Slow-Release vs. Quick-Release Fertilizer: What’s the Difference?

With all the letters, dosages, deficiencies, and chemistry jargon, the world of fertilizers can be quite confusing. Thankfully, you can simplify how you feed your garden by applying basic concepts of plant nutrition and natural soil processes. There are thousands of fertilizer products on the market today, but every single one fits into one of these categories: slow-release or quick-release.

The category of fertilizer you choose from ultimately determines how fast your plant can uptake the nutrients and how often you have to reapply. It can also have major impacts on your local ecosystem and the health of your soil.

Let’s dig into the key differences between slow-release and quick-release fertilizers. 

The Short Answer

Slow-release fertilizers are organic, while quick-release fertilizers are synthetic. Imagine slow-release fertilizers as a gradual trickle of nutrients over a longer time period, while quick-release fertilizers give one megadose that lasts for a short time.

The majority of slow-release fertilizers are organic, which means they contain carbon and require microorganisms to break them down into plant-available nutrients. Slow-release fertilizers include ingredients like compost, bone meal, fish emulsion, blood meal, alfalfa meal, and manure. 

On the other hand, most quick-release fertilizers are synthetic, which means they are manmade and do not contain carbon. They bypass microbial breakdown, so they are instantly available to plants but dissipate quickly, requiring more frequent applications. Quick-release fertilizers can include urea, ammonium nitrate, and superphosphate.

Organic slow-release fertilizers feed the soil, and synthetic quick-release fertilizers directly feed the plant. Your gut reaction may be to choose the latter, but once you understand the importance of a thriving soil microbiome in your garden, you may find yourself reaching for organic options because they contribute to lasting sustainability.

Comparison Chart: Slow-Release vs. Quick-Release Fertilizer

The Long Answer

The fundamental difference between these fertilizers is the rate of nutrient delivery to the plant. Slow-release fertilizers gradually make minerals and nutrients available to the plant over time, creating a more balanced soil ecosystem. Rapid-release fertilizers are highly water-soluble and provide an instant dose of nutrients. 

Both types of products come with pros and cons, but the eco-minded organic gardener most often chooses slow-release fertilizers because they pose less environmental harm and require less frequent applications.

Here we’ll dig deeper into the pros and cons of each type, including plenty of examples and scientific data.

Slow-Release Fertilizer

The primary benefit of slow-release fertilizer is that it provides a steady amount of nutrients over time. You don’t need to apply it very often, and you don’t typically need to worry about fertilizer burn. While there are a few synthetic slow-release fertilizers (such as urea-formaldehyde), most truly slow-release fertilizers are organic.

The organic options are more natural and don’t require complex chemistry to understand. Mother Nature has been decomposing leaves, manure, and other dead plant or animal residues for billions of years before humans invented fertilizer. Plants have evolved age-old collaborations with soil microorganisms to decompose and consume these nutrients.

Soil is Like an External Digestive System

Imagine organic plant nutrients (for example, the nitrogen and phosphorus in manure) as little bundles of food with a special lock on the outside. Plants cannot immediately “unlock” the food capsules and “eat” them because soil microbes have the key. Soil microbial communities like beneficial bacteria and fungi are necessary to break down the nutrients into plant-available forms. 

In other words, the soil microbiome is sort of like an external digestive system for plants. Much like the microbiome of the human gut, beneficial soil microbes act like intermediaries to help plants digest their food. This is why slow-release fertilizers are gradually accessible over a longer period of time. It takes time for microbes to decompose the materials.

If your soil is lacking in microorganisms (or you are growing in a container), it may take even longer for plants to access the nutrients because there aren’t many organisms working in the soil. This can be remedied by using compost, worm castings, soil inoculants, and compost teas. Overall, soils with more microbial activity are better at processing slow-release fertilizer nutrients.

If you want to mimic nature and garden organically, slow-release fertilizers are the best products for you. Still, they can come with a few drawbacks to consider.

Examples of Slow-Release Fertilizer

These fertilizers gradually release plant nutrients to maintain balanced plant nutrition over the entire lifecycle of a crop. They can also improve soil health over time. 

Pros

The science-backed benefits of slow-release organic fertilizers have prompted sustainable gardeners and farmers to switch to organic fertilizers for healthier soil and stronger crops. The pros of these products include:

Prolonged Nutrient Supply (Less Frequent Applications)

Organic fertilizers create long-lasting soil fertility, so you don’t have to apply them as often. If you’re a lazy gardener, you’ll be glad to know sometimes you only need to apply them once or twice per year. They provide sufficient nutrition for your plant over a longer time period, which means you don’t need to repeatedly fertilize. 

