Corn College Report: What You Must Know About Phosphorus

February 22, 2010 10:03 AM

When you grow corn, the first nutrient you think about is nitrogen (N). But phosphorus (P) is equally important. The right timing and placement can boost yield—up to 30 bu. to 40 bu. per acre in years of late planting, such as 2008 and 2009.

In July, Farm Journal Field Agronomist Ken Ferrie explained P management to attendees at Farm Journal Corn College. Demonstration plots at the site near Bloomington, Ill., reinforced his points about timing and placement.

Why crops need phosphorus.
To set the stage, Ferrie says, "phosphorus is required for the elongation and division of cells and for the transfer of starches and especially sugars. If a corn plant can't transfer sugars, which are produced in the upper part of the plant, to other areas, the plant stops growing and turns purple. Something similar happens to trees in the fall; the tree stops transferring sugar, and we see the leaves change color.

"By the time a corn plant reaches 25% of its dry-matter weight, it needs to have 75% of its phosphorus on board," Ferrie continues. "If it doesn't have enough phosphorus, the plant will have problems. Nitrogen can be taken aboard throughout the growing season, but phosphorus must be early."

Think of P as fuel for the growing process. Whether it is in plants, microbes or animals, the compounds inside cells, called ATP and ADP, provide energy to drive growth. Those compounds contain P.

There's more to it than that, of course. Ultimately, energy for plants comes from the sun, Ferrie explains. Plants use photosynthesis to turn sunlight into energy. But that process is inefficient without P.

P picked up by plants from the soil is eventually transferred to the grain during grain fill. Then it leaves the field at harvest and is consumed by animals or humans. The P in soil is essential to everything living on Earth.

How phosphorus reaches roots. Roots come in contact with P by interception, mass flow and diffusion, which occur simultaneously. In interception, as roots grow through the soil they come in contact with P.

Definitions You Need to Know

  • Carbon penalty: Phosphorus fertilizer immobilized by soil microbes during decomposition early in the growing season.
  • Immobilization: The process by which soil microbes render phos-phorus unavailable to plants by converting it to the organic form.
  • Mineralization: The process by which soil microbes make phosphorus available to plants by converting it to the inorganic form.
  • Carbon/phosphate ratio: The relative amounts of carbon and phosphate in crop residue. This ratio determines whether net immobilization or net mineralization (or neither) is occurring—mineralization at less than 200/1 and immobilization at more than 300/1.

"Corn roots grow 1" to 1½" per day in the rapid growth stage," Ferrie says. "They're not growing to water or fertilizer; they just happen to find it."

In mass flow, nutrients are carried along with water. The root system actively draws water from the soil.

In diffusion, nutrients move from an area of greater concentration to one of lower concentration. "Phosphorus usually is very low in the soil solution," Ferrie explains. "With only minute amounts available, it is like an IV drip to the plants, so we need diffusion of phosphorus in addition to mass flow."

Plants control which nutrients come in and in what amount, and they have several ways of achieving that. "The main factor to remember is that it's an active metabolic process," Ferrie says. "Picking up nutrients from the soil requires energy, and plants get the energy from phosphorus."

Phosphorus in the soil
. There are two things to keep in mind concerning P in your soil. First, P (like other nutrients) exists in organic and inorganic forms.

"Organic means that the phosphate ion is hooked to carbon, such as a microbe or cornstalk, and it cannot be taken up by plants," Ferrie says. "It must be broken down, or composted, into the inorganic form."

Second, when you apply P fertilizer, once the P is in the soil it changes from high solubility to low solubility. This happens fairly quickly, and it's the reason that only 10% to 30% of applied P is available to plants the first year. Of the remainder, some gets tied up and becomes unavailable, but a high percentage is absorbed by soil microbes.

"We like to think we apply phosphorus for the plants; but actually we are applying it for the microbes," Ferrie says. "The phosphorus drives microbial processes that feed the plant." The activity of soil microbes plays an important role.

Get the Most from Your Starter

Demonstrations by Farm Journal Field Agronomist Ken Ferrie show that applying starter fertilizer can boost yield by 7 bu. to 10 bu. per acre in a normal growing season and up to 40 bu. to 50 bu. per acre with late planting. Starter can make conservation-tilled corn yield as well as moldboard plowed. Applying starter beside the seed also lets you apply higher rates than placing fertilizer in the seed trench.

