Sciency Answers: The great fertilizer debate

Gary has a question:

Are organic fertilizers such as GardenTone and HollyTone really worthwhile using or is putting compost on your garden beds just as good?

Feeding soil versus feeding plants

Compost provides two things when you put it in your garden. A small amount of fertilizer for plants to take up, and a huge amount of food for all the earthworms, bacteria, fungi, and such in the soil. The fertilizer component is released slowly over time as the compost is degraded by soil life and taken up by plants to use to build leaves and flowers. The soil eating the rest of it improves the texture and structure of the soil making it better at holding water, nutrients, and allowing plant roots to grow through it more easily, so the same amount of fertility is utilized much more effectively.

Fertilizers, on the other hand, just provide nutrients for the plants. Because they are not tied up in the complex structures of compost, they are released quickly in higher concentrations to the plant roots. There really isn't much of a difference between organic and synthetic fertilizers here. The compounds the plant roots actually take up are absolutely identical in either case, and, depended how they are formulated, both synthetic and organic fertilizers will be released fairly quickly in high concentrations. Since both lack the bulky organic matter of compost or mulch, neither are going to do anything to improve soil structure and health in the long term.

More isn't always better

Fertilizers need to be used with caution. I grew up in a rural area where most people's "lawns" were really periodically mowed meadows, never fertilized or watered, and full of as many flowers (what the lawn care companies call "weeds") as actual grass. But we got a new neighbor who had lived in the city, who wanted a green, all grass lawn. They bought a bunch of fertilizer, and dumped it on. Their grass turned a vivid shade of green almost over night. And after the next rain storm, so did the drainage ditch all down the street and a good portion of our stream with a mass of algae spurred into growth by the fertilizer bonanza. Because fertilizers are quite concentrated and release their nutrients quickly, it is easy to over do it. You can harm you plants, but long before you do, you'll harm the more delicate life in the soil, and pollute your local ground water and wetlands as all the extra fertilizer leaches out of your soil.

The bottom line

In my own garden, I rely almost exclusively on compost and mulch to provide fertility. I do use more concentrated fertilizers, but only rarely in my container plantings, and very rarely in new beds that haven't yet been beefed up with enough compost for hungry plants like vegetables. In my ornamental beds, I don't even use compost, just regular mulching, because keeping fertility relatively low there keeps my plants a bit smaller and more compact so I don't have to stake them. When I do buy fertilizer, I frankly don't see much difference between synthetic and organic, so I go with price and convenience, which leads me to a slow-release synthetic fertilizer.

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