Anyone who has tended a high-maintenance lawn knows how annoying constant fertilizer application can be. Slow-release all-purpose products allow you to feed your plant and forget about it for a while.

More Balanced Nutrient Ratios

Organic fertilizers tend to be more balanced in their nutrients, while synthetic fertilizers may pose issues with nutrient deficiencies or overdoses. For example, an all-purpose organic blend like Espoma Garden-Tone Organic Plant Food is derived from feather meal, poultry manure, bone meal, greensand, and potash (wood ashes). The blend of these materials creates a balanced NPK analysis of 3-4-4. When the ratios of nitrogen, phosphorus, and potassium are close together, this means the fertilizer provides balanced nutrition.

For comparison, quick-release nitrogen, like synthetic urea, has an NPK ratio of 46-0-0. You can see how this could quickly create a huge imbalance in the soil!

Balanced plant nutrition is important because excess amounts of certain nutrients can actually make other nutrients unavailable to plants. For example, soils with too much potassium can “tie up” micronutrients like boron and manganese. If you apply a quick-release synthetic potassium fertilizer like potassium chloride (0-0-60), it could lead to a deficiency in boron or manganese.

These complex interactions have to do with the mobility or immobility of certain minerals in the soil. Without digging too deeply into soil chemistry, suffice it to say that nature does the hard work for you. Most slow-release fertilizers deliver even ratios of nutrients rather than a spike in a single nutrient.

Support Healthy Soil Organisms

Organic fertilizers don’t just feed your plants; they feed the soil organisms. Remember the external digestion analogy from above? While microorganisms work to digest nutrients for plant roots to uptake, they are simultaneously building soil structure and long-term fertility. The minerals from slow-release fertilizers enhance microbial populations to create a healthy, self-sustaining ecosystem belowground.

A diverse, abundant soil microbiome is directly linked to greater soil resilience and higher plant yields. Tiny soil organisms like bacteria, fungi, protozoa, beneficial nematodes, spiders, worms, and beetles are all vital members of your garden ecology. In addition to fueling plant growth, they help your plants withstand periods of drought, extreme weather, and pest or disease pressure.

Less Risk of Overfertilizing

Overfertilizing can be detrimental to houseplants, vegetables, flowers, and trees. In mild cases, it can reduce flower production or reduce the aromatic fragrance of an herb. In extreme cases, overfertilizing can cause a plant to turn brown and yellow or completely die. 

Gradual-release fertilizers are far less likely to “burn” your plants. These balanced, organic products provide a slow trickle of nutrients over time, so your plants aren’t blasted with massive amounts of nitrogen or potassium at one time.

Keep in mind that you still need to follow package directions. Never dump large amounts of fertilizer on any plant. Slow-release fertilizers can still cause overfertilization symptoms, but it is less likely. 

Natural and Organic

If you love nature, this benefit is very straightforward: slow-release organic fertilizers come from the Earth, while quick-release synthetic fertilizers are manmade and typically come from a chemistry lab at a fertilizer factory.

Minimal Nutrient Leaching (Better for the Environment)

Leaching is one of the biggest issues in the world of modern fertilizers. Rapid-release synthetic fertilizers are highly water-soluble, which means they are easily “flushed” out of the soil with rainfall or irrigation. These quick-release fertilizers (particularly nitrates) flow into groundwater, rivers, and ultimately the oceans, causing widespread devastation for fish and aquatic life. They are the primary reason for massive aquatic destruction, such as the Dead Zone in the Gulf of Mexico.

In contrast, gradual-release organic products are less likely to contaminate the groundwater because they are water-insoluble. They don’t dissolve readily in water. They naturally bind to soil particles and remain more stationary in the garden. Because slow-release materials require microbial breakdown, they are less likely to megadose a waterway with excessive nutrients that can imbalance the aquatic ecosystem. Organic fertilizers cause less pollution than synthetic ones.

Research shows that organic inputs reduce nutrient leaching and runoff by up to 30%. Other studies have confirmed that nitrate leaching is up to five times higher in conventional synthetic-fertilized plots versus organic ones. In other words, organic slow-release fertilizer is 5 times less likely to leach into the groundwater. This means far less pollution risk for the surrounding ecosystem!

However, beware that high amounts of improperly managed manure can also cause nutrient-leaching issues. It’s best to compost or age manure before using it in your garden and protect your piles from excessive rainfall. This is especially important if you live near a water source such as a pond or river.