Ferrie's studies also show the closer you can place starter, the faster the response. The true sweet spot for starter is ¾" beside and ½" below the seed–closer than traditional 2"x2" placement, which can put the fertilizer out of reach of seed roots. The difference shows up early in the season. "But placing fertilizer this close to the seed is difficult," Ferrie warns.

Here are some tips to help you maximize starter efficiency:

  • Be aware of the danger of seed burn if you decide to place fertilizer on the seed. "If you lose 3,000 or 4,000 plants per acre from starter burn, you'll offset the yield advantage from applying starter," Ferrie says.
  • Make sure the addition of starter tanks and attachments on a planter doesn't prevent it from running level.
  • Match the capacity of starter tanks and planter hoppers, so you can refill both at the same time.
  • Quick-fill fertilizer attachments reduce downtime.
  • Install sufficient screens in the plumbing system to prevent fertilizer lines from plugging.
  • If you use fertilizer attachments that go on the front of the row unit, use scrapers to prevent wet soil from building up on depth wheels and resulting in an uneven planting depth.
  • If you mount starter attachments on row units, you will need to add down pressure to keep them in the ground.
  • If you run single-disk fertilizer openers, set the opener so it pushes away from the planting unit; if openers push toward the seed trench, they cause sidewall compaction.
  • As a cutting coulter wears down, change the pitch or replace the cutter to avoid sidewall compaction.


"Phosphate ions have a strong affinity with water in the soil," Ferrie explains. "When we apply diammonium phosphate [DAP] fertilizer to the soil, the phosphate ions dissolve and combine with water to form phosphoric acid, which is what plants need.

"The acid can dissolve bases, such as calcium, iron and aluminum, in the soil. If the acid stays in the soil long, the phosphate ions capture iron, aluminum or calcium cations. Then the phosphate becomes monocalcium  phosphate, which plants can't use."

Monocalcium phosphate must be acidified or dissolved to become available to plants again. Soil microbes do that by producing acidity as a byproduct of their activity in the spring.
That's why P fertilizer products vary in availability to plants.

"Monocalcium phosphate fertilizers tie up faster than ammonium phosphates," Ferrie says.

The phosphorus cycle.
Similar to the N cycle, the P cycle involves soil, plants and microbes. The process includes uptake of P by plants, biological turnover, fixation and solubilization.

P compounds in the soil fall into various classes. Weakly absorbed inorganic phosphates are held on soil particles by calcium ions. Insoluble phosphates are held tightly in soil. These include dicalcium phosphates, which are difficult to release, and tricalcium phosphates, which are held so tightly that they would have to be mined.

A third class, which is the most interesting to crop producers, includes soluble organic or inorganic compounds in the soil solution. They just need a little more processing to become available to plants. These include compounds contained in the bodies of dead soil microbes.

"The biggest source of phosphorus for your plants is dead microbes," Ferrie says. "The microbes decompose organic phosphate compounds into inorganic phosphates that can be used by plants. When they die, each microbe is like a little bag of nutrients. Enzymes around plant roots decompose those little bags and bring those nutrients into plants."

The P cycle is the process by which past years' crop residue is broken down, so the P can go into next year's crop.

When microbes discover crop residue, which is a carbon source, on the surface, they begin to decompose it. Decomposing carbon plays a leading role in the amount of soluble phosphate in the soil at any given time.

Some microbes immobilize, or "fix," P into forms that are not available to plants. Microbes also mineralize fixed forms of P and make it available to plants.

"Both processes occur simultaneously, but we have net gains and net losses of available phosphorus at various times in the process," Ferrie says.

Factors affecting the rate of mineralization include temperature, soil moisture, aeration, compaction, cultivation, the presence of growing plants and the fertilizer you apply.

