What Are Slow Release Fertilizers? (Plus When To Use Them)


Have you have ever burned your plants with too much conventional fertilizer?  If so, you may be interested in learning more about slow release fertilizers and their benefits.  For instance, how they can help to prevent burning plants, and how they can help to avoid pollution of water.

So, what are slow release fertilizers?  Slow release fertilizers contain nutrients that are dissolved in water slowly or released slowly.  Slow release fertilizers make nutrients available to plants gradually over time, instead of all at once.  The rate of nutrient release depends on bacterial activity in the soil, which in turn depends on soil temperature and moisture.

As you can see, the rate of nutrient release for fertilizer depends on what kind you use and also on soil conditions such as temperature, moisture, and bacterial activity.  Let’s take a closer look at slow release fertilizers and what controls their rate of nutrient release.

Note: slow release fertilizers are sometimes labelled as extended release fertilizers.

What Are Slow Release Fertilizers?

According to the Arizona Cooperative Extension, “Slow release fertilizers include products in which the nutrients contained within the product are either slowly soluble, slowly released, or held in a natural organic form (which require mineralization and nitrification in the soil).”

This means that slow release fertilizers add nutrients to the soil gradually over a long time period.  Often, the nutrients in slow-release fertilizers become available in the soil over a period of 6 to 8 weeks or longer.

According to the University of Florida Extension, some slow-release fertilizers can take up to 18 months to completely release all of their nutrients.

Slow release fertilizers are often used for newly planted shrubs or trees, which need more nutrients as they continue to grow and develop larger root systems.

young avocado tree
Slow release fertilizer is useful for newly planted trees, such as this avocado tree.

A slow release of nutrients into the soil helps to avoid leaching of nutrients.  This helps to avoid wasting fertilizer, making farming and gardening more efficient.

Slow nutrient release also prevents pollution of groundwater by fertilizer runoff during rainstorms or flooding.  This is especially helpful in areas prone to runoff, such as sloping ground or land suffering from erosion.

For more information, check out my article on how to prevent soil erosion in your garden.

One disadvantage of organic (or any other) slow release fertilizer is that the rate of release may be so slow that nutrients are not available when plants need them.

Another disadvantage of slow-release fertilizers is that they are more expensive and less water soluble than traditional fast-release fertilizers.

Note that a slow release fertilizer (SRF) is not the same thing as a controlled release fertilizer (CRF).

Slow release fertilizers have a slower release rate than conventional water-soluble fertilizers, but the rate, pattern, and duration of release are not controlled.  The reason is that release of nutrients depends on microbe activity, which in turn depends on soil moisture and temperature.

For more information, check out this article from the University of Florida Extension.

Organic Fertilizers

One type of slow release fertilizer is any type of organic fertilizer.  Organic fertilizers include compost and manure, along with products derived from animals such as bone meal, blood meal, and feather meal.

bone meal
Bone meal is a slow release fertilizer that provides phosphorus to plants.
Image from: https://commons.wikimedia.org/wiki/File:%D0%93%D0%BE%D1%80%D1%81%D1%82%D1%8C_%D0%BA%D0%BE%D1%81%D1%82%D0%BD%D0%BE%D0%B9_%D0%BC%D1%83%D0%BA%D0%B8_2014-05-07_13-57.jpg

These organic fertilizers break down slowly over time so that nutrients become available to plants on a delayed schedule.

For more information, check out my article on organic fertilizer.

Pelletized Coated Fertilizers

Another type of slow release fertilizer is pelletized, coated fertilizer.  These fertilizers have a coating (often sulfur-coated urea or a plastic shell) that breaks down over time to release the nutrients into the soil.

According to Clemson University, slow release fertilizers in this form should have at least one half of the nitrogen in water-insoluble form.

Both organic fertilizers and coated slow release pellets avoid burning plants by adding nutrients slowly.  Compare this to conventional fertilizers, which make nutrients available more quickly, but that present a danger of burning your plants.

For more information, check out my article on over fertilizing your plants.

Does Slow Release Fertilizer Need Water?

No, slow release fertilizer does not need water.  However, watering regularly will cause coated fertilizer pellets to release their nutrients sooner.

By keeping soil moist, you can cause microbial activity to speed up.  In turn, this will lead to faster breakdown of coated and organic slow-release fertilizers.

garden hose
Slow release fertilizer does not need water, but keeping the soil moist will help bacterial activity and cause nutrients to be released sooner.

Of course, if the soil is too cold, microbial activity will slow down, even there is enough moisture.

Although slow-release fertilizer does not require water, it is not a bad idea to water when applying the fertilizer.  This will further reduce the chances of burning plants due to excess salts caused by over fertilization.

