Slow-release/controlled-release fertilizers are a type of fertilizer that regulates the nutrient release rate through specific technological means. The core objective is to match the nutrient release rate with the nutrient needs of crops at different growth stages, thereby addressing the shortcomings of traditional fertilizers, such as “rapid nutrient release, low utilization rate, and easy loss and pollution.” While the concepts are similar, there are slight differences; controlled-release fertilizers offer greater control over the release rate and are considered a more precise type of slow-release fertilizer.
I. Core Technological Principles and Classification
The key to slow-release/controlled-release fertilizers lies in the “nutrient release control mechanism,” which mainly involves the following three technological approaches:
Polymer-Coated Type (Mainstream Controlled-Release Technology)
Principle: Ordinary granular fertilizers (such as urea and compound fertilizers) are coated with one or more layers of high-molecular-weight polymer film (such as polyurethane, polyolefin, resin, etc.). The thickness and porosity of the film determine the nutrient release rate – water penetrates the film to dissolve the nutrients, which then slowly diffuse through the film pores into the soil; as the temperature rises, the permeability of the film increases, and the nutrient release accelerates, and vice versa.
Release Characteristics: This is a temperature-dependent release, matching the pattern of most crops that require more nutrients during high-temperature growth periods. The release cycle can be precisely designed (e.g., 3 months, 6 months, 12 months).
Representative Products: Resin-coated urea, polymer-coated compound fertilizers (NPK).
Chemically Synthesized Slow-Release Type
Principle: Through chemical synthesis reactions, fertilizer nutrients (mainly nitrogen) are combined with organic compounds to form stable, low-water-soluble compounds. These compounds gradually decompose under the action of soil microorganisms or enzymes, releasing effective nutrients.
Release Characteristics: This is a microorganism/enzyme-controlled release, and the rate is affected by soil temperature, pH value, and microbial activity. The release cycle is longer (up to several months to a year).
Representative Products:
Urea-formaldehyde (UF): A condensation product of urea and formaldehyde. During decomposition, formaldehyde is released first, and then gradually converted into ammoniacal nitrogen. The nitrogen utilization rate is more than 30% higher than that of ordinary urea. Ammonium polyphosphate (APP): A slow-release fertilizer containing nitrogen and phosphorus. After hydrolysis, it releases orthophosphate ions for plant absorption. It is suitable as a phosphorus source in compound fertilizers and can also chelate micronutrients in the soil.
Others: Urea-formaldehyde (CDU), isobutylidene diurea (IBDU), etc.
Matrix-based composite type (traditional slow-release technology)
Principle: Fertilizer nutrients are mixed with porous matrices such as bentonite, zeolite, and humic acid. The adsorption and pore structure of the matrix are used to delay nutrient dissolution; or sulfur coating technology is used (a sulfur layer is coated on the outside of urea granules), and nutrients are controlled-released through the slow breakdown of the sulfur membrane and microbial decomposition.
Release characteristics: The release rate is relatively gentle, and the cost is lower than polymer-coated and chemically synthesized types, making it a low-cost slow-release solution.
Representative products: Sulfur-coated urea (SCU), humic acid compound slow-release fertilizer, zeolite-loaded fertilizer.
II. Core Advantages (compared to traditional quick-acting fertilizers)
Significantly improved nutrient utilization: The utilization rate of traditional quick-acting nitrogen fertilizers is only 30%-40%, and a large amount of nitrogen is lost with rainwater or volatilized into the air; the nitrogen utilization rate of slow-release/controlled-release fertilizers can reach over 60%, and some controlled-release fertilizers can even exceed 80%, reducing nutrient waste.
Labor-saving and time-saving, enabling one-time fertilization: Traditional fertilizers require multiple top dressings according to the crop growth period, while slow-release/controlled-release fertilizers can be applied to the soil at once during sowing or transplanting. The nutrients are slowly released to meet the needs of the crop throughout its entire growth period, especially suitable for large-scale planting scenarios such as field crops, fruit trees, and lawns, reducing labor costs.
Reduced environmental pollution: The slow release of nutrients avoids excessively high concentrations of nitrogen and phosphorus in the soil, thereby reducing environmental problems such as soil compaction, water eutrophication (such as agricultural drainage polluting rivers and lakes), and atmospheric ammonia volatilization, meeting the needs of green agricultural development. By synchronizing nutrient supply with crop demand through controlled release cycles, the peak release period of fertilizer nutrients can be precisely matched with the peak nutrient demand period of crops (such as the tillering stage of rice and the large bell stage of corn), avoiding seedling burn in the early stages and nutrient deficiency and premature aging in the later stages, thereby improving crop yield and quality.
III. Product Forms and Applicable Scenarios
Form Characteristics: Mostly granular, with a larger particle size (usually 2-4 mm), high physical strength, and not easily broken, suitable for mechanized sowing and fertilization.
Applicable Crops and Scenarios
Field Crops (corn, wheat, rice): Choose controlled-release compound fertilizers with a release cycle of 3-6 months, applied as a base fertilizer in one application, reducing the need for topdressing.
Fruit Trees, Flowers, and Seedlings: Choose long-acting slow-release fertilizers with a release cycle of 6-12 months (such as urea-formaldehyde, polyammonium phosphate), suitable for the nutrient needs of perennial crops.
Protected Agriculture and Potted Plants: Choose polymer-coated controlled-release fertilizers to precisely control the release rate and avoid excessively high nutrient concentrations in potted plant soil.
IV. Precautions for Use
Not suitable for topdressing: Slow-release/controlled-release fertilizers release nutrients slowly. If crops show acute nutrient deficiency symptoms, fast-acting fertilizers should be used for topdressing.
Appropriate fertilization depth is necessary: It should be applied to the soil near the crop roots (avoid surface application) to allow sufficient water penetration to trigger nutrient release; coated fertilizers should not be crushed during use, otherwise, the coating structure will be damaged, and the controlled-release effect will be lost.
Higher cost than ordinary fertilizers: The price of slow-release/controlled-release fertilizers is 1.5-3 times that of ordinary fertilizers, but considering the benefits of “labor saving + increased efficiency + reduced pollution,” the long-term use is more cost-effective.
Production of Slow-Release Fertilizers: Granulation and Beyond
Manufacturing coated or chemically synthesized slow-release fertilizers involves specialized adaptations of standard npk fertilizer production technology. The core npk fertilizer manufacturing process begins with precise formulation using a npk blending machine or a bulk blending fertilizer machine. For creating the base granules, an npk granulation machine employing specific npk granulation machine technology—such as a rotary drum for agglomeration—forms the uniform particles. This entire suite of fertilizer raw material processing machinery and equipment is the foundation of the npk fertilizer production line, which must be designed to achieve the required NPK compound fertilizer production capacity and granule quality.
Following granulation, the process diverges for slow-release products. The base granules are fed into a coating drum, a critical piece of additional npk fertilizer granulator machine equipment, where polymer or sulfur coatings are applied. For chemically synthesized types like urea-formaldehyde, the reaction occurs earlier in the process, and the resulting material is then granulated. The integrated npk fertilizer production process thus combines standard equipment like the fertilizer mixer machine and npk fertilizer granulator with specialized coating or synthesis units to create a value-added product with controlled nutrient release profiles.
Therefore, producing advanced slow-release fertilizers requires an expansion of the traditional npk fertilizer machine line. It demonstrates how core granulation and blending technologies serve as a platform for innovation, enabling the creation of products that deliver both agronomic efficiency and environmental benefits.