What Is Slow-Release Fertilizer | When Best To Use It?

When it comes to choosing fertilizer, the term ‘slow-release’ is likely a familiar one. As home gardeners and farmers, we have a basic understanding that these disperse nutrients at a slower rate than fast-acting options. You may also already know that they come in either granule or spike form. 

But what slow-release fertilizers actually are, how they’re made, and why they were developed is a little more complex.

In the 1930s, the West was experiencing a great depression. In which a weakened ability to sustainably feed the masses revealed itself. Here’s a look at just how the agricultural industry reacted.

What Is Slow-Release Fertilizer?

In 1936, the first “controlled release” fertilizer (CRF) technology was invented. By the 1960s, a successful wax/polymer coating had been added. Creating a product that contained 75% water-insoluble nitrogen.

Meaning that more vital nutrients would be retained, instead of being lost to runoff and evaporation. Then released into the soil, in a controlled manner. A release that’s dependent on beneficial microbe activity in the soil and moisture availability.

In time, other companies caught on and developed their own offering. A “slow-release” fertilizer (SRF), is very similar to the controlled version in terms of coating and rate of nutrient release.

Yet, the release was dependent more on soil and weather conditions. The nitrogen content in this new product was also more water-soluble than in the controlled version and thus has a relatively quicker effect on plants. 

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What Effects a Fertilizer’s Rate of Release 

CRF and SRF products are both designed to gradually release nutrients over time. Naturally, controlled fertilizers are going to be more effective in fertile soil.

With enough moisture, the coating on these becomes soft enough for microbes to eat through. Nutrients are then converted into an easily absorbed form for plants. 

For most home gardeners, slow-release fertilizers are going to be more advantageous. Especially if your soil is less-than-fertile. When moisture availability and air and soil temperatures align, an SRF’s coating breaks down to release nutrients. 

Fertilizer Particle Size 

Particle (or granule) size is a key factor in the effectiveness of slow-releasing fertilizers. Influencing two primary aspects of performance. How fast the coating breaks down and the rate of nutrient delivery, once it does.

As expected, larger particles take longer to break down than smaller ones. And regardless of size, each particle becomes less effective the closer together they are applied. Making following application directions is crucial to good results. 

Soil and Weather Conditions 

Soil and weather conditions can change from day to day, depending on where you live. Which makes the rate of nutrient delivery in both SFR and CFR options just a variant. Thus, a predictable rate is impossible to establish. 

This is also true for purely organic forms of slow-releasing fertilizer. From which nutrients are released based on the rate of decomposition. A process that is either hastened or delayed by soil and weather conditions. 

Ingredients and Coating 

All synthetic, slow-release products are typically water-soluble materials covered in a semi-water insoluble coating. Resulting in a slower conveyance of nutrients than liquids. 

The thickness of this coating varies from brand to brand, as well. Some include sulfur for plants that prefer a lower pH.

Recently, some companies have switched to a non-eco-friendly, plastic coating that doesn’t break down and permanently remains in your soil. So be sure to read the ingredients list prior to purchase. 

Slow-Release Vs Instant-Release 

Slower-paced fertilizers are typically preferred over those that tend to inundate plants all at once. Fast results may seem like a no-brainer. But, large boosts can be harmful to your plants and accidental over-feeding with liquids is common. 

Yet, liquids do have their benefits. When grown in poor soil, plants immediately receive the nutrients they need to thrive. Even if you regularly use slow-release fertilizers, a quick boost of specific nutrients may be needed as buds set or in times of drought. 

Slow-Release Vs Controlled-Release 

  • I’ve discovered a distinct difference between these when applied to trees and shrubs on slopes. While SRFs are commonly recommended here, controlled release options are far less water-soluble. Allowing for available nutrients far longer. 
  • This makes sense if you get a lot of rainfall per year and have a sloped garden. On the other hand, they require really fertile soil in order to reach maximum effect.
  • Those categorized as “slow-release” aren’t dependent on microbe activity and work really well on even terrain that gets lots of summer sun and is watered well. 

Why Use Slow Release Fertilizer?

