Phosphorus1 is a chemical element2 found on Earth in numerous compound3 forms, such as the phosphate ion4 (PO43-), located in water, soil and sediments5. The quantities of phosphorus in soil are generally small, and this often limits plant growth. That is why people often apply phosphate fertilisers6 on farmland. Animals absorb phosphates by eating plants or plant-eating animals.
The role of phosphorus in animals and plants
Phosphorus is an essential nutrient7 for animals and plants. It plays a critical role in cell8 development and is a key component of molecules9 that store energy, such as ATP10 (adenosine triphosphate11), DNA12 and lipids13 (fats and oils14). 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 weathering15 cause rocks to release phosphate ions16 and other minerals. This inorganic17 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 organic18 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 bacteria19 that break down organic matter20 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.
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 (adsorbed21) to soil particles, making it unavailable to plants. Desorption22 is the release of adsorbed23 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 (pH24) 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 vegetation25 to replace phosphorus. Therefore, farmers replenish the phosphorus ‘pool’ by adding fertilisers or effluent26 to replace the phosphorus taken up by 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 acid27 is used to convert the insoluble28 rock phosphate into a more soluble and usable form – a fertiliser29 product called superphosphate30.
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 fertilisation31. For example, adding lime32 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 leaching33 and water run-off34. This phosphate ends up in waterways, lakes and estuaries35. Excess phosphate causes excessive growth of plants in waterways, lakes and estuaries leading to eutrophication36.
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 inference37, imagination and creativity. Often they use models to help other scientists understand their theories. The phosphorus cycle38 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.
- phosphorus: A chemical element with the symbol P and atomic number 15. Phosphorus is essential for life, playing a critical role in cell development and in producing ATP, DNA and lipids in plants and animals.
- element: A substance made of atoms that all have the same atomic number. Elements cannot be split into simpler substances using normal chemical methods.
- compound: A pure substance made up of two or more different elements chemically combined.
- ion: An atom or group of atoms that has gained or lost electrons and has an electrical charge.
- sediments: Material that settles to the bottom of a liquid. In geology, it describes the solid fragments of inorganic or organic material that come from the weathering of rock and are carried and deposited by wind, water or ice.
- fertiliser: Compounds that are given to plants to promote growth.
- nutrient: A substance that provides nourishment for growth or metabolism.
- cell: 1. Building block of the body. A human is made of millions of cells, which are adapted for different functions and can reproduce themselves exactly. 2. A simple electrolytic device that enables chemical energy to be transformed into electrical energy.
- molecule: Two or more atoms bonded together. The molecule of an element has all its atoms the same. The molecule of a compound has two or more different atoms.
- adenosine triphosphate (ATP): A chemical used by cells to store and transport energy.
- adenosine triphosphate (ATP): A chemical used by cells to store and transport energy.
- DNA: Deoxyribonucleic acid (DNA) is a molecule that contains the instructions needed for an organism to develop and function. These instructions are stored as a code made up of four chemical bases: adenine (A), guanine (G), cytosine (C) and thymine (T).
- lipid: Any of a group of organic (carbon-containing) compounds, including fats, oils and waxes that are insoluble in water, oily to the touch and, together with carbohydrates and proteins, constitute the principal structural material of living cells. Lipids are characterised by their insolubility in water and solubility in organic solvents.
- fats and oils: Dietary fat is made up mainly of triglycerides, which are a combination of glycerol and fatty acids. The fatty acids are either saturated or unsaturated and are used by the body as a major energy source. Some fatty acids are essential and must be included in the diet.
- weathering: Chemical, mechanical (including freezing and thawing) and biological processes that break down rocks into smaller pieces. Weathering does not include the transport away of broken-down material.
- ion: An atom or group of atoms that has gained or lost electrons and has an electrical charge.
- inorganic: Atoms and molecules that are of inanimate (non-biological) origin.
- organic: 1. Molecules that contain carbon and that have a biological origin. 2. Grown using natural processes with nutrients from natural sources.
- bacteria: (Singular: bacterium) Single-celled microorganisms that have no nucleus.
- organic matter: The decomposed remains of living organisms and their waste products.
- adsorption: The binding of molecules or particles to a surface. This is different from absorption – the filling of pores in a solid. Adsorption is usually weak and reversible.
- desorption: Molecules or particles are released from a surface – the reverse of adsorption.
- adsorb: When molecules stick to a solid object as a thin film.
- pH: A measure of the acidity or alkalinity of a substance, based on a scale of 0 to 14. Acidic solutions have pH values less than 7, whereas alkaline solutions have pH values greater than 7.
- vegetation: Plant life.
- effluent: The outflowing of water from a system – often refers to the discharge of sewage, but can also be natural, for example, the outflowing of a river to the sea. Agricultural effluent refers to the treated and untreated wastewater collected during the management of livestock.
- acid: A hydrogen-containing substance that is capable of donating a hydrogen ion to another substance.
- insoluble: A substance that does not dissolve in liquid.
- fertiliser: Compounds that are given to plants to promote growth.
- superphosphate: New Zealand’s premium fertiliser. Superphosphate provides the essential nutrient phosphorus and first went on sale in England in 1843. It is produced by treating rock phosphate with sulfuric acid.
- fertilisation: (Reproduction) The joining of male and female sex cells (gametes), resulting in combining genetic material.
- lime: Chemically, lime is the compound calcium oxide. Commercially, lime could refer to ground-up calcium carbonate marketed as AgLime.
- leaching: When a compound becomes dissolved in water and moves from one place to another, for example, a fertiliser in the soil dissolves in rain water and ends up in a stream.
- run-off: Water carried away from land to streams and rivers.
- estuary: A partially enclosed body of water where freshwater mixes with saltwater from the sea.
- eutrophication: A process where waterways, lakes and shallow sea areas receive excess nutrients that stimulate excessive plant or algae growth. This excessive growth depletes the available oxygen in the water and causes other organisms to die off.
- inference: Conclusion from facts and reasoning.
- phosphorus cycle: Phosphorus, which is essential for life, is found in rocks. From there, it cycles through water, sediments and soil. In soil, it is made available to plants (and animals through plants). As organisms decay, it is cycled back through water, sediments and rocks.
