There are many types of fertilizers available in the market. Is the slow-release or the fast-release type better?
Slow-release fertilizers are better for houseplants and vegetables if you want to establish sustained, lasting growth, and reduce the risk of root burning. To target a specific nutrient deficiency problem and large lawn area, fast-release fertilizer is a better option.
In this article, we will find out which is better by looking at the differences in the timing of nutrient release and the nutrient profile.
1. Fertilizer type
Fast-release fertilizers are water-soluble conventional chemical fertilizers, such as ammonium nitrate, urea, ammonium phosphate, potassium chloride, and exist commercially in liquid and powder form.
Slow-release fertilizers, also known as “controlled-release fertilizers”, contain the same chemicals in the form of small pellets or granules coated by a layer of synthetic or natural polymer, such as polyethylene, sulfur, starch or clay, which acts as a physical barrier to slow down the release of nutrients.
Slow-release fertilizers can also be organic or natural sources, such as compost, vermicompost, and green and animal manure which require soil microbes to decompose over time.
Osmocote slow-release plant food
2. Rate of nutrient release
Fast-release fertilizers release nutrients almost immediately once they are in contact with moisture.
On the other hand, slow-release fertilizers, depending on the coating used and also the soil moisture, generally take between one to a few months to release the nutrients to the plants.
The most common coatings use polymer coatings which are made of a plastic material that dissolves gradually, allowing the nutrients to be released slowly over time. They can also be formulated to release nutrients in response to environmental conditions such as temperature or moisture, providing more precise nutrient delivery to plants.
Sulfur coatings, on the other hand, are made of a sulfur compound that reacts with the soil’s moisture, creating a barrier that slows down the release of nutrients. They are more effective in acidic soils and release nutrients at a slower rate than polymer coatings.
There are also other types of coatings used in slow-release fertilizers, such as resin coating, oxide coating, and starch coating.
Depending on the brand, slow-release fertilizers generally release the nutrients over a period of 3 to 6 months so they are best applied at least once at the beginning of the growing season, in late winter and late spring. This gives them time to start releasing nutrients into the soil before the growing season begins.
Quick-release fertilizers should be applied multiple times, usually once every month, during the plant’s active growing season usually in spring and summer.
3. Nutrient profile
Fast-release fertilizers are ideal for targeting specific nutrient deficiency problems.
The most commonly used fast-release fertilizer is the “Cal-Mag” fertilizer. It is a liquid fertilizer with concentrated Calcium and Magnesium minerals to target plants with Calcium deficiency, which generally manifests as black rot on new leaf tips and margins during the growing season.
The same “Cal-Mag” liquid fertilizer or Epsom salt can also be used to target Magnesium deficiency which manifests as a yellow mosaic pattern on older leaves with the presence of black rot as well.
Another commonly used fast-released fertilizer is urea or nitrate fertilizers to target problems with nitrogen deficiency which manifests as a yellow mosaic pattern on older leaves but with the absence of black rot.
A fertilizer with higher portions of potassium can target potassium deficiency which manifests as black rot on older leaves and leaf drop.
Here are some fast-release fertilizers that can target specific nutrient deficiency problems.
Problem | Solution |
Calcium Deficiency (Black rot on new leaf tips & margins) | Cal-Mag  |
Magnesium Deficiency (Yellow mosaic on older leaves, black rot) | Cal-Mag (same as above), or Epsom salt  |
Nitrogen Deficiency (Yellow mosaic on older leaves, no black rot) | rePotme Orchid food  |
Potassium Deficiency (Black rot on older leaves, leaf drop) | Better-Gro  |
Also, fast-release fertilizers can target certain plant types.
For example, most plants can take up both ammoniacal nitrogen and nitrate nitrogen, but acid-loving plants such as blueberries, strawberries, and azaleas, etc. prefer ammoniacal nitrogen and orchids prefer nitrate nitrogen for growth and development.
On the other hand, applying slow-release urea that is sulfur-coated can increase microbial activities in the soil because both sulfur and urea help increase soil acidity, which is better for microbes. Sulfur is also an essential mineral for all plants.
Which is better?
Fast-release fertilizers are ideal when you are targeting a specific nutrient deficiency problem and when the cost is a concern, especially for fertilizing a large space such as lawn.
On the other hand, slow-release fertilizers are better for houseplants, vegetables and small garden space as you don’t need to worry about timing and fertilizer burn.
Using slow-release fertilizers is more expensive and may not be very practical for those who need to fertilize a large space because of the higher cost of producing polymer-coated pellets.
The synthetic coating on the pellets may be difficult to degrade, causing environmental problems and the sulfur coating can increase the acidity in the soil over time.
Also, because of more time needed to release nutrients, slow-release fertilizer pellets (or liquid) can be washed off the soil by rainfall or irrigation before the nutrients can be released to plants.
Because fast-release fertilizers release nutrients immediately, getting the right dosage is very important. It is easy to cause fertilizer burn and damage plant roots, especially plants such as orchids which have delicate roots.
Fast-release fertilizer is also not a very efficient way to fertilize plants because much of it applied is washed off and not absorbed by plants. Studies have shown that only 30-35% of chemical fertilizers are used by plants.
Also, the application of fast-release fertilizers should be timed according to the nutrient type and quantity needed by the plant in different development stages, whether it is developing foliage, roots, or flowers and fruits.
For example, snap beans have a slow-fast-slow growth stage pattern where a small quantity of nutrients is needed in the early development stage, followed by a greater requirement in the middle stage and a smaller requirement again in the late stage.
Happy gardening!
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Reference
Controlled-Release And Slow-Release Fertilizers As Nutrient Management Tools
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