Phosphorus is the best nutrient we've got for making strong stems on storm-tossed corn. Tomatoes and other crops need it for early fruiting. Bulbs depend on it for vigorous flowering. But phosphorus, the "P" in N-P-K, is a reluctant garden guest. It's almost immobile. It's easily changed into forms plants can't use. And it's sort of a wallflower, slow to solubilize, to dissolve and join the throng of nutrients in the soil.
Plants can't use phosphorus until it's been dissolved. That's why the folks at my local garden center tell me to use slow-release P sources (rock phosphate, bonemeal or manures) for long-term availability. They say to save the soluble synthetics -- triple superphosphate (TSP), ammonium phosphates and others -- for fast feeding.
But it's not that simple. No matter which phosphorus fertilizer we use, slow-release or soluble, our plants will recover only a small portion of the P we add during the year we add it. The rest will be "banked" in various forms, to be withdrawn at different rates -- or not at all -- depending on soil characteristics. The fertilizer you choose is important. But if you don't want your corn's P check to bounce, you've got to know how to manage your phosphorus account. Neutralize garden pH
As soon as the P in slow-release fertilizers has been dissolved, or as soon as the P in soluble fertilizers like triple superphosphate has been released, it's up for grabs in a competition between two teams. We gardeners cheer for the root hairs; we usually lose to the oxides.
Plant root hairs have to touch floating phosphorus compounds before they can capture P. But phosphorus doesn't like to travel. Oxides -- iron and aluminum in acid soils, calcium in alkaline soils -- are everywhere. Root hairs are not. Once grabbed or "fixed" by these oxides, even the P in synthetics is unavailable to plants until it has been once more, very slowly, dissolved. Soluble P, no matter what its source, is usually locked into a long-term CD in just a few days.
This fixation is more likely to happen at pH extremes. Liming acid soils reduces iron and aluminum fixing of phosphorus. Using sulfur and adding organic matter to alkaline soils reduces fixation by calcium. But it's a hard balance to strike because naturally occurring phosphorus (most soils have at least some) and the P in slow-release fertilizers dissolve fastest in acid soils. Dr. Willard Lindsay, professor of agronomy at Colorado State University in Fort Collins, says that a pH of 6.2 is best for phosphorus utilization. Fine-tune your fertilizer
To your pH, that is. Rock phosphate, for example, takes "forever" to dissolve in most alkaline soils. In acid garden soils, however, it's a good, inexpensive P source. Rock phosphate offers a different kind of savings there, too. Soluble P fertilizers are fixed quickly as iron and aluminum phosphates, but "banked" rock phosphate is simply undissolved. According to Dr. Larry Hammond, manager of rock phosphate marketing at Texasgulf Export Corporation in Raleigh, North Carolina, in acid soils, undissolved rock phosphate is generally more available than "fixed" soluble fertilizers.
Texas Gulf's rock phosphate is unusual among the rock fertilizers. The reason is "reactivity," says Dr. Norman Chien, soil chemist at the International Fertilizer Development Center in Muscle Shoals, Alabama. Chien explains that rock phosphate ores from Missouri are the slowest to dissolve (have the lowest reactivity) and show the least plant response of all the domestic ores. North Carolina rock phosphate, on the other hand, has the greatest tendency to dissolve and the fastest plant response of all the world's commercially mined phosphates. Hammond says North Carolina rock phosphate dissolves so fast that it's available almost as quickly as triple superphosphate. (Few manufacturers identify their ore's origin. Gardeners can buy North Carolina rock phosphate, though, in Fertrell products and in some Erth-rite preparations.) Add organic matter
Whatever your soil type, whichever fertilizer you chose, phosphorus will dissolve only when a P "sink" has been created -- when root uptake causes the concentration of P in the soil solution to fall. But sometimes the flow of P doesn't keep up with demand. By adding organic matter, we can flush P out of its hiding places. Organic decomposition releases acidic compounds that quicken phosphorus dissolution. Organic matter itself also provides significant quantities of slow-release P.
Willard Lindsay at CSU says that, although adding organic matter may not immediately affect overall pH in highly alkaline western soils, it will create small pockets of greater soil acidity. And "anything you do in the microenvironment to lower pH will increase P solubility," he adds. Organic matter also releases compounds that compete with P for fixation, leaving more phosphorus available for root uptake.
