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Mike McGrath

Slow Release Fertilizer - What is It & Why is it Important?

Table of Contents

What is Slow Release Fertilizer?

Slow release fertilizers, sometimes also called time release fertilizers, release a small amount of nutrients into the soil over weeks, months and sometimes even years.

This plant fertilizer may be made of natural, organic material, or synthetic material. Organic slow release fertilizer releases its nutrients as it decomposes. Synthetic slow release fertilizers are often coated with a polymer or resin that breaks down when exposed to water, heat, and other elements.

Nutrients Found in Slow Release Fertilizers

Both quick-release and slow-release fertilizers contain three major nutrients: nitrogen, phosphorous and potassium (sometimes called potash). These three nutrients play different roles in plant growth and development.

  • Nitrogen aids in leaf development and plant growth. Plants given a nitrogen boost may become a more vivid green.
  • Phosphorous helps with the plant's root growth and the production of fruit, seeds and flowers.
  • Potassium helps promote the plant's root development, which in turn helps the plants stand up to drought and disease pressure.
  • Fertilizers may also contain other micronutrients, plant materials, manures, animal byproducts (such as fish emulsion or bone meal) and mined minerals that feed the soil.

Why Use Slow Release Fertilizer?

Using a slow release fertilizer for vegetables, fruits, ornamental plants, and your lawn can have many benefits. Because these fertilizers release nutrients at a slow rate, plants grow at a more uniform rate. This, in turn, means that the plants will develop stronger root systems and experience less stress, so they are better able to handle disease and pest pressures. This is one reason that slow release fertilizers are favored for seed-starting and new plantings. Also, because the nutrients are released slowly, the plants are less likely to experience "burn" from the fertilizer.

Slow release plant fertilizer tends to be better for the environment, too. This is because it is less likely to leach. Finally, because it releases its supportive minerals over time, organic slow release fertilizers are applied less often than fast-release options.

How to Use Slow Release Fertilizer

Slow release fertilizer can be used on all types of plants, including vegetables, fruits, turf, shrubs and trees, and ornamental plants. When selecting the best slow release fertilizer for your needs, take a few things into consideration.

You can choose a synthetic, slow release fertilizer or an organic slow release fertilizer (also known as all-natural). Gardens Alive offers a complete line of all-natural fertilizers.

Because different plants have different nutrient needs, slow release fertilizers have different N-P-K (nitrogen, phosphorous, and potassium) ratios. For example, the recommended fertilizer for strawberries has higher levels of nitrogen while other ones for fruiting plants may have higher levels of phosphorous. This makes it very important to select slow release fertilizers for specific plants and to read label instructions carefully.

Gardens Alive offers slow release fertilizers for specific vegetables, lawns and turf grass, container gardening, fruit plants, and more. This helps take some of the guesswork out of choosing the right slow release fertilizer.

How to Read Fertilizer Numbers

When reading fertilizers, the NPK ratio is listed on the package. For example, the NPK ratio on Vegetables Alive! Fertilizer is 4-5-4. The first number is nitrogen, the second is phosphorous, and the third is potassium. This means the fertilizer contains 4 percent nitrogen, 5 percent phosphorous and 4 percent potassium. Because sweet corn requires more nitrogen, the NPK ratio on the slow release nitrogen fertilizer Sweet Corn Alive! is 6-3-3.

In slow-release, all-natural fertilizers the NPK numbers are often lower than in quick release fertilizers. That's because the numbers represent the nutrients that are immediately available. Slow-release formulas make nutrients available for weeks and often months. This allows for continuous feeding of the soil.

Slow Release Fertilizer vs Quick Release Fertilizer

While slow release fertilizer offers many benefits, fast release fertilizer has its appeals. The primary appeal is that fast release fertilizer gives the plants an immediate boost and you see quick results. For example, if you want a lawn to green up and burst with growth quickly, then a fast release fertilizer will do this.

There are some downsides to the quick release fertilizer. Because the nutrients are released rapidly, they may leach rapidly which can be harmful to the environment. Fast release fertilizers don't feed as long and require more frequent applications. Over application can burn plants, so you have to be more careful about applications.

