Nitrogen is an element plants require in significant amounts for building proteins, creating chlorophyll, and supporting vigorous growth. Traditional nitrogen fertilizers, like urea, are highly water-soluble, meaning they quickly release a large dose of nutrients into the soil. This rapid availability can lead to nutrient waste and potential harm to the plant. Slow release nitrogen fertilizer (SRNF) is formulated to meter out its nutrient content gradually over an extended period, providing a steady supply that better aligns with a plant’s ongoing nutritional needs.
How Slow Release Works
The fundamental difference between slow release and conventional fertilizers lies in the mechanism controlling the nitrogen release rate. These mechanisms fall into two main categories: those using physical barriers and those using chemically altered compounds.
Physical Barriers
Barrier coatings, such as sulfur or various polymers, encapsulate the water-soluble nitrogen core, creating a physical shell. Release begins when water permeates the coating through microscopic pores or imperfections, dissolving the inner nitrogen granule. The dissolved nitrogen then diffuses out into the soil. The rate of release is largely governed by soil temperature, where warmer conditions increase the diffusion speed.
Chemically Altered Compounds
These compounds rely on chemical reactions or biological breakdown to free the nitrogen. Products like urea-formaldehyde (UF) consist of long chains of nitrogen compounds that are not readily water-soluble. For the nitrogen to become available to plants, these chains must be broken down by soil microbes through a process known as mineralization. Isobutylidene Diurea (IBDU) relies primarily on hydrolysis—a chemical reaction with water—to convert the compound into usable urea. IBDU’s release is controlled by the material’s low solubility, making it less reliant on the variable activity of soil microorganisms than UF products.
Major Types of Slow Release Formulations
The market offers several distinct product types, each utilizing a specific mechanism to achieve slow release properties.
Coated Fertilizers
Coated fertilizers use a physical barrier approach. Sulfur-Coated Urea (SCU) is one of the oldest commercial forms, where granules are coated with elemental sulfur. The sulfur slowly degrades or breaks due to mechanical stress or microbial oxidation, releasing the nitrogen load inside. Polymer-Coated Urea (PCU) is a more advanced coating technology, using a thin, semi-permeable plastic membrane to precisely control the rate of water entry and nutrient diffusion. This technology allows manufacturers to engineer products with specific release durations, such as 90, 120, or 180 days, based on the coating’s thickness and composition.
Chemically Modified and Organic Sources
Chemically modified fertilizers are condensation products of urea and an aldehyde, creating complex molecules that break down slowly. Urea-formaldehyde (UF) is common; its release rate is determined by the length of the polymer chains formed during manufacture. Shorter chains release nitrogen faster, while longer chains require more microbial activity. Isobutylidene Diurea (IBDU) is another reacted product offering consistent release driven by water and soil acidity. Natural organic fertilizers, such as compost, manure, and feather meal, are inherently slow-release because their nitrogen is bound within organic matter and becomes available only as soil organisms decompose the material.
Practical Advantages Over Conventional Fertilizers
The controlled release of nitrogen provides several practical benefits over single-application soluble fertilizers.
Plant Health and Efficiency
By delivering nutrients gradually, slow release products significantly reduce the risk of fertilizer burn (phytotoxicity), which occurs when high concentrations of soluble salts injure plant tissue. This steady nutrient supply increases the plant’s nutrient use efficiency, ensuring a larger percentage of applied nitrogen is absorbed for growth. Furthermore, the need for frequent applications is reduced, often allowing a single application to feed a plant for an entire growing season, saving labor and application costs.
Environmental Protection
A major advantage is the minimization of environmental impact associated with nitrogen runoff and leaching. Soluble nitrogen sources, particularly nitrate, are easily washed out of the root zone by rain or irrigation, contaminating groundwater and surface water. Slow release formulations prevent the rapid saturation of the soil solution, keeping the nitrogen contained or bound in a less mobile form until utilized. This reduced loss profile makes them a desirable choice for sandy soils, high-rainfall regions, or areas with strict nutrient management regulations.
Choosing and Applying Slow Release Fertilizer
Selecting the correct slow release product requires careful examination of the fertilizer label’s guaranteed analysis. The label lists the total percentage of nitrogen and often breaks this down further to show the percentage of Water-Insoluble Nitrogen (WIN) or Controlled Release Nitrogen (CRN). This percentage indicates the portion of nitrogen designed to release slowly, with a higher percentage signifying a more sustained feeding period. For urea-formaldehyde products, the Activity Index (AI) is sometimes listed, which predicts long-term nitrogen availability based on polymer chain lengths.
The effectiveness and longevity of the fertilizer are heavily influenced by environmental conditions. Release from all forms is accelerated by higher soil temperatures, as heat increases both the rate of water diffusion through coatings and the activity of soil microorganisms. Soil moisture is also a factor, particularly for coated products, as water is required to dissolve the inner nutrient core.
When applying granular products, uniform spreading is important to prevent uneven feeding. In high-value settings like container gardening or turf, the granules should be lightly incorporated or top-dressed to ensure contact with the soil. For deep-rooted plants, specialized application methods, such as deep-root injection, can deliver the material directly into the active root zone.