When It Comes To Phosphorus Deficiency, Think Organically!
Gardeners are very much aware of the close relationship between organic matter and nitrogen, but the decomposition of organic matter in the soil is of equal importance to the amount of phosphorus available to plants. In both sandy and clay soils, phosphates can become quickly unavailable – “locked up” – in the absence of rotting organic matter. The carbonic and nitric acids present as organic matter break down and help unlock the phosphorus.
Without phosphorus, plant growth slows. Crops won’t mature when they should. Scientists are not positive yet about what phosphorus does, but they think the following explanation is accurate. The cellular material that mysteriously pulls the cell interior apart to form two new cells contains phosphorus. If the content is low, the rate of cell division is slow.
The subject of phosphorus deficiency gives us an excuse to inveigh against a press release issued by our land grant colleges periodically. The purpose of the release seems to be to hit organic gardening a good lick just on general principles, gain attention, and prove to the old alma mater that here is one researcher who has not wavered from the path of chemical righteousness. An expert is Invariably quoted to the effect that “Food grown on poor soil is no less nutritious than food grown on good soil. The variation is only one of quantity.”
How can responsible people utter statements like that? Outside a very, very, narrow context, that is plain, unvarnished error. Phosphorus deficiency is one of many examples that will refute such a declaration. Cows pastured on soil lacking phosphorus have poor bone development. Such a cow can look like the living death walking around. Check the University of Minnesota and ask to see their photos of cows grazed on poor, phosphorus-deficient soils in northern Minnesota. If we humans didn’t get adequate phosphorus in our diets, we’d start looking like those knock-kneed, mangy cows.
Discovering Phosphorus Deficiency
The quickest, surest indicator of phosphorus deficiency in vegetable plants is a reddish-purple discoloration of leaves, leaf veins, and stems. The coloring comes from an over-concentration of sugar resulting in the formation of anthocyanin, a purple pigment – the same reddish purple you see in autumn leaves. The excessive sugar production is triggered by the scanty supply of phosphorus.
Corn leaves, including sweet corn, may first appear a darker green than usual when young if phosphorus is lacking. Then the leaves and stalks become purplish. Knee-high corn with purple-fringed leaves almost always reveals that your land needs phosphorus. Defectively shaped ears are another sign – the crooked ears caused by slow silk emergence at pollination time. Crooked and incomplete rows of kernels on the ear is another sign.
Tomatoes starved for phosphorus get the characteristic reddish-purple color on the underside of their leaves, the color first appearing in spots on the web of the leaf, spreading to the entire leaf, and finally affecting the veins. Leaves are small and stalks too slender. Cole vegetables turn reddish purple too, on leaf edges. But don’t confuse that with purple cabbage! Also some sweet corn, like Early Sunrise, has a purplish hue in the stalks, natural to the variety. Remember, too, that cane sorghum shows reddish blotches on the stalk because of high sugar content, but it’s supposed to have high sugar content.
Supplying Phosphorus If Necessary
A good, built-up organic soil seldom if ever shows phosphorus deficiency because where high organic matter content is maintained, the natural phosphorus in the soil remains more available. But every soil, no matter how good, should have phosphorus added to it periodically if the soil is heavily cropped. Phosphate rock, either raw or the colloidal type, is the accepted organic fertilizer to apply. Rock phosphate is not cheap, but a two ton per acre application once every four years won’t break you up either.
Rock phosphate is not as readily soluble in water as superphosphate, though modern crushers can now render the former to a very fine powder that allows the phosphorus to be released much faster than used to be the case. Moreover, in the presence of lots of organic matter, more phosphoric acid becomes available from slow release sources like phosphate rock than the superphosphate salesmen will admit.
Organicists don’t use superphosphate for a number of reasons. They figure the extra processing necessary to convert rock phosphate into superphosphate represents a use of energy unnecessary to organic agriculture. The sulfuric acid used to make the conversion causes a build-up of a type of bacteria in the soil which feed on fungi that break down cellulose.
Bone meal is another source of phosphorus that makes sense on a small garden or on a few choice shrubs around the house. There are three kinds of bone fertilizer products: raw bone meal, steamed bone meal, and bone black, all with a phosphorus content of over 20 percent, though exact content will vary according to the age, diet, and kind of animal the bones come from. Some disagreement exists among gardeners over the benefits of bone meal. Some say they get no results from it; others wouldn’t garden without it. The difference of opinion probably springs from the fact: that it takes a long time for the meal to decompose and release the phosphorus in it. Also, gardeners who really care enough to use bone meal usually already have soil in good shape.