How should fertilizers be stored properly? Do chemical fertilizers lose their effectiveness over time?

　　Whether at agricultural supply stores or on farmers’ fields, fertilizers left over after the growing season often end up piled up for extended periods because they can’t be sold or used up. This raises the question: Do fertilizers lose their effectiveness over time? And how does the rate of degradation vary among different types of fertilizers?

　　Does fertilizer lose its effectiveness if left for too long?

　　From the perspective of fertilizer characteristics, as long as chemical fertilizers are stored properly—ensuring that the packaging remains intact, is protected from rain, kept dry, and not stored together with alkaline substances—they can generally be stored for an extended period. Consequently, the state currently does not impose a shelf-life requirement for chemical fertilizers.

　　For example, fertilizers such as urea, superphosphate, and potassium sulfate can be stored in a cool, dry warehouse for up to 20 years; apart from the formation of clumps, their nutrient content and other quality parameters remain unchanged.

　　Some fertilizers lose their effectiveness over time, while others remain stable; this depends on the specific type of fertilizer. The following is an overview of several major, commonly used varieties:

　　1. Baking soda does not lose its effectiveness; it only loses volume.

　　In winter, when temperatures hover around 0°C, the fertilizer’s effectiveness remains virtually unchanged. Farmers can take advantage of the seasonal price drop in ammonium bicarbonate during this period and purchase it in preparation for spring plowing.

　　When summer temperatures rise above 30°C, ammonium carbonate volatilizes extensively. In its dry form, it becomes damp as ammonia evaporates, leading to a reduction in weight. Therefore, the bag openings should be tightly sealed to prevent moisture ingress and minimize losses.

　　However, ammonium bicarbonate does not lose its effectiveness over time; its fertilizing power remains strong, and its efficacy does not decline when calculated on a dry‑weight basis. Farmers should not equate weight loss with loss of efficacy.

　　2. Urea only loses its potency; it does not become ineffective.

　　When dry, it can be stored for long periods. At temperatures as high as 122°C, urea decomposes, with the resulting ammonia and carbon dioxide volatilizing and causing a loss of weight. Therefore, even if a warehouse catches fire, the remaining urea retains its properties. However, urea is highly susceptible to moisture; exposure to water or wet conditions can lead to leaching and a corresponding loss of weight.

　　3. The fertilizer efficiency of calcium superphosphate remains unchanged.

　　If exposed to rain, its fertilizer efficacy will deteriorate. Calcium superphosphate produced from phosphate rock with high iron and aluminum content should not be stored for more than one year, because after approximately 170 days, the calcium carbonate it contains can gradually convert into ferric phosphate and aluminous phosphate—insoluble compounds—resulting in reduced fertilizer effectiveness.

　　4. Potassium chloride and potassium sulfate are very stable.

　　Potassium chloride and potassium sulfate are highly stable compounds that tend to cake when exposed to moisture; after breaking up the clumps, they can be applied without any loss of fertilizer efficacy. However, when applying these fertilizers, the dosage should be calculated on a dry‑weight basis.

　　5. Diammonium phosphate is sensitive to high temperatures.

　　At elevated temperatures—such as when the ambient temperature exceeds 30°C—diammonium phosphate gradually converts to monoammonium phosphate, with a small amount of ammonia volatilizing. Therefore, when storing diammonium phosphate, the fertilizer bag should be tightly sealed to prevent ammonia loss.

　　6. Ferrous sulfate deteriorates when it absorbs moisture.

　　Ferrous sulfate is a strong oxidizing agent; when exposed to moisture, it readily converts to ferric sulfate, which crops cannot absorb. As a result, its fertilizing efficacy is poor and it is the most prone to losing effectiveness.

　　7. Fertilizers containing microorganisms must not be used if they have expired.

　　The viability of microbial strains in biofertilizers is subject to a time limit. According to the biofertilizer standard, the expiration date of the microbial strain must be clearly indicated on the packaging; under normal storage conditions, the shelf life typically does not exceed two years.

　　Microbial fertilizer should not be used after its expiration date. It must be stored in a cool, dark, well-ventilated place at an optimal temperature of 4°C to 10°C to prevent loss of efficacy.

　　Some microbial strains require specific temperature ranges; for example, Trichoderma harzianum must be stored in a constant‑temperature incubator at 2–8°C and has a shelf life of one year. In contrast, certain Bacillus species can remain viable for up to a year and a half or even longer, depending on the quality of production—particularly the level of spore formation. Conversely, strains with poor spore‑forming ability may lose viability in less than six months. Therefore, vegetable growers should carefully select microbial fertilizer products.

　　Do not be tempted by low prices and opt for expired products, as they contain very few live microorganisms and have lost their efficacy. In general, bio‑fertilizers that are more than two years old should be chosen with caution.

　　How should fertilizers be stored scientifically?

　　With the peak fertilization season now behind us, how should the remaining fertilizer be stored? Farmers, please take note: under no circumstances should fertilizer purchased with hard‑earned money lose its effectiveness or see its efficacy diminished due to improper storage. Specifically, keep the following four points in mind:

　　1. Prevents moisture absorption and spoilage

　　Ammonium bicarbonate readily absorbs moisture, leading to nitrogen loss through volatilization; ammonium nitrate is highly hygroscopic, prone to caking and deliquescence; calcium cyanamide and superphosphate also tend to cake upon exposure to moisture, compromising their effectiveness. Therefore, these fertilizers should be stored in a dry, cool location, with ammonium bicarbonate particularly requiring tightly sealed packaging to prevent contact with air.

　　2. Fireproof and sun-resistant

　　After exposure to sunlight or high temperatures, nitrogen fertilizers experience accelerated volatilization losses; therefore, they should be stored away from direct sunlight and kept strictly free of open flames and sparks.

　　3. Prevention of Volatile Losses

　　Ammonia water and ammonium bicarbonate are highly volatile and prone to loss; they must be stored in tightly sealed containers. Nitrogen fertilizers and superphosphate must never be mixed or stacked with alkaline materials (such as lime or wood ash) to prevent nitrogen volatilization and to avoid reducing the efficacy of phosphate fertilizers.

　　4. Corrosion and Toxicity Prevention

　　Superphosphate is corrosive; avoid contact with skin and metal utensils. Ammonia solution is highly corrosive to copper and iron, so it should be stored in ceramic, plastic, or wooden containers. In addition, fertilizers must not be stored together with seeds, nor should seeds be packed in fertilizer bags, to prevent adverse effects on seed germination.

　　Scientific and effective storage of fertilizers not only ensures their quality but also helps farmers avoid losses caused by improper storage, thereby safeguarding their interests.