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.)

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Living With the Roots of an Oak Tree

I laugh (and cry) at the little orange fences they put around oaks during construction. They are nearly worthless. While it keeps vehicles for hitting and scaring the bark, it in no way protects the feeding roots.




As a maintenance gardener I’ve had to dig up a few oaks. And just below the ground the roots have a covering much like bark. Bark absorbs very few nutrients. In a study of 25-year-old apple tree in the UK found that the first four feet from the trunk accounted for less than 10% of all the water and nutrients absorbed.

The wobbly-orange fences are, at best placed at the edge of the foliage. A good start as it may help to keep water off the trunk’s base. But this is a far cry from protecting the majority of the root system’s feeding roots.

In the photograph to the left, the construction company dutifully surrounded the oak’s dripline. But all the machinery and the house itself cover what used to be the leaf litter, duff, and organic mater (humus) that once both fed and protected the young feeding roots.

The tiny root hairs, that live for just a day or so, tend to grow up into the fertile strata of the humus and duff zone. They, like all plants, want to be the first to capture liberated nutrients as they become avaiable. As one researcher put it—“roots grow up not down”. While a bit overstated, they fact is the youngest rootlets and their root hairs reside well beyond the canopy of the tree, where all the construction is taking place. (Learn more in my book Roots Demystified,)

(See my Blog of March 31st to see diagrams of the extent of a roots wanderings.)

At one-half to thee times, or more of the width of the dripline roots gather nutrients and moisture far from the trunk or even the canopy.

The photo (on the above right) of the chairs face south along the Big Sur coastline are a good attempt to protect to crown of the young California live oak from moisture in the summer. The gravel does allow rain in the winter to percolate down along with oxygen and harmful gases escape. The compacted gravel is a bit hard on the feeding roots, but the tree seemed health. Perhaps this is one solution to living among oak woodlands.

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