Cons

The main drawbacks to gradual-release fertilizers are their timing, solubility, and a few ingredients with controversial sourcing.

Insoluble in Water

Slow-release organic fertilizers are mostly insoluble, which means they don’t readily dissolve in water. While there are some great liquid products (like liquid fish emulsion), most slow-release fertilizers are granular, pelleted, or ground up like compost. 

The insolubility is why these products are less likely to leach into waterways (which is a good thing!) But it can also be problematic for some gardeners, specifically container gardeners, houseplant growers, and people using fertigation (liquid fertilization through drip lines). 

Soil microbes are required to convert organic fertilizers into water-soluble forms. If your soil is lacking in soil microbes, slow-release fertilizers may have a difficult time breaking down. Plants could display signs of deficiencies because they are unable to uptake the fertilizer. This is yet another reason to prioritize amending with organic materials like compost and cover crops!

If you are growing indoors in containers without much microbial activity, consider using a more readily available organic fertilizer like Espoma Organic Indoor Plant Food.

Slower-Acting

The biggest drawback to organic fertilizers is that they require more patience. If your plant is yellowing due to a lack of nitrogen, a slow-release fertilizer won’t perk it back up in a day. It may take two to four weeks, depending on conditions. 

Take note that some organic fertilizers like seaweed and fish emulsions release nutrients more rapidly than something like compost. This is because they are in liquid form and readily soluble. If you need a fast-acting fertilizer but still want to use organic products, choose a marine-based fertilizer like liquid fish or kelp. These options are great for helping a houseplant recover or supplying nutrients to a dying tomato plant.

However, just because they are organic doesn’t mean they can’t burn the plant. Never add large amounts of these fertilizers to your plants. Follow package instructions and avoid over-fertilizing, as this can still “burn” the crop.

Some Unsustainable Resources

Some organic fertilizers, like bat guano or greensand, have sustainability and sourcing issues. Bat guano is controversial because of the bat diseases spread by guano collectors in bat caves. In conjunction with habitat damage and urbanization, these diseases are contributing to global declines in bat populations.

As an alternative, we recommend using worm castings because they are cheap, readily available, and easy to collect from a DIY home worm bin. You can also install bat boxes in the trees surrounding your garden.

Greensand is another organic fertilizer with potential environmental concerns. It is a completely natural slow-release potassium fertilizer that contains a wealth of trace minerals. However, it is mined from 80 million-year-old marine deposits that are nonrenewable. Once all the greensand is mined, there won’t be any left because it takes so long to mineralize belowground. Kelp meal is a great renewable alternative.

Quick-Release Fertilizer

The biggest problem with quick-release fertilizer is the potential for environmental pollution. It can also cause issues with nutrient “burn” from overfertilizing your plants. However, some landscapers and container gardeners still prefer these products because they rapidly release nutrients into the root zone, effectively bypassing the need for microbial decomposition.

Here are the main pros and cons of synthetic plant nutrients, with examples and scientific research.

Examples

Quick-release synthetic fertilizers are inorganic compounds. In chemistry, inorganic means a compound does not contain carbon. Coincidentally, these products are also not suitable for organic farming because they contain manmade synthetic chemicals. 

Most of these fertilizers are made of byproducts from the petroleum industry. This may not be readily advertised on the package, but you should be able to look at the ingredients of a product blend to find the scientific names of the ingredients.

Pros

The key benefits of rapid-release fertilizers are quick absorption and advantages for potted plants.

Quick Absorption

The name of these fertilizers is a major giveaway: they quickly release nutrients into the plant root zone, which means the plants can rapidly absorb the “food.” There is no need for synthetic fertilizers to go through the “soil digestive system” to get to the plants. These chemical compounds are already “available” for plants to uptake right away. 

If your plant is yellowing, necrotic, or lacking in flowers, a quick flush of fast-release fertilizer could provide very quick recovery, sometimes as fast as a few days to one week. Conventional farmers regularly use quick-release fertilizer because it boosts their crops instantly. However, organic farmers are not allowed to use synthetic products in their fields, so they must plan more to ensure that soil fertility is sufficient before planting.

If your soil is extremely poor, a quick-release fertilizer provides quick relief for nutrient-deficient plants. But you must beware that it won’t create lasting fertility the way that organic fertilizers do. In fact, the “quick fix” could actually cause your garden to have more problems over time.

Solubility in Water

The solubility of quick-release fertilizers is a benefit for growers who need to instantly boost a plant or brighten a dying lawn. These products readily dissolve in water or come in a liquid form. They are easy to mix in a watering can or flow through a fertigation system.