Mike McGrath

Slow Release Fertilizer - Expert Advice

Q. How does slow release fertilizer work? Should I use it instead of regular fertilizer? If I use slow release, should I still fertilize once in a while? Thanks,

    ---Brenda in Claremont, CA

A. Thank YOU, Brenda; this is the perfect time of year to discuss the feeding of plants. AND more and more garden writers seem to be recommending "slow release fertilizers".

Let's begin by directly answering the question for a change. In the world of packaged fertilizers, "slow release" typically describes a product whose layers are designed to melt away in sequence, supposedly releasing a steady stream of active ingredient, just like "time release" medications.

But in a garden, this release has to be highly variable; controlled by the factors that specifically activate the layers in a certain product to fall apart. You'd think the motivating factor would be 'passage of time', but only the use of nuclear elements as the wrapping material would provide that much of a standard and predictable rate of decay. (Thankfully, chemical companies haven't marketed "Atomic Plant Food" yet. I hope I haven't put any ideas in their heads. I can see the ads now: "Guaranteed rate of release! Plus: your plants glow in the dark, so you can tend them at night!)

With our current, non-nuke fertilizers, heat will play a factor in the unraveling of the layers. So a full sun location, an especially warm clime, a hot summer, or the substance sitting on the surface as opposed to being covered with cooling soil would release the fertilizer faster. Same with water; a wet year or poorly drained soil would cause faster release.

So the speed at which a 'time release' fertilizer IS released will vary from garden to garden. But the vast majority of such fertilizers are chemically-based; and the essence of an organic, natural and/or sustainable gardening philosophy is the avoidance of such things. No matter how they're released, chemical fertilizers are artificial and salt based. The unnaturally fast growth they cause is very appealing to pest insects and disease; and studies show that chemical fertilizers inhibit a plant's ability to produce the naturally occurring compounds that can protect it from such attacks.

Plus, as Bill Wolf, one of the first advisors to the National Organics Standards Board, used to love to point out, organic fertilizers are naturally slow-release; always making their food available to plants over a much longer period of time than their chemical counterparts.

The mined mineral known as rock phosphate, perhaps the best natural source of phosphorus (the plant nutrient represented by the middle number on the label of any packaged fertilizer) is a great 'single ingredient' example of this. This organic bloom-boosting nutrient is only rated as having 2 to 3% available phosphorus on the label. But its total content is actually a whopping 25 to 30% phosphorus, which will become available to your plants gradually, over a time span of several years—as opposed to a couple of months for even the slowest 'time-release' chemical fertilizer. This release is so slow and steady that you won't need to add any more phosphorus to your soil for a good three to five years afterwards, making it much more cost effective than even the cheapest chemical version as well.

Greensand, a mined mineral that comes from 70-million-year-old marine deposits, is another great example. This super-cool source of natural potassium (the third number on a fertilizer label) will only be marked as containing 1% of this important all-around plant nutrient. But, again, that's just the amount it releases right away. The total potassium content of this prehistoric treasure is actually 7 to 10%, released slowly over a five to ten year period. Plus it's an excellent source of important trace minerals, which you won't find in any chemical fertilizer.

Large-scale gardeners and organic farmers typically buy these nutrients in bulk every three or four years. Everyday backyard gardeners will find them (or other natural sources of those nutrients) in packaged organic fertilizers designed to be used once a year or so.

When using ANY granular-type fertilizer, spread it on top of bare soil as directed, then cover it with an inch of soil or, even better, compost, so that the nutrients are exposed to the soil life that helps carry them into your plants in the most beneficial form.

Of course, the ultimate once-a-year slow release fertilizer is compost! University studies—and the experiences of countless organic gardeners—continually reaffirm that a two-inch layer of compost applied to the surface of the soil over the course of a season provides all the fertilizer that almost every plant requires to look and perform its best. And, of course, it also improves the structure of your soil and adds much needed life to the garden.

The only exceptions to the two-inch rule that come to mind are heavy feeders like sweet corn, which needs more nitrogen (good sources include composted horse and poultry manures), and areas in the Deep South where the intense heat burns organic matter up more rapidly. Apply up to four inches of compost a year in locales where winter is short and somewhat sweet.

Just remember that you can't go wrong when you "feed your soil, not your plants." No one ever fed the Great Plains, the endless stretches of wildflowers or the hardwood, evergreen or rhododendron forests.

So, if chemical fertilizers tempt you, "consider the lilies of the field; how they grow...even Solomon in all his glory was not arrayed like one of these."

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