Marin Master Gardeners
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Marin Master Gardeners

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Before you fertilize, stop and think: Do you really need to?

June 8, 2019
Nanette Londeree

It’s spring and the garden is growing like crazy, especially with the bountiful rain in May and early June. Do you think it is time to feed the lawn, the tomatoes and veggies, roses and other flowers? Before you haul out bags, boxes or bottles of plant food and sprinkle, spread or pour them on, consider whether you really need to. Are the products beneficial to the plants? To your wallet? And to the environment?

We routinely describe fertilizing as “feeding” plants, but that’s not accurate. Plants feed themselves, producing their own food, sugars and carbohydrates, through the process of photosynthesis. Most of the 18 chemical elements plants need for healthy growth are already in the soil or the air and don’t need to be added regularly.

Rather than fertilizing on a calendar-based schedule, watch your plants and give them only what they need, at a time when they can use it. Understanding the needs of specific plants and identifying nutrient deficiencies in the soil are first steps in figuring what, if anything you should be adding. Also consider the cost of the fertilizer and any potential impact it may have on the environment.

Test your soil to see what levels of nitrogen, phosphorus and potassium (N-P-K), the chemical elements most quickly used by plants, are present before adding more. Too much fertilizer can damage plants or cause other problems. Excess nitrogen, especially fast-release forms, can produce lush growth that’s attractive to pests like aphids. In lawns it stimulates growth that requires more water, more frequent mowing and can lead to increased opportunities for disease. And that surplus fertilizer can end up running off into storm drains, ultimately making its way into bodies of water like the bay, where it can have a negative impact on fish and other aquatic animals.

Slow-release fertilizers seem like a boon to the gardener — saving time and effort with a single application for the season. Pellet-like granules of multi-element fertilizer are coated with a substance that slowly breaks down allowing the release of a small, steady amount of nutrients over time. However, if you use a drip system for irrigating, this type of fertilizer won’t do you much good unless the granules are placed directly below the emitter. If you water often with a hose, you may speed up the fertilizer release process and give the plant more than is needed. And if you apply too early in the season, the nutrients can get flushed out of the soil by spring rains before the plant has had the opportunity to use them.

Some of the most expensive products, that are also tough on the environment, are multipurpose ones that tout one-step care for feeding and protecting against weeds or disease and insect pests. Why treat your plants or lawn with pesticides if they don’t need it? Would you give your family daily doses of antibiotics just in case one might be exposed to an infection? Imagine the cost of that, the toll it would take on their body, and the possibility that it may weaken their natural defenses and make them less able to fight off disease. Using pesticides in a preventive manner is basically the same approach for your plants. You’ll save a lot of money by staying away from combination products and be kinder to the environment at the same time.

If you do need to provide specific nutrients to your soil, consider using natural fertilizers. Often sold as meals, these agricultural and farming by-products release nutrients slowly over a longer period, allowing plants to absorb them more efficiently. Compost, with a relatively low N-P-K ratio, does double duty, providing nutrients and improving soil quality. If you’ve got lawn, leave the clippings after mowing (called grass cycling); they will break down and put nitrogen back into the soil.

Use fertilizer wisely; just give your plant what it really needs, when it can best use it.

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