Phosphorus helps cells to elongate and divide, which is how plants grow. This unique role makes P a very important nutrient.
Adequate P levels are required to transfer starches, especially sugars, within a plant. It’s important to note that in certain seasons, the right amount of P can boost corn yields by 40 bu. per acre.
“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,” explains Ken Ferrie, Farm Journal Field Agronomist. “When that happens, plants take on a purple appearance.”
Corn plants need P early.
“Because phosphorus is so important to early growth and development, the majority needs to be taken up early in the growing season,” Ferrie says. “This is different from nitrogen, which plants can take up throughout the season.”
Plants have several ways of regulating nutrient uptake.
“It’s key to remember that taking up nutrients is an active metabolic process,” Ferrie says. “It requires energy, and plants get that energy from phosphorus.”
The role of P in plant development involves two compounds in cells: ATP and ADP.
“Phosphorus is constantly cycling between ATP and ADP and back,” he says. “In this process, energy is released for growth. Think of it as a spring winding up and letting loose. It takes energy, which comes from the sun, to wind the spring or create ATP. When the plant converts ATP to ADP, that’s like letting go of the spring, and energy is released for growth and development. Then, the process starts all over again.”
How P Gets In Plants. Plant roots reach P in three ways: interception, mass flow and diffusion. These processes occur simultaneously.
Roots intercept P as they grow. During their rapid growth stage, corn roots grow 1'' to 1.5'' per day.
In mass flow, plant roots draw water from the soil and literally pull the soil water to the plant. The water carries nutrients to the plants.
In diffusion, P leaks into soil water with help from soil microbes. The P ions move, or diffuse, from organic phosphate stored in the soil to inorganic phosphate that plants can use.
Organic Versus Inorganic Form. Like other nutrients, P exists in organic and inorganic forms.
“In the organic form, phosphate ions are hooked to carbon chains inside microbes or crop residue,” Ferrie says. “That phosphorus cannot be taken up by plants. It must be broken down, or composted, into the inorganic form.
“Soil microbes run this entire process,” he continues. “Soil phosphorus will not diffuse into the soil solution until the microbes are up and running at full speed. That process is extremely sensitive to soil temperature. The microbes that release P don’t become very active until soil temperature reaches 60°F to 65°F.
“Even soils testing high in phosphate will have purple corn when the soils are cold,” Ferrie explains. “Unlike soil phosphorus, the phosphorus in the starter was not sensitive to temperature or microbial activity, so it was available to plants.”
After you apply P fertilizer to the soil, the P changes from high solubility to low solubility.
“The process happens fairly fast,” Ferrie says. “That’s why only 10% to 30% of applied phosphorus is available to plants the first year.”
Some of the remaining P gets tied up and becomes unavailable to plants. But a high percentage of it is consumed by soil microbes.
Hungry Microbes. “In reality, we are applying phosphorus for the microbial processes that feed plants,” Ferrie says. “When we apply diammonium phosphate (DAP) fertilizer, the phosphate ions dissolve and combine with water to form phosphoric acid, which is what microbes and plants need. But the acid can also dissolve bases such as calcium, iron and aluminum in the soil.”
“If the phosphoric acid remains in the soil very long, the phosphate ions can bond to other ions and become unavailable to plants,” he says. “In some soils, this requires changing your fertilizer program to mitigate a tie-up and ensure that phosphorus is available when plants need it.”
Knowing that, you can see why P fertilizer products vary in their availability to plants.
“In calcaric conditions, monocalcium phosphate fertilizers tie up faster than ammonium phosphates,” Ferrie says.
Another nutrient, zinc, is required for phosphorus uptake.
“You must maintain the proper phosphate-to-zinc ratio,” Ferrie says.
Measuring P. As with all nutrients, managing P starts with a soil test. But it’s more complicated than reading numbers on the lab report.
“Soil tests do not tell you exactly how many pounds of phosphorus are in your soil,” Ferrie explains. “The results are indicators you can use to try to predict how much phosphorus will come out of soil and how easily. Soil is a dynamic medium. Microbes are constantly changing phosphorus from plant-available to plant-unavailable states and back again.”
There are various ways to test soil for P availability. Ferrie says each test has pros and cons, depending on your area and soil types.
Even though phosphorus seems to be a moving target, with correct management, there are significant opportunities to improve yield.