Phosphorus dynamics in water and soil: A study of 3s

By Dusty Sonnenberg, CCA, Field Leader, a project of the Ohio Soybean Council and soybean check-off

At the 2022 Ohio No-Till Conference, Warren Dick, retired soil scientist at The Ohio State University, discussed a 10-letter word that has everyone’s attention – phosphorus. Largely blamed for many of the water quality issues we hear about today, phosphorus is one of the primary nutrients in crop production. There is a phosphorus (P) cycle, much like there is a water cycle. Animal manure, commercial fertilizers, biosolids, and plant residue are added to the soil. These all contain phosphorus. There is also atmospheric deposition of P in rain and dust. The phosphorus is mineralized in the soil and becomes soluble P that can be taken up by the crops.

Dick describes understanding phosphorus as a series of 3s. 

“There are three forms we find phosphorus in the environment,” Dick said. “There is mineral P that interacts with iron, aluminum and calcium. There is the soluble P, which is small but the most impactful. There is also the organic P that is the result of mineralization.

“There are three functions of P in plants. Phosphorus stimulates the early formation and growth of roots. It stimulates flowering and seed development, and it is necessary for the energy and enzyme reactions of many plant processes.

“There are three pathways of phosphorus removal. These include plant uptake, removal by harvest, and then leaching and soil erosion.”

Phosphorus exists in three pools in the soil. Solution P or available P is the pool where plants can absorb P. 

“This is always the smallest of the P pools,” Dick said. “The labile P or active P pool is the weakly retained P by the soil. This is the P absorbed to mineral surfaces or soluble inorganic or organic compounds. It is the portion of P that when solution P decreases, labile P will fairly rapidly replace the solution P. The stable P pool is also known as the nonlabile or fixed P pool. It is the largest portion of total P in the soil. The weathering process is very slow in converting this to available P for plant growth.”

There are also three mechanisms of P uptake. 

“Diffusion is the first mechanism,” Dick said. “It is the movement of molecules through the soil due to concentration gradients. The distance P travels by diffusion in soil is extremely small. Therefore, an active large root system is important. Phosphorus is supplied to roots primarily by diffusion and root interception. This is why P is often times banded in the soil. It is concentrated in a band and will diffuse to the nearby roots.”

Mass flow/bulk flow is the movement of nutrients to root surfaces through soil water movement. “Mass flow to roots is driven by plant transpiration; however mass flow is not a major pathway of P movement to plants,” Dick said. “Root interception is the growth of root structures into new soil that contacts plant-available P. Root growth is important because it provides additional root surface area for P uptake.”

There are three ways to increase P availability once P is already in the soil. These include soil/rhizosphere pH, improved rooting and biological activity, and saturation of fixation sites. “There is a sweet spot for maximum P solubility in equilibrium with P minerals at about a soil test pH of 6.5,” Dick said. “At the same time, the minimum amount of P retention or fixation within the range of common soil pH values is also around 6.5.”