The phosphorus cycle
Phosphorus is a chemical element found on Earth in numerous compound forms, such as the phosphate ion (PO43-), located in water, soil and sediments. The quantities of phosphorus in soil are generally small, and this often limits plant growth. That is why people often apply phosphate fertilisers on farmland. Animals absorb phosphates by eating plants or plant-eating animals.
The role of phosphorus in animals and plants
Phosphorus is an essential nutrient for animals and plants. It plays a critical role in cell development and is a key component of molecules that store energy, such as ATP (adenosine triphosphate), DNA and lipids (fats and oils). Insufficient phosphorus in the soil can result in a decreased crop yield.
The phosphorus cycle
Phosphorus moves in a cycle through rocks, water, soil and sediments and organisms.
Here are the key steps of the phosphorus cycle
Over time, rain and weathering cause rocks to release phosphate ions and other minerals. This inorganic phosphate is then distributed in soils and water.
Plants take up inorganic phosphate from the soil. The plants may then be consumed by animals. Once in the plant or animal, the phosphate is incorporated into organic molecules such as DNA. When the plant or animal dies, it decays, and the organic phosphate is returned to the soil.
Within the soil, organic forms of phosphate can be made available to plants by bacteria that break down organic matter to inorganic forms of phosphorus. This process is known as mineralisation.
Phosphorus in soil can end up in waterways and eventually oceans. Once there, it can be incorporated into sediments over time.
The phosphorus cycle
Phosphorus moves in a cycle through rocks, water, soil and sediments and organisms.
Most phosphorus is unavailable to plants
Since most of our phosphorus is locked up in sediments and rocks, it’s not available for plants to use. A lot of the phosphorus in soils is also not available to plants.
The availability of phosphorus in soil to plants depends of several reversible pathways:
Bacteria : Bacteria convert plant-available phosphate into organic forms that are then not available to plants. Although other bacteria make phosphate available by mineralisation, the contribution of this is small.
Adsorption : Inorganic (and available) phosphorus can be chemically bound (adsorbed) to soil particles, making it unavailable to plants. Desorption is the release of adsorbed phosphorus from its bound state into soil solution.
pH : Inorganic phosphorus compounds need to be soluble to be taken up by plants. This depends on the acidity (pH) of the soil. If soils are less than pH 4 or greater than pH 8, the phosphorus starts to become tied up with other compounds, making it less available to plants.
Many plant crops need more phosphorus than is dissolved in the soil to grow optimally. In addition, crops are usually harvested and removed – leaving no decaying vegetation to replace phosphorus. Therefore, farmers replenish the phosphorus ‘pool’ by adding fertilisers or effluent to replace the phosphorus taken up by plants.
Phosphorus availability to plants
While much of the phosphorus in soil is not readily available, there are a number of reversible pathways that release small amounts of phosphorus to plants.
Phosphate fertilisers replenish soil phosphorus
Many farmers replenish phosphorus through the use of phosphate fertilisers. The phosphorus is obtained by mining deposits of rock phosphate. Locally produced sulfuric acid is used to convert the insoluble rock phosphate into a more soluble and usable form – a fertiliser product called superphosphate.
In New Zealand, superphosphate is made using rock imported mainly from Morocco.
Adjusting the pH of the soil for efficient plant uptake of phosphate should be done prior to fertilisation. For example, adding lime reduces soil acidity, which provides an environment where phosphate becomes more available to plants.
Water pollution by fertilisers
When fields are overfertilised (through commercial fertilisers or manure), phosphate not utilised by plants can be lost from the soil through leaching and water run-off. This phosphate ends up in waterways, lakes and estuaries. Excess phosphate causes excessive growth of plants in waterways, lakes and estuaries leading to eutrophication.
Steps are being taken in agriculture to reduce phosphate losses in order to maximise the efficiency of fertiliser and effluent applications.
Nature of science
Scientists make observations and develop their explanations using inference, imagination and creativity. Often they use models to help other scientists understand their theories. The phosphorus cycle diagram is an example of an explanatory model. Diagrams demonstrate the creativity required by scientists to use their observations to develop models and to communicate their explanations to others.
