How To Tell if Fertilizer Is Slow Release (9 Tips)

Slow-release fertilizer is a great way to improve overall soil fertility and offer your plants nutrients in a sustainable way that ensures consistency in their growth. Slow-release fertilizers are also a great way to fertilize your plants without burning them.

Here are nine tips to help you identify a slow-release fertilizer:

  1. Check for a coating that ensures slow nutrient release.
  2. Slow-release fertilizers need to be applied less frequently.
  3. Nutrient release is mostly controlled by microbial activity.
  4. The percentage of nitrogen control-released is higher.
  5. Slow-release fertilizer is not mixed with pesticides.
  6. The water solubility is low, and they’re primarily solid.
  7. They are formulated to improve water retention in the soil.
  8. The fertilizers are more expensive.
  9. They can be applied at any time of the year.

In this article, I’ll explain each of these tips in a little more detail, so you can identify the right slow-release fertilizer for your garden. Read on!

1. Check for a Coating That Ensures Slow Nutrient Release  

As the name suggests, slow-release fertilizers are a type of fertilizer that ensures that nutrients are supplied to plants at a slower rate. 

Most fast-release fertilizers work by releasing plant nutrients into the soil immediately after application. Slow-release fertilizers counter this nutrient release by coating the plant nutrients with a formulation that prevents immediate nutrient breakdown.

The most common coating for slow-release fertilizers is sulfur-based urea or polymer sulfur urea. Other coatings include gypsum, bentonite, and starch, bound together with paraffin wax. These coatings degrade slowly in the soil and slow down the nitrogen release. The coatings are what truly differentiate slow-release inorganic fertilizers from fast-release inorganic fertilizers. 

While the slow-release does mean that plants won’t have immediate access to nutrients, they are able to grow more consistently without the risk of fertilizer burn. They are the best type of fertilizers for older plants, trees, shrubs, and newly repotted plants.

2. Slow-Release Fertilizers Need To Be Applied Less Frequently

One of the main benefits of slow-release fertilizers is that they don’t need to be applied to the soil as frequently as fast-release fertilizers. 

Fast-release fertilizers supply nutrients to plants immediately and are particularly useful when used as starter solutions for seedlings or newly repotted plants. However, when used regularly to support plant growth, they need to be applied fairly frequently. 

Most fast-release fertilizers need to be applied as often as once every two weeks during the growing season, especially for vegetables, fruits, and flowering plants. However, Some types of slow-release fertilizers like fertilizer spikes can last between a month and several months.

Fertilizer spikes formulated for trees can even last the entire growing season.

3. Nutrient Release Is Mostly Controlled by Microbial Activity

Slow-release fertilizers can be affected by water, but the nutrient release is determined by microbial activity. The microbes consume the sulfur, gypsum, bentonite, or starch-based coating on the inorganic slow-release fertilizers, which releases the plant nutrients into the soil. 

Organic fertilizers like compost don’t have a coating, but they also release nutrients into the solid through microbial activity. Water may leach the nutrients from compost, but the maximum nutrient release is achieved by organisms. 

The microbial activity is not only helpful for nutrient release, but it also improves the overall soil quality as the presence of slow-release fertilizers attracts organisms. These organisms such as microbes and worms aerate and fertilize the soil, improving its quality. 

Other factors that influence the nutrient release of slow-release fertilizers are temperature and the soil pH value. 

4. The Percentage of Nitrogen Control-Released Is Higher

When it comes to identifying slow-release fertilizers, check the percentage of nitrogen labeled as ‘slow’ or ‘controlled’ release on the package. When the percentage is high, that’s an indication that the majority of the nutrients will be released to the plants over time rather than all at once. 

Percentages of control-released nitrogen in slow-release fertilizer are usually between 50-80% but they can be higher. The best kind of slow-release fertilizers have about 80% or higher control-released nitrogen. 

5. Slow Release Fertilizer Is Not Mixed With Pesticides

Fertilizers and pesticides are both applied to improve plant health and yield but work in different ways.

In some cases, inorganic fertilizers are mixed with specific pesticides like herbicides. Combining fertilizer with a herbicide in one product means that you can enrich the soil, supply nutrients to your plants, and kill off any weeds present. 