Soda Springs Phosphate, Inc. in Soda Springs, Idaho, has implemented this idea for growers by combining organic matter with rock phosphate right in the fertilizer bag. Their "Lignosulfonate Rock Phosphate," released in 1991, is a finely ground, then granulated, rock phosphate bound by lignosulfonate, a paper industry waste consisting mostly of carbon and sulfur. Their product, already certified for organic growers in some states, is sold under the name "Idaphos" by Midwestern Bio-Ag in Blue Mounds, Wisconsin. Get in tune with the season
Norman Chien says that while plant nutrients such as nitrogen "go to the root" (that is, move easily in the soil solution), "the root has to go to P." Early in the season, however, root growth is minimal. Most crops will show a P deficiency in cold soil, says Robert Parnes, a consulting soil scientist in Mechanic Falls, Maine, and author of Fertile Soil: A Grower's Guide to Organic and Inorganic Fertilizers (Press, date). For organic gardeners, Parnes recommends poultry manure for spring application. Organic P sources, he says, are generally more available than mineral phosphorus (such as rock phosphate) early in the growing season. (Bonemeal might also be a good spring choice for this reason.) Dr. Larry Murphy at the Potash and Phosphate Institute in Manhattan, Kansas, points out nothing beats synthetic phosphates in immediate availability and suggests that, for early spring feeding, applying TSP or other soluble synthetics "would be a highly valuable practice." A synthetic foliar spray such as Nutra Phos, which provides both immediately available and "controlled release" P with other nutrients, can be effective on young seedlings. But Al Mitlehner at Uniroyal Chemical in Middlebury, Connecticut, makers of Nutra Phos, cautions that their products are "prescription-oriented," designed to correct specific nutrient deficiencies. They may not be for everyone. (And because they contain synthesized phosphates, they are not for organic gardeners.) Nutra Phos is available is some garden centers in California, and through agricultural suppliers elsewhere in the country. Increasing P levels
Though ideally you can build up P levels over time in the soil until you don't have to worry about it being available, some of us will have to make short-term deposits to our phosphorus account. Let's take a closer look at some of the choices. Slow-release fertilizers
Phosphete rock, a mineral derived from the remains of marine animals, is mined from deposits in North Carolina, Florida, Idaho and a few other western and southern states. Most processors strip- mine the ore, then clean, dry and finely grind it. Rock phosphate's average analysis is 0-30-0; some high-quality Florida rock averages as much as 0-36-0.
Florida phosphate deposits also yield colloidal or "soft" phosphate. This substance analyzes 0-18-0 and is less available than most rock products. It can be purchased alone, and is the P source in many less-expensive natural fertilizers. Some organic growers swear by it, though most soil chemists consider it a poorer substitute for rock phosphate. Rock and colloidal phosphates are the cheapest natural sources of fertilizer P for the organic gardener.
Bonemeal is one of the most expensive. This slaughterhouse by- product is chemically similar to rock phosphate but is more available. For this reason, Bob Cantisano of Organic Ag Advisors in Colfax, California, recommends it to organic gardeners for potting soils, where the kind of biological activity that encourages rock phosphate dissolution is low. It's a better choice than rock phosphate for alkaline soils, too, especially when organic matter levels are low. Bonemeal products come with various analyses, such as 3-20-10, 3-30-0 or 1-11-0.
Poultry manure is a less expensive but low-analysis natural alternative for alkaline soils and for spring feeding -- as long as your garden soil is biologically healthy. The P in poultry and other manures, including bat guano, is made available by soil microorganisms. In soils that are very low in organic matter and microbes, manures are relatively inert. Soluble fertilizers
Soluble or immediately available P fertilizers start with acid-treated rock phosphate. Superphosphate is manufactured by treating rock phosphate with sulfuric acid. It was a favorite of gardeners until not too many years ago and is still a fairly common ingredient in complete synthetic fertilizers. It analyzes 0-20-0.
Triple superphosphate is the most common source of fertilizer P today. TSP is made by treating rock phosphate with phosphoric acid, and though it is no more soluble than superphosphate, it is more potent. TSP's analysis averages 0-46-0. It's the acid treatment that makes superphosphate and triple superphosphate unacceptable to organic growers.
Some types of nitrogen not only increase plant growth, but also seem to increase P uptake by plants. That's one reason for the widespread use of ammonium phosphate fertilizers. Another is their price -- they're cheap. You'll see them in 50-pound bags in the farm store, in lawn fertilizers, and even in popular liquid fertilizers such as Miracle-Gro. They come with various analyses -- 10-53-0 and 18-46-0 among them. A similar product, ammoniated superphosphate, also enhances P uptake, but packs less bang for the buck. It analyzes an average 4-16-0.
MINDING YOUR P'S...AND CUES Once you've figured out which P fertilizer is best for your garden, how do you use it? Here are some tips.
A soil test will tell you how much P you need to add to your garden. If springs in your region are cool, collect your soil samples the year before, while plant roots are active. Samples collected from cold spring soils will probably show a phosphorus deficiency, even if your phosphorus reservoir is high.
Don't bother side-dressing with P during the growing season. Field tests on agricultural crops have shown no significant response to occasional side-dressing with phosphorus, even when soluble fertilizers such as TSP were used. Soil scientists suggest applying the total P recommended in your soil test all at once.
Phosphorus fertilizer is like real estate -- the most important factors are location, location, location. Try to incorporate your P fertilizers in the root zone, either by turning the garden after the fertilizer has been applied (this is especially important if you're using rock phosphate), or by scratching it into the seeding row. This "banding" is especially important for spring plantings in cool soils.
Copyright NGA
Reprinted with permission HouseNet, Inc.
Your P Portfolio
by Shelly Stiles