There are a number of perks that come with using a slow-release fertilizer vs a fast-acting liquid:

  • Vegetables and annuals tend to grow at a similar rate, giving a consistent look to your garden
  • Plants grow stronger roots when they have time to receive and process nutrients at a slower pace
  • This slower pace allows plants to develop increased resistance to disease, pests, and environmental stresses
  • Slow-release granules are recommended over liquids for fertilizing delicate seedlings and propagated cuttings
  • Plants are far less likely to experience fertilizer burn
  • Slow releasing fertilizers run far less risk of soil and water source contamination

This is what they do for your plants! For you, they free up time. Slow-releasing fertilizers last longer and don’t have to be applied as often. Allowing you the chance to enjoy those warm, sunny days in your garden, with less work. 

Plant Health 

Slow-releasing fertilizers can keep your soil healthy by stimulating microbe activity. One perk of being a solid material is some brands opt to add those beneficial critters right in the granules!

Slow releasing nitrogen feeds are safer to use on your lawn than quick releasers because they reduce the risk of brown patches in excessively hot summers. And increase tolerance against wear and tear.

There’s really no limit to what kind of plants these can be applied to. But, of course, organic options will always be far better for overall soil and plant health than synthetic options. 

Environment and Nutrient Penetration 

Slow and controlled release fertilizers can stay active in the soil, nourishing your plants, anywhere from 3 weeks to a full growing season, depending on the brand.

Nutrients are quickly absorbed when released and in doses that mimic what plants might naturally have access to in their native habitats. 

However, it’s always best to learn what your fertilizer granules or spikes are coated with. Polymers blended with wax or some other degradable materials are easier on the environment and your outdoor spaces. Plastic-coated particles don’t break down and may eventually leak unwanted chemicals into your soil. 


We typically associate the word “cost” with money. So, let’s cover that first. Yes, liquid fertilizers do cost less, upfront. But, they also run a higher risk of accidental yet irreparable damage to lawns and plants.

Spending a few extra pennies up front, on a slow-release fertilizer, will be a lot cheaper in the long run. Not only can they save you money long-term but save the life of your plants. Saving you a lot of time and the angst of losing previously thriving plants. Needless to say, the contamination and waste savings to the environment would be incalculable. 

What Is Slow Release Fertilizer Made From?

Slow-release feeds can be either organic or inorganic, in nature. Inorganic options break down at different rates of speed, depending on what they’re made from and how. But organics are made from purely natural materials that slowly decompose at similar rates. Which are driven by environmental factors.

The base of synthetic, granular alternatives starts with the ground and powdered urea (nitrogen) and is mixed with other nutrients (either organic or lab-produced). This dry mixture is then compacted into granule form and coated with either a wax or plastic-based polymer.

Slow releasing organics are natural materials that can be applied raw, such as worm castings and seaweed. Or in a decomposed state, like compost, leaf mold, and manure. 

Active Ingredients 

Albeit in different ratios, most fertilizers contain nitrogen, phosphorus, and potassium. These are the primary nutrients that all plants need for optimal health and productivity. 

Some are formulated to focus on a single nutrient. A 0-0-60 fertilizer, for example, is simply potassium. Others include micronutrients and trace elements that the primary ones depend on to fulfill their functions. These would be calcium, sulfur, magnesium, iron, manganese, zinc, copper, and boron.

No active ingredient is necessary to facilitate nutrient release. As this is dependent on microbe activity and environmental factors. 

Synthetic Vs Organic 

It’s important to note one key difference between these two. Not all dry, synthetic fertilizers are slow releasing. But all dry, organic fertilizers are. This is because all organic matter decomposes in a similar way and at a similar rate. Slowly.

Deciding between synthetic and organic is simply a matter of preference, gardening style, and end-result expectation. However, a basic understanding of how each works can support an informed decision.

Because synthetic options are human-made, what nutrients go into them and how much can be manipulated. Creating a plethora of NPK ratios designed for different plants, at various stages of growth.

Individual, organic materials have fixed amounts of naturally formed nutrients. To create a specific NPK, you would simply determine the amount needed and mix different organics together.

Understanding Fertilizer Labels N-P-K 

Every slow-release fertilizer label reflects a three-numbered ratio. This is the N-P-K of that product, indicating its content percentages of nitrogen, phosphorus, and potassium.

