gardeninacity

Most of the gardeners I know, read, and talk to have a strong bias in favor of an organic approach to soil fertility. I share that bias. In almost all of my garden, all I do is add mulch with some compost here and there. (And I’m planning on cutting back on the compost after I got back the results of my soil test.) Most of my plants are native wildflowers, cultivars of same, and vigorous exotics that just don’t need fertilizers if grown in the right kind of soil.

Even flowers with a reputation for being “heavy feeders”, such as Clematis and Bleeding Heart (Dicentra spectabilis) do fine in my garden with just a few shovelfuls of compost.

But I have a confession to make: I do use some synthetic fertilizer. I’ve used it for three things: container plantings, my vegetable garden, and my roses. The first two have a rational basis: constant watering makes nutrients wash out of containers, and vegetable plants really are heavy feeders. As for the roses: OK, I won’t do this again, but I had just planted my first rose bushes and I wanted SO BADLY for them to do well and the roses on the package looked so happy …

Anyhow, I feel a definite sense of guilt when purchasing synthetic fertilizer. At Home Depot I asked for a plain brown wrapper for my container of Osmocote. But is the guilt warranted?

As I mentioned in an earlier post, I just finished a course at the Chicago Botanic Garden on Soil Basics. Very worthwhile course, very much geared toward gardeners and not scientists (very easy on the chemistry, etc.). From the class and assigned as well as supplemental reading I drew the following conclusions:

• Synthetic fertilizers are greatly overused in home landscapes, and as such they can do substantial environmental damage.
• Organic fertilizers and soil conditioners improve soil structure, generally contain micronutrients at appropriate levels, and are much less likely to create excess concentrations of nutrients and cause nutrient runoff.
• Synthetic fertilizers are not inherently bad. The degree of concentration and the extent to which a fertilizer is fast acting are more important than whether the fertilizer was created through an industrial process or through the decay of organic materials.

Some have argued that synthetic fertilizers should not be used at all because they damage or destroy the soil food web – the vast number of bacteria, fungi, and other critters of varying size that are essential to soil fertility. This position is laid out in Teaming with Microbes by Jeff Lowenfels and Wayne Lewis. I found that particular argument unconvincing, though otherwise I found this to be an excellent book.

Synthetic fertilizers usually make nitrogen available to plants in the form of nitrates, a kind of salt. “Fertilizers are salts,” say Lowenfels and Lewis, and these salts “suck the water” out of soil microbes, drive away worms, and cause the overall soil food web to decline.

One problem with this argument is that nitrate salts are also produced by bacteria and fungi breaking down organic matter. Moreover, Jeff Gillman over at The Garden Professors cites a peer reviewed study showing that synthetic fertilizer was actually more effective than aerated compost tea at growing microbial populations in soil samples (though it also found that compost was better at helping poor soils retain nitrogen).

So that is my semi-informed, amateur take on the issue. What about you? Are you organics-only when it comes to fertilizer, or do you use a mixed approach?