How Long Does It Take Slow Release Fertilizer to Work?

Slow release fertilizer can take anywhere from a few weeks to 18 months to work.  Compost will probably take a longer time than most pelletized fertilizers to break down and release its nutrients.

The time it takes for slow release fertilizer to work depends on many factors, including microbial activity, temperature, and moisture in the soil.

If your soil lacks organic material or is high in salts due to overuse of conventional fertilizer, then it may be difficult to encourage microbial growth.  This will slow down the nutrient release rate of slow release fertilizers.

Cold or dry soil will also slow down microbial activity, which will cause nutrients to be released even more slowly.

Sandy soil is more likely to stay dry, despite frequent watering.  Adding compost to your soil or mulching can help with water retention.

dry soil
Dry soil will cause slow release fertilizers to delay the release of nutrients.

If you have a problem with dry soil, check out my article on how to treat dry soil.

Can Slow Release Fertilizer Burn Plants?

Yes, slow release fertilizer can burn plants.  However, this is much less likely than if you were using conventional fertilizers.  The reason is that organic fertilizers do not contain concentrated salts like conventional fertilizers do.

nitrogen burn on blueberry plant
Fertilizer burn can scorch the leaves of plants, especially when too much nitrogen is present.

No matter what type of fertilizer you use, there are a few steps you can take to minimize the chance of burning your plants.

  • First, get a soil test before applying any fertilizer or other soil amendments.  That way, you will know exactly which nutrients your plants need, and whether you need any fertilizer at all.
  • Next, if a soil test reveals the need for fertilizer, and choose an appropriate one based the nutrient analysis (NPK content) on the label.
  • Then, follow the instructions on the fertilizer package.  Measure your garden and calculate carefully to find the area you need to fertilize, and use the correct amount of fertilizer, using a spreader if necessary to even out the distribution of fertilizer.
  •  Finally, apply fertilizer to moist soil, and water it in after application.

For more information, check out my article on how to do a soil test and my article on NPK ratios in fertilizers.

What Are Some Slow Release Fertilizers?

There are many organic fertilizers that will serve as slow release fertilizers.  Their nutrient content (NPK profile) varies considerably, so figure out what you need to add to your soil before deciding on one of these.

fish
Fish meal is made from fish remains and is a good slow release fertilizer than contains plenty of nitrogen.

The table below summarizes various slow release organic fertilizers and their NPK content.

FertilizerPercent
Nitrogen
by Weight
Percent
Phosphorus
by Weight
Percent
Potassium
by Weight
Alfalfa
(Lucerne)
2.50.52
Bat Guano5.5 to 84 to 8.61.5
Blood Meal12.51.50.6
Bone Meal212 to 160.6
Compost1.5 to 3.50.5 to 11 to 2
Feather Meal1500
Fish Meal104 to 60
Hair12260
Hoof and Horn Meal9 to 141.5 to 20
Manure0.5 to 6.50.2 to 40.4 to 3

There are also several synthetic materials with varying nutrient profiles that can be used as slow release fertilizers.

The table below summarizes various slow release synthetic fertilizers and their NPK content.

FertilizerPercent
Nitrogen
by Weight
Percent
Phosphorus
by Weight
Percent
Potassium
by Weight
Guanylurea3700
Magnesium
ammonium
phosphate
8400
Oxalic
acid
diamide
31.800
Potassium
calcium
phosphate
017 to 2221 to 22
Urea
aldehyde
3000

For a full list of synthetic slow release fertilizers, check out this table on the University of Florida Extension website.

How Often Should I Apply Slow Release Fertilizer?

At the very least, get a soil test once a year, in the spring, and apply fertilizer if needed.  The last thing you want to do is keep fertilizing with nitrogen-heavy fertilizer when your plants really need more phosphorus or potassium!

soil
Get a soil test before adding any fertilizer or amendments to your garden.

Other than that, there is no need to apply slow release fertilizer more often.  An exception is if the plants in your garden show signs of nutrient deficiency (such as yellow leaves, poor flowering, stunted growth, etc.).

In that case, you may want to opt for a faster-release fertilizer to save your plants.

Conclusion

By now, you have a much better idea of what slow release fertilizers are, and why you might want to use them in your garden.  You also know of some types that you can try to see if they work for you.

I hope you found this article helpful – if so, please share it with someone who can use the information.  If you have any questions on slow release fertilizers, please leave a comment below.

jonathon.david.madore

Hi, I'm Jon. Let's solve your gardening problems, spend more time growing, and get the best harvest every year!

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