Thursday, April 15, 2010

Organic Phosphorus? Maybe Not

Purplish strips at the edges of corn leaves usually indicates a deficiency in phosphorus. Some organic gardeners use colloidal phosphate as the solution to this deficiency, but consider this: colloidal phosphate is often strip-mined in Florida, washed with water (and Florida has a big problem with supplies of fresh water), loaded on train cars, and shipped to places as far away as California and Washington, where it’s sacked up and shipped to your local garden-supply store.

[As a side note: Florida mines 75 percent of the phosphorous used by American farmers and about 25 percent of the entire world production. Phosphate ore must be chemically processed with sulfuric acid. When sulfuric acid reacts with the phosphate it produces a slightly radioactive byproduct known as phosphogypsum. According to the Florida Department of Environmental Protection, there are a billion tons of phosphogypsum stacked across the state and 30 million tons are generated each year.]

And the total amount of phosphorus (P2O5) in the sack is only 16 percent of all the bagged-up bulk, of which a mere two percent is available the first gardening season since the phosphorus is locked up in a mineralized form that requires the activity of soil microbes, soil bacteria, and exudates. (Exudates aid in the nutrient-release process by releasing; sugars, organic acids, and other compounds to dissolve minerals into a soluble state and stimulate the soil’s microbial action.) Unnecessary water use, exploitation of limited resources and wasted energy—all wrapped in a single bag. Add to this the fact that there is a limited supply of easily mined colloidal phosphorus. It’s much like oil: will we have enough in the future? Will we be able to find enough new supplies if the current mines are exhausted? Some say the U.S. supply will be gone by 2035. The world supply make be depleted in 50-100 years.

Choosing to buy commercial colloidal phosphate means making a very important environmental decision. This is especially clear when one compares the environmental cost of imported amendments to the energy-efficiency of “growing” phosphorus at home by planting legumes and tilling the young foliage into the soil. Thus the gardener has two choices: (1) import nutrients, organic or not, to force an intensive yield, or (2) use wider spacing when planting and/or rotate heavy-feeder crops with a season of legumes to cut down on the competition for available nutrients. Whichever you choose, don’t grow corn in the same spot every year, as it will exhaust much of the nitrogen. Instead, alternate corn crops with green manures—that also increase available phosphorus.

(The illustration above is an excavation done by John Weaver in the '30s. It is a view looking down on the expansive root system of corn. Each square is one-foot square. As seen in my book Roots Demystified, Change Your Gardening Habits to Help Roots Thrive.)


Let me know what you think. Visit my web site to learn about my new book on drip irrigation and other gardening books. Thanks, Robert


Fred Hoffman said...

And, as you know, kelp is another one of those popular organic fertilizers that is a limited natural resource. Also, as WSU Horticulture professor Linda Chalker-Scott points out in her book, "Sustainable Landscapes and Gardens", the addition of phosphates at transplanting may actually inhibit the plant's establishment. Plant roots in a low phosphorus environment exude organic acids, allowing mycorrhizal fungi to penetrate roots and fomr networks, allowing the plant to uptake water and nutrients. In a high phosphorus environment, the roots do not exude those acids, thus the mycorrhizal connections do not form as readily. Yesterday at a meeting I attended at the Sacramento Tree Foundation, we discussed the topic of whether transplants need any extra fertilizer. The concensus: several inches of a high quality organic mulch around the plant will suffice.

Robert Kourik said...

Hi Fred, Thanks for all the new info. Especially from the Tree Foundation.