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."