However, this combination only works when the fertilizer is fast-release, as the release mechanism of both the fertilizer and the pesticide will be the same. For herbicides to be effective, they must be released when the plants start growing so that weed growth can be nipped in the bud. 

Slow-release fertilizers are designed to release their components slowly, so they are purely fertilizers without any pesticides mixed in. 

6. The Water-Solubility Is Low, and They’re Primarily Solid

As I’ve mentioned earlier, the binding agent for the coating on slow-release fertilizers is usually paraffin wax, which prevents water-solubility. This low level of water-solubility is why these fertilizers’ nutrient release is not activated by water.

Unlike other fertilizers, which can and often should be diluted with water, especially if you’re making starter solutions, most slow-release fertilizers don’t need to be diluted. They are almost always solid fertilizers, with a few exceptions like compost tea, which is made by steeping solid compost in water. 

The low-water solubility ensures there is less nutrient leaching or run-off, which makes slow-release fertilizers more environment-friendly than regular fertilizers. 

7. They Are Formulated To Improve Water-Retention in the Soil

Slow-release fertilizers are often formulated such that they indirectly improve the water retention capacity of the soil. Compost and inorganic slow-release fertilizers hold water well, so when mixed in with the soil, they do the work of retaining water. 

The gradual release of nutrients like nitrogen also improves water retention. Typically, nutrient absorption is thirsty work for plants. They need to absorb a lot of water along with the nutrients to prevent desiccation that leads to fertilizer burn and even death. 

When nutrients are released gradually, as with slow-release fertilizers, the plants and trees don’t need to suck up all the water at once in an unsustainable way. Additionally, the nutrients are made available to plants in the necessary quantities that are easy to absorb without needing excess water. 

8. The Fertilizers Are More Expensive

Slow-release fertilizers tend to be more expensive than bags of urea simply because of the technology involved in creating the coating that makes the fertilizers slow-release. 

While the coating itself may not be expensive to make, formulating the coating is expensive. The formulation must be carefully done to ensure that the coating doesn’t poison the soil, degrade unexpectedly, or be overly affected by moisture or soil pH levels. 

Slow-release fertilizers also have greater longevity, as mentioned earlier. For instance, fertilizer spikes are frequently packed with enough nutrients to last the plants for the entire growing season. 

The concentration of nutrients and quality of the coating means that slow-release fertilizers are typically more expensive than other fertilizers. However, the longevity and long-term benefits like improved soil quality and lack of environmental damage make these fertilizers worth the price. 

9. They Can Be Applied at Any Time of the Year

The most important thing to remember about slow-release fertilizers is that they don’t work immediately. The nature of slow-release fertilizers means that they are not what you apply to plants when they need an immediate nutrient boost. 

That said, slow-release fertilizers can be applied to the soil at any time of year, even if it is not during the growing season. As the nutrients are released in small quantities, the plants will only take up the nutrients that are necessary for their growth. The rest is incorporated into the soil by microbial activity. 

Therefore, you can mix in compost or other inorganic slow-release fertilizers into the soil before winter and allow the fertilizer to improve soil quality.

In contrast, regular fertilizers must only be applied during the growing season. At any other time of year, fast-release fertilizers will cause unsustainable, leggy growth. They also leave the plant vulnerable to disease and pest attacks as it is unable to use up all the nutrients for growth. 

Slow-release fertilizers like fertilizer spikes can be applied to the soil in early spring, in preparation for the growing season, and in late fall to support plants through the winter. You can also feed houseplants with slow-release fertilizers throughout the year. 

Key Takeaways

The primary indicator that a fertilizer is slow-release is the presence of a coating that is typically broken down by microbial activity to ensure the slow release of nutrients. Additionally, these fertilizers are usually not water-soluble, have a higher percentage of control release nitrogen, and are solid rather than liquid. Slow-release fertilizers improve soil quality and water retention as well and tend to be more expensive than other fertilizers. 

Alexander Picot

Alexander Picot is the principal creator of, a website dedicated to gardening tips. Inspired by his mother’s love of gardening, Alex has a passion for taking care of plants and turning backyards into feel-good places and loves to share his experience with the rest of the world.

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