For example, a 16-4-8 fertilizer has 16% nitrogen, 4% phosphorus, and 8% potassium. In addition to assorted, secondary macronutrients, depending on the manufacturer’s formula. 

Every plant requires these elements in order to thrive. Just in different proportions or NPK ratios. 

Each of these primary nutrients plays a vital role in cell formation, photosynthesis, and nutrient distribution. All work in tandem, with other secondary elements, to create healthy, lush, and productive plants. 


Nitrogen plays a vital role in fueling the growth of sturdy stems and vibrant foliage, by expediting the production of chlorophyll. This gives foliage its green color and ensures that photosynthesized energy is available to convert starches and carbohydrates into food. 

The resulting healthy plant structure then feeds and supports blooms and abundant crops. This multi-tasking macronutrient also fuels the proteins and enzymes that regulate water and nutrient uptake and disbursement. 


Potassium is responsible for the circulation of water, nutrients, and photosynthesized food. By doing so, it activates 80 different enzymes, formed by phosphorus and nitrogen, that support bud, flower, and fruit formation. 

Proper amounts of potassium contribute to a plant’s resistance to disease and extreme heat and drought by helping them conserve moisture. And is especially effective in encouraging tolerance for even the coldest winters when plants are dormant. 


Phosphorus plays a key role in photosynthesis by triggering captured energy to convert starches and carbohydrates into food.

In food crops, phosphorus contributes to bud and fruit production. But, acid-loving plants, helps to regulate soil pH. So that they can easily access other vital nutrients. 

Phosphorus then moves to the cellular level. Facilitating the construction of those same proteins and enzymes that nitrogen and potassium use to regulate water and nutrients. 

Types of Slow Release Fertilizer 

Control Release (CRF) – these synthetic, slow-release options are not water-soluble. Eliminating the possibility of nutrient loss by evaporation. The release of nutrients is controlled by soil microbe activity.

Slow Release (SRF) – Organic or synthetic, concentrated granules or spikes offer a steady stream of nutrients over an extended period of time. With nutrients being released based on ambient temperatures. 

Organic – Dry organic fertilizers are naturally slow-releasing. Liquids are fast-acting. Both contain nitrogen, phosphorus, and potassium and nourish plants and improve soil quality.

Inorganic – manufactured with minerals and chemicals, these can result in substantial plant growth. But, can also contaminate soil and do little to increase fertility. 

Pellets or Granules 

Slow-releasing, granules, and pellets are easy to use and can provide consistent nourishment for nine months or more. With the first results show in just a few weeks, depending on the brand. These are also less likely to cause root burn.

These are applied by evenly casting around the base of shrubs and plants. No watering is needed to activate them and is less likely to leach into nearby surface or groundwater. 

Liquid Concentrate 

Most liquid fertilizers come in concentrated form and need to be diluted prior to use. Then, applied with a watering can or hose attachment, around your plants.

Quick releasing nutrients are then immediately carried to the root system through the soil or leaves. Some are also formulated to be used as foliar sprays.

These offer a beneficial boost, once or twice a month, while plants are experiencing a rapid plant growth rate. 

Fertilizer Spikes 

Spikes are one of the most convenient to use, especially if you have a container garden. Pre-measured in various sizes, they’re easy to push into loamy soil.  And being slow-releasing, they run little risk of burning.

Nutrients are released from the spikes by increased moisture and carry the added benefit of stimulating resistance to disease and pests. Unused portions can then be stored mess-free until needed. 

Slow Release Fertilizer Vs Natural Organic Material 

There are some clear advantages to using organic fertilizers vs synthetic options:

  • Compost is rich in naturally formed nutrients and microbial activity. Which can be freely made by composting kitchen scraps and garden waste. 
  • Organics contain the same purposeful and healthy level of nutrients as in the wild and therefore run little risk of plant damage

There are some advantages to using inorganic slow releasers, too. Especially over liquids.

  • Nutrients are released at a slower rate, so they pose far less risk of soil and waterway contamination. As well as plant damage.
  • Fewer applications are necessary and can fertilize plants for longer periods of time. 

