Available or Unavailable: The Behavior of Phosphorus in Your Soil

Introduction: Phosphorus availability

Phosphorus availability is key to the plant’s proper uptake and utilization of phosphorus for key growing phases, however, research suggests that the available phosphorus in the first 6” of topsoil is far less than the amount needed for optimal plant growth. Kansas State University Extension suggests that the total available phosphorus content is as little as 200lbs or 0.0001% of the total topsoil in an acre1. In addition, the movement of phosphorus in the soil is very little. According to Penn State University Extension, Phosphorus moves only about ⅛ of an inch per year(2). This means that available phosphorus is not going to move around the soil solution for growing roots to readily take up. What makes this nutrient so immobile and unavailable in the soil solution?

Nutrient movement and holding

CREDIT: LSU Extension

Soils carry plant nutrients in varying amounts through the interaction of electro-magnetic charges between soil particles and different nutrients3. Clay particles and organic matter, for example, carry a negative change. Plant nutrients such as calcium, magnesium, potassium, and ammonium (cations) carry a positive charge. These positive and negative charged substances are attracted to each other and will bind together temporarily until they are pulled into the soil solution to replace nutrients taken with roots or lost through leaching. Phosphorus is an anion.

Anions are those nutrients that carry a negative charge. Since the soil components that hold electrical charges are also negative, it is very difficult for phosphorus to bind to soils for uptake from plants. The capacity for any particular blend of soil to hold positive nutrients is the cation exchange capacity (cec) of the soil. Soil pH is a closely related topic. Soil pH is the measurement of the ratio of acid forming cations to base forming cations. The relative proportion of acid forming to base forming cations is known as the base saturation. As acid forming cations increase, pH drops. As basic cations increase, pH rises. Knowing these soil characteristics, we can understand better how phosphorus interacts in various soils. We know that phosphorus is negatively charged, so what does this tell us about its behavior in soil? The next puzzle piece in understanding phosphorus availability is the workings of the phosphorus cycle.

The phosphorus cycle

Any discussion of phosphorus availability starts with the phosphorus cycle. The phosphorus cycle is simply the movements phosphorus takes through any particular ecosystem as it is stored, used, or lost. The most basic cycle of phosphorus availability follows two main steps: a very large fixed pool of phosphorus that is continuously being broken down into very small amounts of available phosphate and the movement of phosphorus from the available pool to the soil solution to replace what is taken up by the plant(4).

The phosphorus cycle begins with multiple inputs of phosphorus into the biome. These inputs come from organic and inorganic forms. Organic forms of phosphorus inputs include animal manure, living organisms in the soil that die and breakdown, and organic matter release from residue. Inorganic phosphorus includes all phosphates in fertilizers such as MAP, DAP, and Phosphoric Acid. From here, organic and inorganic sources of phosphorus can take a few paths.

One way phosphorus is created is through the mineralization process. This occurs when bacteria breaks down carbon-based biomass with hydrogen and dihydrogen phosphate released into the soil solution as a result. The reverse process, immobilization, occurs when microorganisms consume inorganic phosphates that are converted back into organic forms. Here we see the main process in the phosphorus cycle, phosphorus becoming available and unavailable. When phosphate is made available, plant roots can take it up and use it for key plant processes. Adsorption in a second subcycle in the phosphorus cycle. With adsorption, inorganic phosphorus is pulled from the soil solution and forms temporary chemical complexes with iron and aluminum on the soil surface. Here a new bond is created making the phosphorus unavailable. The opposite process, desorption,makes the phosphate available again. While adsorption and desorption are quick, reversible processes, precipitation and dissolution driven by mineral weathering are a more permanent process of phosphorus solubility. Precipitation is the process by which phosphate particles bond with secondary minerals like iron or aluminum to form completely new compounds. Organic phosphates and the new compounds are released through weathering. But, weathering is a very slow process. Because weathering occurs over years, it is a poor form of creating available phosphorus.

Some questions to ask yourself:

Is low phosphorus availability a limiting factor in any of my crops?

What do I need to look for to measure phosphorus deficiency?

What methods are at my disposal to alleviate poor phosphorus availability?

Citations

1. https://www.agronomy.k-state.edu/services/soiltesting/documents/fertilizer_recommendations_and_references/Phosphorus%20Fact
   s.pdf
2. https://extension.psu.edu/what-is-an-acre-furrow-slice-of-soil
3. https://extension.uga.edu/publications/detail.html?number=C1040&title=Cation%20Exchange%20Capaci
   ty%20and%20Base%20Saturation#:~:targetText=As%20such%2C%20the%20CEC%20is,soil%20solutio
   n%20for%20plant%20uptake.&targetText=Plant%20roots%20can%20remove%20nutrients,away%20from
   %20the%20clay%20particles
4. https://www.aces.edu/blog/topics/crop-production/understanding-phosphorus-forms-and-their-cycling-in-th
   e-soil/#:~:text=Precipitation%20is%20a%20slow%20process,release%20rate%20is%20very%20slow.