ammonium-based fertilizers, on the other hand, are prone to volatilization in alkaline soils, so they should be applied deeply and covered with soil.
Mr. Zhou from Jingzhou City, Hubei Province asked: Why is calcium superphosphate acidic? Can it be used as a foliar fertilizer?
A: Calcium superphosphate is an acid-acting phosphate fertilizer made by reacting phosphate rock with sulfuric acid. It contains a small amount of free acid, hence its acidity. Despite being acidic, it is water-soluble and safe for foliar application. For example, spraying 1–3% solution on wheat, corn, and rice, or 0.5% on cotton, tomatoes, and cucumbers, can improve yield and quality. The best time to spray is during the late growth stage or before and after flowering.
(Senior Consultant of Sinofert Company, Professor Wang Xingren, China Agricultural University)
Farmers Daily
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