A fertilizer primer

hand holding fertile soil
Organic fertilizer helps boost soil and plant health.

It’s been a long time since I wrote about fertilizer so I’m going to do that now because people are always asking about what to feed their plants. First, though, it’s finally time for seed sharing and I’ve got the Little Free Seed Library outside my house all stocked up. As always, the top shelf of our library is reserved for seeds in the spring and fall. If you’d like to pick up or drop off some seeds, the library is located on our boulevard on the corner of 45th Street and Washburn Avenue in Linden Hills.

Coin-size envelopes are in there so you can package up seeds to take home. If you have seeds to share, please bring them in their original packets or label them in envelopes or baggies in some way so people can clearly see what’s available.

Now, let’s talk plant food. People often ask me why their plants look spindly and sad when mine are so lush and happy. The answer is simple: I feed them. All living things need sustenance, and plants are no exception. Lots of people who tell me they don’t have a green thumb would find that the opposite is true if they added food to their plants’ usual diet of plain old water. This is especially true for plants in containers in which soil nutrients are quickly depleted and not replenished as they are in gardens with healthy soil.

If you’re new to fertilizing, here’s a quick tutorial. On every package you will find three numbers such as 5-5-5 or 10-5-5. Those numbers are always in the same order and represent the percentage of three nutrients. The first is nitrogen (N), which promotes green, lush growth. The second is phosphorus (P), which is good for developing healthy roots and is important for flower and fruit development. The last is potassium (K), which plants need for healthy overall growth.

People used to advise gardeners to always use a “balanced” fertilizer, meaning all three numbers would be the same, like 10-10-10. But newer research has shown that balanced fertilizers often provide far more phosphorus and potassium than plants really need. Instead, look for products with numbers more like 5-1-2 or 5-1-3. That way, you’ll avoid having too much phosphorus and potassium build up in your soil. And you’ll also help protect the environment from those excess nutrients, which often leach or run into nearby waterways where they promote the growth of algae and harm fish and other creatures.

Organic or synthetic?

When you shop for fertilizer, you’ll find two types to choose from—organic and synthetic. Organic options are derived from ground-up rocks, animal waste, plant parts and other organic matter. A few examples include greensand, blood and bone meals, cottonseed meal, fish emulsion, seaweed extracts, compost and manures. Synthetic fertilizers are made of synthesized nitrogen, phosphorus and potassium through processes that rely on some kind of chemical reaction involving non-renewable resources (synthetic nitrogen is produced using natural gas).

I use organic fertilizers on my plants because they are generally a more sustainable choice—exceptions include rock phosphate, which is strip-mined, and bat guano (poo), which is harvested from caves at bats’ expense. Organic fertilizers also help foster a healthy environment for beneficial microbes in the soil. And, while they usually contain smaller amounts of N-P-K than synthetic fertilizers do, organic options can be more nourishing to plants because they are taken up slowly as needed where synthetic fertilizers are slurped up fast, like a human gulps down a Coke.

It’s not always easy to find a good selection of organic fertilizers. So I’d like to give a shout out to Midwest Supplies. Located in Minneapolis, this fine little store has a wide array of choices and knowledgeable staff to answer any questions you may have. As I wrote in a column a few years back, my organic concoction of choice is a mix of fish emulsion and blackstrap molasses. I got the idea from Dean Engelmann, co-owner of Tangletown Gardens, who uses the mixture himself.

Unsulfered blackstrap molasses is commonly used in organic horticulture. The sugars in it feed beneficial microbes, helping to boost soil and plant health. Dean recommends combining the two at a 1-to-1 ratio. I use a five-gallon bucket for this, first adding fish emulsion with the amount of water recommended for the area I’m feeding (read the label). Then, add the same amount of molasses as you did fish emulsion, and give the mixture a good stir because the molasses gets blobby.

It’s not a great idea to let this stinky brew sit around, so make only what you need at one time. To use, just dip a plastic pitcher or whatever into the bucket and pour the liquid fertilizer wherever you need it. I mostly use it on container plants and vegetables, but if something else in the garden looks like it could use a good meal, go for it. One thing: do your best not to spill this stuff on your shoes. They will smell like dead fish forever.


Meleah Maynard is a Minneapolis-based writer and editor who blogs at Livin’ Thing

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  • Becky

    Hello! As a fairly new organic gardener, could you tell me what the NPK ratio of this combo would be? Thx so much!

  • Alex Dubois

    The liquid fish will vary in strength depending on the source. Alaska Fish will be 5-1-1 whereas Dramm will be close to 3-3-3, again depending on the product. Molasses typically runs somewhere around 1-0-4 but also contains a whole host of trace minerals and some sugars. It is typically used at a ratio of 1-2 oz per gallon of warm water.

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