More Precise Nutrient Ratios

Advanced plant chemistry has made modern fertilizers extremely precise about their nutrient ratios. While the mineral nutrients of compost can vary widely based on the ingredients, the nutrients inside a synthetic fertilizer are very pinpointed. 

Good for Houseplants

Many houseplants and orchid growers prefer synthetic fertilizers because they are the industry standard for nurseries. The rapid release of nutrients is easy for promoting big blooms or helping a plant recover. However, as you’ll see below, this can also come with some major risks, like fertilizer burn or constant reapplications.

Cons

Quick-release fertilizers pose major issues for environmental sustainability, leaching, salt buildup, and the potential to burn your plants.

In my opinion, the risks of rapid-release fertilizer are not worth it. Organic gardeners should avoid these products. Nature already gave us all the fertility we need in the form of healthy soil amended with plant and animal byproducts!

Leaching Environmental Pollution

The high solubility of quick-release fertilizers means they are highly prone to leaching. When rain or irrigation water hits your front lawn or garden soil, synthetic fertilizers can easily runoff or leach out. Where do they go? Into the groundwater, where they can cause major environmental pollution that harms wildlife and contaminates drinking water supplies.

The Dead Zone in the Gulf of Mexico is a prime example of the ecological disasters caused by modern synthetic fertilizers. When excessive amounts of nitrates are applied to farm fields in the Midwest and Mississippi River basin, the rain washes excess fertilizer into the groundwater and streams. This water, full of fertilizer runoff, ends up in the Mississippi River, which flows to the Gulf of Mexico.

When the nutrient-heavy water reaches the gulf, it causes a problem called eutrophication. The excessive amounts of synthetic quick-release fertilizer fuel a sudden bloom in algal growth. This algae covers the ocean surface, effectively killing everything beneath it. The hypoxic (oxygen-devoid) zone below kills off all the fish and other aquatic life. As of 2023, the Dead Zone is over 4,000 square miles in size. In past years, it expanded to over 8,000 square miles.

If you care about oceans and clean, safe drinking water, consider avoiding synthetic quick-release fertilizers! 

Frequent Applications Required

The most annoying thing about quick-release fertilizers is their neediness: they must constantly be reapplied! The rapid release also means rapid decomposition. As plants use up the nutrients, they quickly become hungry for more. 

While slow-release organic fertilizers build up soil fertility over time, synthetic products create a constant need for reapplication. Once you become accustomed to feeding plants with synthetic nutrients, you are stuck on a sort of “fertilizer treadmill”: you have to keep buying more fertilizer and apply it many times throughout the season. This isn’t only higher maintenance; it results in a higher risk for issues like overfertilization or leaching.

Salts

The residual salts from excessive synthetic fertilizer use can harm soil health. In other words, synthetic fertilizers contain salts that hurt soil microbes and reduce soil fertility over time. This is why modern industrial agriculture faces such tremendous challenges with soil health and sustainability. After decades of synthetic fertilizer application, the soil of some crop fields becomes completely degraded and no longer farmable. 

Risk of Nutrient Burn

Over-fertilizing is a major risk with quick-release fertilizers because these products tend to be highly concentrated. The result is nitrogen “burn” or fertilizer “burn” because the plant receives an overdose of nutrients that it cannot process. Imagine it as similar to taking an outrageous amount of vitamins at one time (bad idea!) 

When a plant is doused in a concentrated synthetic fertilizer, it can cause rapid wilting, yellowing, browning, leaf drop, or plant death. While you can over-fertilize with any product, it’s particularly risky with rapid-release nutrients because they megadose the plant all at once.

Synthetic With Petroleum Byproducts

The majority of synthetic fertilizers are man-made from petroleum byproducts. Synthetic nitrates are even manufactured using a similar process to the nitrogen bomb! After World War I, the government had a lot of leftover ammonium nitrate that was originally used for explosives in the war. They realized that it could be used as a nitrogen fertilizer and began selling it to farmers. 

Most modern-day quick-release fertilizers are still made with the same Haber-Bosch process used in WWI. This method for producing synthetic nitrates uses high pressures and high temperatures to synthesize ammonium nitrate. This process alone is estimated to contribute to nearly 2% of global carbon dioxide emissions every year.

So, if you’ve ever heard of fertilizer plants exploding or barns catching on fire, now you know why! Quick-release nitrogen (specifically ammonium nitrate) is highly flammable and explosive because it was originally developed for wartime use.

If you want to keep petroleum and explosive ingredients out of your garden, choose slow-release fertilizers instead.