Phosphorus finders

Through a delicate but resilient natural process, soil fungi partner with bacteria and plants to ferret out and deliver phosphorus from the soil back to the plant roots. It's a partnership between soil, plants, and microorganisms with the potential to help reduce applications of phosphorus fertilizer, regenerate soils, and grow resilient crops.

"Very little phosphorus gets lost in this natural process, and very little water is used," says Pennsylvania-based soil scientist and consultant Kris Nichols. "Over time, nature has created a highly efficient, resilient system by which plants are able to access the nutrients they need from the soil when they need them."

However, the key is having an abundance of microorganisms in the soil, with mycorrhizal fungi being of critical importance. These hair-like organisms are adept at seeking out the small deposits of phosphorus in the earth.

They do it because their mode of survival, says Nichols, is to get food from the plant root.

"In order to get that food," she says, "they have to first give the plant something. So they give some of the carbon they get from the plant to the bacteria. The bacteria release enzymes that help break the bonds binding phosphorus to other minerals in soil particles.

"Then, because the phosphorus is now solubilized in water (the moisture in the soil), it can be absorbed into the fungal hyphae," she says. "They take it back to the plant roots, where they penetrate the root cells, delivering the phosphorus and taking carbon from the root for themselves. It's a pipeline for delivering nutrients from the soil into the plants. The plants signal for the process."

Fertilizer treadmill

Nichols has done her share of researching the dynamics of soil biology and the mechanisms needed to regenerate soil health. She served as a research soil microbiologist with the USDA and, later, as the chief scientist at the Rodale Institute. Through her present work as a consultant with KRIS Systems Education & Consultation, she works with farmers and others aiming to restore biological life in soils by helping them design farm plans.

Her research and firsthand experience with farmers point first to the vibrant potential for biologically active soils to be self-sustaining in their growth of crops, reducing the need for farmers to depend on synthetic fertilizers.

Yet, while myriad microorganism are native to the soil, they are, to a great extent, underpopulated, with their work "outsourced" by farmers' repetitive use of fertilizers, she says.

"By applying fertilizers, we've stopped the activity of the soil biology," says Nichols. "The plants have stopped signaling their needs, and the biology can't do their job. Since they can't work, they don't eat, and they don't reproduce. Over time, the populations of soil microorganisms have declined."

The result is an increased need for growers to apply fertilizers.

"In recent decades, we've seen applications of fertilizers increase as soil health declines," she says. "Here in the United States, our soils have decreased in organic matter and in biological activity. Soil structure tends to be poor, and water infiltration has declined. We've also seen increases in pests and diseases in crops."

A decline in fertilizer-use efficiency is a telling yardstick of soil's increasing dependence on fertilizers to grow crops.

"An overall look at recent data done by the University of Minnesota shows that it takes more fertilizer today than it took in 1960 to grow a bushel of grain," says Nichols. "More nitrogen, in particular, is needed, but it's also true of phosphorus. Back in 1960, there may have been more biological life to cooperate with soils in growing crops."

Of the phosphorus fertilizer farmers apply, only about 30% ends up in the plants, says Nichols. The rest stays in the soil or ends up being carried away with soil particles via wind and water erosion.

"Phosphorus is not as easily water soluble as nitrogen fertilizer, but it is contaminating our waterways on soil particles," she says.

The inefficient plant use of applied phosphorus occurs because of the narrow window of opportunity for plants to uptake the nutrient. In the natural system, where the fungi deliver the phosphorus to the plant roots, the fungi and bacteria only supply phosphorus when the plant has the need. Little excess-available phosphorus is lost, and the response occurs in a timely manner.

When humans try to meet this need by artificial applications, says Nichols, the timing and placement of the applied phosphorus is not necessarily synchronized with the needs of plants. Thus, much of the fertilizer goes unused.

The good news is, this unused phosphorus that remains in the soil could be contributing to a store of phosphorus.

"Here in North America, our soils have historically been rich in phosphorus. In addition to that, farmers have been applying synthetic fertilizers for decades," says Nichols. "If only 30% ends up in the plants and some is lost to erosion, some of the phosphorus should still be available in the soil."

Added to this possible existing supply are the continual natural deposits of phosphorus being made.

"Animals, birds, and insects are constantly depositing phosphorus in the soil," says Nichols. "All these add to that bank of phosphorus. Our goal is to make more of that phosphorus available to plants on a continuous basis. The soil fungi and bacteria can do that without a concern of 'mining' the soil."