Making sense of fertilizers

Judith Adam

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A furry helper among the fertilizers. (Photo by Brendan Zwelling)

I’ve never thrown away partially used containers of fertilizer products, but just keep pushing them to the back of the shelf. If they’re stored cool and dry, the nutrients remain fully potent indefinitely. However, the shelves in both basement and garage are now full, and some sorting needs to be done. These are organic and manufactured (sometimes referred to as synthetic) fertilizers that I’ve used for indoor and outdoor plants. Some are liquid concentrates; others are water-soluble crystals. They all contain the basic plant foods—nitrogen, phosphorus, potassium—and some have added micronutrients.

Some packages indicate the fertilizer should be used for specific plants (like roses or tomatoes) or for specific purposes (like encouraging buds and blooms). There are also some general, all-purpose fertilizers. Here’s my collection:

Orchids 19-31-17

Tomatoes 18-18-21

Bloom builder 5-30-5

Flower bloom 8-16-8

Perennials, annuals, roses 12-12-12

Annuals 15-30-15

Roses, flowers 5-7-2

Roses 9-18-9

Vegetables 14-14-14

All-purpose 20-20-20

All-purpose 8-12-9

Bonsai 0-10-10

Fruit tree 10-15-15

There’s quite a range in analysis, some with lower amounts of the essential nutrients, as compared to others with almost twice as much. There are also differences in the contents of micronutrients contained in some. No wonder gardeners might be confused when selecting fertilizers. Should we buy the package with a specific plant picture (like tomatoes) or the all-purpose fertilizer? Is it better to buy fertilizer with higher or lower nutrient analysis?

These are all good fertilizers, and if package directions are followed, they will benefit plants. Looking at the fine print on the tomato fertilizer label listing the full range of nutrients included, I see that it contains more magnesium than other fertilizers. That would be because tomatoes (as well as roses, potatoes and peppers) require magnesium to produce quality flowers and fruits. Magnesium is a part of the chlorophyll in all plants, and essential to photosynthesis and plant metabolism. (Magnesium deficiency is one of the causes for blossom-end rot in tomatoes.)

The tomato fertilizer is also high in potassium (the third number), which promotes strong cell walls and disease resistance. That would be important for any soft-skinned fruit or vegetable. The orchid fertilizer probably also contains elements those plants specifically require, although other kinds of plants would be well nourished with it, too. My thought is that fertilizers for specific plants will certainly benefit them, but can also be used for other garden plants with good results.

When evaluating my collection, I considered their nutrient analysis, which ranges from reasonably low (5-7-2 for roses) to more than twice as strong (all-purpose 20-20-20). The middle nutrient—phosphorus—contributes to root growth and bud formation, and fertilizers designed for flowering plants (like summer annuals and roses) often have a higher middle number. Yet some accomplished rosarians use 20-20-20 with gorgeous results. It’s a matter of personal preference, and perhaps a bit of experimentation will help to form an opinion of which is the best fertilizer analysis to produce prolific flowering.

I’m more concerned with the nitrogen content (the first number in the analysis), because in large amounts it can cause stress to plants, and excessive runoff in ground water. I prefer to keep the nitrogen number below 15 for spring feedings (when plants want to grow and can use more of the nitrogen), and then below 10 for midsummer feedings. That doesn’t mean that my package of 20-20-20 is useless; I’ll just use less of the concentrate when diluting it with water.

I probably have a lifetime supply of fertilizer, and only need to adjust the dilution rates for the type of plant and season. I haven’t solved the shelf storage problem, but at least I can cook up a meal for plants with greater efficiency.


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5 thoughts on “Making sense of fertilizers”

  1. I have several fertilizers which I’ve had for years. 19-31-17, 20-20-20, 10-52-10 and 8-7-6. Can you till me which are good for vegetables, which are good for flowering plants (like geraniums and petunias) and for general green house plants. I’ve read so much about fertilizes and I’m still at a loss and need some guidance.

  2. What a confusing topic indeed.

    I am trying Miracle Grow 18-18-21 (Tomato fert.) on both my Orchids (as per American Orchid Society) and Alstroemeria and Tomato plants.

    It’ll be interesting to see the results.

    I also have:
    Miracle Grow: 24-8-16 (All Purpose), 15-30-15 (Bloom Booster)
    Shultz: 19-31-17.
    I’ve heard the theory or alternating fertilizers to get the best of what each offers.

  3. Judith,
    Thank you so very much for making sense out of and enlightening us about the types of fertilizers and their uses. Once again, your informative writing skills and common sense content makes it easy to understand another gardening mystery – fertilizers! Thanks, Judith, for sharing your knowledge, it is very appreciated!
    Deanna Groves

    • Hi Deanna,

      Thanks for your kind message. I don’t know when we’ll get to use fertilizers, if the weather doesn’t get more spring-like. There was ice in the birdbath, again, this morning.

      — Judith


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