Posted in: Agronomy and Soil HealthFertility

Understanding crop available phosphorus

The behavior of Phosphorus (P) in the soil has long puzzled scientists. Curiously, Phosphorus is not particularly mobile. In western Canadian soils, there is generally an extremely low probability of leaching.[1] This poses a challenge, since this key nutrient must be absorbed through the plant’s root system.

Before we get into the conundrum around the behavior of Phosphorus, it is important to realize that Phosphorus exists in three states within the soil: as Soluble P (plant available), Slowly Soluble P and Insoluble P. As we’ll see, as the soluble P is consumed, it is replenished from reserves of slowly soluble P.

Why isn’t phosphorus mobile in the soil?

Plants absorb ionic forms of Phosphorus that are made available in the soil solution. This includes H2PO4 (a readily available form) and HPO4=. These forms of Phosphorus are anions (which are negatively charged) and therefore are not attracted and retained by the negatively charged soil.

So, by all logic, Phosphorus anions should be mobile in the soil – as is the case with Nitrogen anions. In fact, the opposite is true.

Researchers believe this is due to the fact there are extremely low levels of available phosphorus in the soil solution at a particular time – typically less than 0.00005 grams/L. According to estimates, normal crop growth requires the phosphorus in the soil solution to be replaced twice per day (on an average). [2] Meeting that need comes down to the ability to replenish the soil solution to keep pace with the plant’s absorption of Phosphorus through its roots.

This is achieved through an ‘equilibrium reaction’. Essentially, whatever Phosphorus is removed from the soil solution by the plant will be replaced by new P from the available phosphorus reservoir in the soil.

Slowly Soluble/Insoluble P                 Soluble/Plant Available P

– P minerals & compounds                        – Phosphorus in ionic form

– Organic P


                                                          ←←←←←←←SOIL SOLUTION P

Factors affecting crop available phosphorus.

Mobility isn’t the only challenge when it comes to getting available phosphorus to the plant. There are several factors at play.[3]

Aeration: Oxygen is required for mineralization and the breakdown of organic matter, which is necessary for naturally occurring P to become plant available. Plants also require oxygen to absorb phosphorus and other nutrients.

Compaction: This reduces aeration in the root zone, which affects absorption of P and other nutrients.

Moisture: Phosphorus ions are carried in the soil solution. Water is necessary to increase availability of P in the soil (including P supplied through fertilizer).

Soil Particle Size: Phosphorus becomes tied up in clays. It is somewhat more mobile in sandy soils.

Temperature: In many types of soil, higher temperatures facilitate the breakdown of organic matter. Extreme high or low temperatures can reduce the plant’s ability to absorb Phosphorus.

Soil pH: The ideal pH range for maximum availability is 6.0 to 7.0, which is optimal for H2PO4 ions. In highly acidic or basic soils, chemical reactions can cause the phosphorus to become tied up in an insoluble phosphate.

Other Nutrients: Other nutrients promote healthy root development, which aids in the absorption of P. Ammonium plays a role in P uptake.

Arbuscular mycorrhizal fungi (AMF): AMF is a microbe that forms a symbiotic relationship with plants. Its hyphae extend the reach of the roots, allowing the plant to access Phosphorus that would otherwise sit beyond the reach of the root system. (Click here to learn more about this fascinating and highly beneficial relationship).

Micronutrient Deficiencies: Several micronutrients are necessary for plants to respond to phosphorus fertilizers.

Placement: Because Phosphorus has limited mobility, fertilizers must be applied as near to the seed as possible – but far enough away to prevent injury. Banding applications are considered to be the most effective.

Next: Building Phosphorus in the soil.

Achieving and maintaining adequate levels of plant-available phosphorus is vital to the success of all crops. There are several strategies for maximizing crop available phosphorus, which we will explore in Part 3 of this series.

Want to find out more about improving soil quality? 

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[1] With the levels of P typically found in western Canadian soils, the possibility of leaching is extremely low. However, some leaching can occur depending on the content in the soil and soil solution movement

[2] “Soils – Part 6: Phosphorus and Potassium in the Soil.” Plant and Soil Sciences eLibrary Pro: USDA National Institute of Food and Agriculture. ©2017.

[3] The Agronomy Handbook, Midwest Laboratories. Editors: Don Ankerman, B.S., Richard Large, Ph.D.

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