Blood and Bone Meals 

Blood and bone meal (derived from animal bi-products) are soil additives that are rich in macro and micronutrients for healthy, strong growth. They’re also attractive food sources for soil microbes, who convert all those nutrients into an easy-to-absorb form. 

Bone and blood meal are excellent multitaskers. Providing organic nourishment for increased crop yields and abundant blooms. While naturally increasing soil fertility to benefit present and future growing seasons. 

Garden Compost 

Well-aged compost is a highly prized source of nutrients and is very simple to make. Kitchen scraps, yard waste, and raked leaves can all be collected and left to age.

Working compost into your soil can drastically improve the structure, loosen dense clay, and give body to light, sandy soil. 

Considered more a soil amender than fertilizer, compost still releases nutrients in a slow and steady manner. Resulting in long-term soil and plant health. 

Animal Manure 

Conversely, manure is considered a slow-release fertilizer. Contains limited amounts of nutrients that are gradually released as chemical changes occur. Due to these limits, and because manure alone can burn roots, farmers and home gardeners often add it to their compost bins. 

If you choose to add pure manure to your garden, fall and winter are the best seasons. Allowing the acidic nature of manure to mellow before introducing young plants to it. 

How Best To Use Slow Release Fertilizer 

Slow releasing fertilizers, that both nourish plants and improve soil quality, are the best type to use for long-term growing success. This is the very definition of organic fertilizers.

Discovering their nutrient value and how you can effectively combine them, does take some reading and experimentation. But, in my experience, it’s time well spent. 

However, if your goal is to simply focus on one growing season at a time, then pre-made fertilizers with focused NPCs work just as well. Applying these according to the manufacturer’s directions and avoiding over-application will equal success. 

Trees and Shrubs 

In the wild, trees, and shrubs rarely want nutrients because of the natural nutrient recycling process around them. Growing these in planned landscapes, where this doesn’t necessarily exist, may result in the need for a slow-release fertilizer.

This is especially true for new saplings that need nourishing encouragement to form strong roots and new shoots. When planted on slopes, where run-off is common, slow-release fertilizers will support proper tree fertilization


Flowering perennials in the ground benefit from slow-release fertilizers that have NPK formulas designed for their unique needs. Some require a lower pH, like hydrangeas and rhododendrons. While others prefer it more neutral. Such as with shade plants.

Annuals also benefit from a slower feed with an NPK that hastens maturity for a longer bloom time. In pots, granules and spikes support proper nourishment despite increased runoff that occurs with more frequent watering.  

Vegetable Gardens 

With so much planting, weeding, and pest control to do, we shouldn’t have to spend a lot of time fertilizing. This is why using a slow-release method makes such sense.   

Switching between granules that have higher nitrogen and higher phosphorus content is the key to a successful tomato fertilizer, or when choosing a fertilizer for pumpkin, squash, and watermelons. These will nourish your crops for much longer and may even give you time to sit and enjoy all your hard work. 


Yellowing houseplant leaves (or chlorosis) is a common sign of nutrient deficiency. Using spikes (a 10-10-10 fertilizer NPK is best) can sufficiently nourish houseplants for months at a time. Allowing you to enjoy the beautiful weather without worrying about having to feed them every week. 

In autumn and winter, many houseplants enter a dormancy period. Unless you live in a temperate climate with lots of sunshine during these months, they typically don’t require fertilizing. 

Lawn and Grass 

Fertilizers for grass with a nitrogen-focused granular formula work well for lawns and grasses when applied just before a big growth spurt. This phase will vary from region to region and whether your grass is a warm or cool-season variety.

Grasses grow best when nourished slowly, allowing roots and blades time to absorb and process nutrients to their greatest benefit. The result will be a lovely green lawn that’s tolerant of drought and wear and tear. 

Final Thoughts On Slow-Release Fertilizer

Slow-release fertilizers are effective at gradually and continually providing nutrients to your garden, trees, flowers, lawns, and houseplants over a sustained period of time. 

This method of fertilizing offers, convenience, time-saving, and a wide range of applications, and offers an effective approach throughout all stages of the growing season and beyond.