Fast Facts

  • Half of the world's phosphorus deposits are in North Africa; the U.S. has 30% of the world reserve and Russia has 15%.
  • Phosphorus was isolated as an element in 1669.
  • In pure form, phosphorus burns if exposed to air.
  • The average adult has 2 lb. of phos-phorus in his or her teeth and bones.
  • In the Earth's crust, phosphorus is the 11th most abundant element.
  • An acre of soil contains 2,000 lb. to 3,000 lb. of phosphorus in the top 6", but 80% of it is in the organic form, which is unavailable to plants.
  • In soil, zinc is a key factor in phosphorus uptake. You must maintain the correct phosphorus/zinc ratio.
  • Phosphate ions have a negative charge; when they bond to positively charged ions, such as calcium, in the soil, the phosphorus becomes unavailable to plants.

In your grandfather's time, rock phosphate was the only phosphorus fertilizer available; it required a long time to become available to plants. Today's phosphates become available to plants more easily.

When crop residue is present for food in this process, the microbe population explodes.    

"Bacteria have life cycles of an hour, so you can imagine how fast a generation can turn," Ferrie says. "Microbes need phosphorus for energy, and they get it from the soil. If they suck all the phosphorus out of the soil, the plants growing there are in trouble."

The carbon/phosphate ratio determines whether that happens.

"If the ratio is less than 200/1, we gain phosphate," Ferrie says. "If it is between 200/1 and 300/1, we neither gain nor lose. Above 300/1, we have a net loss of soluble phosphate in the soil solution. That's because microbes are removing phosphate from the soil faster than they are mineralizing phosphorus back into the soil solution."

Eventually, the phosphate taken up by the microbes is returned to the soil and made available to plants. In the meantime, however, plants may run short of P.

"It takes more phosphorus to prevent a temporary shortage when growing continuous corn because there is more residue to decompose than there is in a corn-soybean rotation," Ferrie says.

Soil Testing for Phosphorus

"Soil tests do not tell you exactly how many pounds of phosphorus are in your soil," explains Farm Journal Field Agronomist Ken Ferrie. "The results are simply indicators that we use to try to predict how much phosphorus will come out of soil and how easily." Soil is a dynamic medium in which microbes are constantly moving phosphorus from plant-available to plant-unavailable states and back again, he adds.

There are multiple ways to test soil for phosphorus availability. "Each test has pros and cons, depending on your area and soil types," Ferrie says. "Ask your Extension specialist which test will work best for you."

Paying the carbon penalty.
Think of residue as carbon upon which microbes feed. Think of decomposition as the burning of carbon.

Instead of smoke, the carbon is released as carbon dioxide, causing the residue pile to shrink. The nutrients, such as P, remain, like ashes left from a fire. Soil microbes become really active at temperatures more than 60°F.

"In Illinois, we often get five or six weeks of such temperatures in the fall," Ferrie says. "In southern Illinois, Tennessee and Kentucky, where winters are warmer, all the carbon may be burned in the winter. In Wisconsin, Michigan and Minnesota, by the end of harvest the soil is usually too cold to drive decomposition, so you may not get any decomposition until spring."

The carbon penalty—the temporary deficit of soil P that exists until microbes switch from net immobilization to net mineralization—is what makes starter fertilizer so important in continuous corn.

Early season phosphorus.
At the beginning of the growing season, soil P can be temporarily immobilized by microbes—or if the weather turns cold after planting, the microbes can shut down for a while, causing corn plants to just sit there and turn purple. Yet you need to push 75% of the plant's total P into it by the time the plant accumulates 25% of its dry-matter weight.

That's where starter fertilizer comes in. "Starter is insurance against cold weather after planting," Ferrie says. "With normal weather, we have found starter phosphorus provides a 7 bu. to 10 bu. per acre yield increase. In 2008, with a wet spring and late planting, applying starter produced 40 to 50 more bushels per acre."

Because starter concentrates a source of plant-available P near the roots, it is more efficient than broadcast fertilizer.

"The optimum Bray P1 soil test reading for phosphorus is 20 to 30 ppm [parts per million]," Ferrie says. "Using starter, you can operate at 15 to 20 ppm. Applying phosphorus as starter can be helpful on rented farms, if you don't expect to have control for more than one season." (See the sidebar "Get the Most from Your Starter.")

If spring 2010 shapes up to be a repeat of the previous two seasons, be all about P. Time it right, place it right—and your yields will climb despite the slow start.

You can e-mail Darrell Smith at


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