The Truth about Organic Fertilizers and Weed Control

Truth about Organic Gardening by Jeff GillmanJeff Gillman knows all about the hot controversies within the gardening world in this era of eco-consciousness and has this complaint: “Everyone seems to want to pass judgment.” As a professor of horticulture at the University of Minnesota, he’s concerned that most gardening information is written by people who follow a particular school of thought and are “unable to see beyond their biases.” So he wrote The Truth about Organic Gardening (Timber Press) to help gardeners “see beyond dogma” about organic versus synthetic gardening products, one of the primary areas of disagreement and confusion. I say amen to that.

Soil Enrichment and Fertilization

So let’s examine some common myths about fertilizers. First there’s the notion that nutrients in synthetic fertilizers are different from the ones in organic fertilizers, or that synthetic fertilizers are “chemicals” and organics are not. Not so — they’re all chemicals. Organic ones are slower to break down and take effect, so they last longer. They also require a larger quantity to be used and therefore cost more for the same quantity of nutrients. Synthetics are quick and cheap and concentrated, which accounts for their popularity.

Another common belief is that synthetic fertilizers are made from petrochemicals, but Gillman says that’s rarely the case. Synthetics do use a lot of power in their production, but that’s usually natural gas.

And here’s a surprise: organic fertilizers can be brought to the market through the decidedly unsustainable practice of mining. For example, rock phosphate is mined in Florida and North Carolina and those mines do considerable damage to the land there. (Potassium used in synthetic fertilizers is mined in the Western states, with similar deleterious effects.) So Gillman recommends using unmined organic fertilizers that re-use nutrients from other living sources — fertilizers like compost, bonemeal, blood meal, seaweed extracts, alfalfa meal, and fish emulsions.

Another myth about organic fertilizers is that they don’t leach into our groundwater the way synthetics do, and Gillman disagrees, saying they’re just as likely to leach into our groundwater “if they’re used in the over-aggressive way that most people fertilize their lawns.”

In working with homeowners as a gardening coach I’ve noticed the widespread assumption that everything in the garden needs regular applications of fertilizer. Gillman knows better, though, explaining that we should focus instead on making soils more fertile so they’ll support healthy plants, and that means not just the usual N-P-K in most packaged fertilizers but also the right pH, bacteria, fungi, and organic matter. And the best source of all of that is good old compost. So for vegetable gardens he recommends tilling into the soil a half-inch of composted manure; just make sure the compost has been cured long enough, especially if it’s manure (otherwise it can contain high levels of human pathogens).

But how about the rest of our plants, like shrubs, trees, and perennials? Gillman told me in an email that they don’t even need compost, just a good organic mulch every year. That’s been my own practice for decades and my plants seem happy enough. Even roses will bloom without “rose food” but if you want maximum floral performance he suggests two applications of alfalfa meal per season around your roses. One clear exception to the mulch-only rule for ornamentals is plants in pots because their nutrients are leached out by the frequent watering they need. Gillman’s favorite fertilizers for pots are fish emulsion and the ones based on seaweed. And of course you’re using a good potting soil, right?

Now the plant that homeowners ask gardening experts about more than all the others is turfgrass, so I nagged Gillman for some advice about lawns. He couldn’t resist first blaming the corporate members of Overfertilizers Anonymous for creating a national addiction to bright green monocultures of perfect lawns.  Gillman’s own favorite fertilizer for lawns is corn gluten meal, which is commonly used to prevent weeds, so it’s a twofer!

And that’s a nice segue to weeds – in the next column.

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