01 November 2011

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.

Have a question? Get a sciency answer! E-mail me: engeizuki at gmail dot com

7 comments:

Laura said...

this is so helpful! thank you.

Liz said...

I like this post. I really don't use much fertilizer--a little on my veggies and I'm good. Also it's a great idea to get a soil test for things--I did have a phosphorus deficiency that was preventing my lawn from growing. But I only added a little bit--I'd prefer my lawn not grown than utrification in the waterways.

Susan in the Pink Hat said...

I would take issue with "There really isn't much of a difference between organic and synthetic fertilizers." There's a huge difference in processing and manufacturing. Chemical based fertilizers are usually salts chemically bonded to the nutrients to stabilize their shelf-life and provide a quick release to the plants. They usually hold no organic matter in themselves and if used exclusively over a long period of time increase the soil's salinity. Most organic fertilizers are made out of organic byproducts, or, they are mined or processed without the salt stabilizers. In the long run, you'd be better off with the organically-based fertilizers.

Greensparrow said...

Susan, those are good points. The ecological impacts of the manufacturing processes are beyond my expertise. I know in a lot of analyses of the total carbon and ecological footprint of a product, packaging and transportation are a bigger factor than the manufacturing itself, but how that shakes out for specific fertilizer products, I don't know.

Yes, organic fertilizers will have some organic matter, but in such tiny amounts as to be insignificant compared to the effects of adding compost and mulch.

I live in a very wet climate, where soil salinity is not an issue, so I don't know much about it. I know in areas where it is a problem, some fertilizers need to be avoided to prevent salinity build up, but that includes both some synthetic and organic (calcium chloride, for example) fertilizers.
We can both agree, though, that in almost all cases, adding organic matter like compost is a better option than any sort of concentrated fertilizer.

Hanna at Orchid Care said...

Rather than risk polluting water with toxins and the air with putrid odors. I use only compost in my garden and it does rather well without fertilizers. I’ve got two dogs and I am always concerned about their safety and the harm fertilizers can do to them if ingested.

So, organic or otherwise, fertilizers are not welcome on my property.

Tovah Martin said...

Any thoughts on "soiless mixes" for potting houseplants? I've been working with soil mixes with compost and, I've got to say, the plants look great compared to compost-free mixes. Also, for the plants that haven't recently been repotted, I've been fertilizing with fish emulsion on the houseplants and it seems to have the exact same results as slow release non-organic.

Greensparrow said...

Tovah,
I was told once by an old greenhouse grower that back in the day they used soil based mixes, and only switched to soilless media to keep weight down for shipping. He also said and that good old fashioned soil was a lot easier to grow in. I know when I was working in Japan, they used actual soil for all their container growing. I've never done a side-by-side comparison, and don't know if there is any research comparing the two. I should look into that more.