Phosphorus (P) is the 15th element of the Periodic table and the 12th most abundant element in Earth’s crust. Chemically, it is classified as a non-metal. It has two major elemental forms – white and red phosphorus, and neither of the two occurs naturally due to their immense reactivity. At room temperature, this element is a colorless, transparent solid that gently glows in the dark.
However, this simple, mechanistic description doesn’t grasp the power and importance of this element, especially when carbon-based life is concerned. Phosphorus is one of the crucial elements that sustain life on our lovely planet. When it joins up with four oxygen atoms, it creates a phosphate ion (PO43-). This simple combination of five atoms is an integral component of many essential molecules, including nucleic acids, ATP, and phospholipids.
A bit of history
Phosphorus was first isolated in the 17th century by a German merchant named Hennig Brandt. Alchemy was quite popular at the time among the educated middle and upper class, and the discovery of this element is actually an unexpected outcome of Brandt’s quest for the Philosopher’s stone. After boiling and distilling 50 buckets of urine, Herr Brandt ended up with a strange substance that could glow in the dark – white phosphorus. He hid his discovery for about six years, hesitant to show it to his colleagues, as intellectual theft was common amongst alchemists.
Brandt’s hesitance proven to be justified, not because the people weren’t ready for his incredible discovery, but because his colleagues were quick to use it to earn some silver entertaining European nobility with the light show. The phosphorescent glow is a result of oxygen reacting with phosphorus ions and creating short-lived molecules – HPO and diphosphorus dioxide (P2O2). These molecules emit a faint, greenish glow that is visible in the dark. Because of its luminescence, this element was given the Greek name phosphoros (Φωσφόρος) meaning the bringer of light, as a reference to the planet Venus.
Phosphorus in the industry
During the years following Brandt’s discovery, phosphorus was mostly used for arguable medical practices. That was until the second half of the 18th century, after the discovery that bones contain significantly more phosphorus than urine. Instead of an arduous task of extracting a tiny amount of this element from gallons of urine, now people could produce phosphorus in abundance cheaply and simply.
The highly reactive white phosphorus is flammable when exposed to air – a very useful quality for the production of matches and various military equipment (bombs, grenades, artillery shells, etc.). By the middle of the 20th century, phosphorus already achieved its famous, gruesome reputation, wreaking havoc in European cities during WW1 and WW2. Luckily, after WW2, the use of phosphorus was limited in weaponry, and people started exploring more pacifistic uses for this element. Today, the largest portion of phosphorus compounds we make goes to the production of fertilizers.
Why is phosphorus important to plants?
From the perspective of plant physiology, phosphorus is one of three primary macronutrients, along with nitrogen and potassium (NPK). It makes up about 0.2-0.8% of the plant’s dry mass. However, don’t let this seemingly small percentage fool you – this element is an important part of many essential compounds vital for plant growth, development, and reproduction.
The specific role of phosphorus in plants has a lot to do with the structure of its ions. Phosphate ions have four oxygen atoms, out of which three can form various types of chemical bonds (hydrogen, ionic and covalent bonds). Being able to connect with many elements in different ways, phosphorus is a structural component of many important compounds:
- Phospholipids – key components of the plasma membrane
- Nucleic acids – DNA and RNA
- Nucleotides – AMP, ADP, ATP, GTP, UTP
- Coenzymes – NAD, NADP, FAD, FMN
- Sugar phosphates
- Phytates – reserves of phosphorus in the seeds
- Phosphatidyl derivatives – lecithin, choline
- Esters of phosphoric acid
Summary of phosphorus’ most important functions in plants:
- As one of the main components of phospholipids, phosphorus is vital for active transport through cell membranes
- Compounds included in the Krebs’ cycle, glycolysis and WDH pathway (like phosphate esters and phosphorylated sugars) contain phosphorus, so this element also plays a role in photosynthesis and respiration
- Carbohydrate and protein metabolism depends on the phosphorus content, and a deficiency usually leads to an increased concentration of low-molecular-weight compounds in plant tissues and low-quality yields
- It is essential for starch production and transport of photosynthates (products of photosynthesis)
- pH value regulation – both organic and inorganic phosphates act as buffers in the cytoplasm, keeping its pH value within a favorable range
- Mechanisms of plant resistance to low temperatures and disease partially rely on phosphorus compounds
- It is vital for seed germination, development of seedlings, and the root
Adenosine triphosphate (ATP)
The legendary science fiction writer, Isaak Asimov, once said: “Life can multiply until all the phosphorus has gone, and then there is an inexorable halt which nothing can prevent.” The compound he probably had in mind is adenosine triphosphate (ATP). ATP is an organic compound and its parts include adenine (nucleic base), the sugar ribose, and three phosphate ions. It is the main energy currency in all living organisms.
ATP works by releasing and accepting phosphate ions. When ATP is hydrolyzed, it releases a phosphate ion, turning into ADP (adenosine diphosphate). This reaction is followed by the production of energy that living cells use to fuel their metabolism. If more energy is needed, ADP will hydrolyze and release another phosphate ion, becoming AMP (adenosine monophosphate). These phosphate ions are replenished through chemical reactions aided by enzymes, preparing new ATP molecules to start the process again.
Phosphorus in the soil
Just like nitrogen, phosphorus also has its natural cycle. However, compared to nitrogen, the phosphorus cycle is fairly simpler. This element is naturally found in the soil as a result of the decomposition of various minerals (more than 170 minerals contain this element), as well as plant and animal residues. Phosphorus is present in both organic and inorganic compounds, and various species of microorganisms take part in the decomposition of those compounds. Microbial activity transforms the previously unavailable phosphorus into ions that can be taken up by plants.
Plants absorb phosphorus and incorporate it into their cells and tissues. Animals eat the plants, and when both die and decay, they return the phosphorus to the soil. A portion of phosphorus compounds gets washed up with the rain, ending up in rivers, lakes, and seas. They slowly get incorporated into the sediment, creating minerals and rocks.
How do plants absorb phosphorus?
There are three types of phosphates according to the level of dissociation: primary phosphate (H2PO4–), secondary phosphate (HPO42-), and tertiary phosphate (PO43-). They are more famous by their name orthophosphates. Plants are able to absorb the first two types. Which one will they take up, depends on the pH value of the substrate. In slightly acidic soils, primary phosphates are more abundant, while secondary phosphates are usually dominant in alkaline soils (pH>7). These ions reach the roots through diffusion or symbiotic, mycorrhizal fungi help to deliver them.
Soils that are prone to phosphorus deficiency are calcium-rich soils. When it interacts with P, Ca forms an insoluble compound that makes P ions unavailable to plants. Commercial plant substrates usually have a balanced content of these nutrients, so cases of deficiency due to a lack of phosphates in the soil are rare.
The problem with P deficiency usually occurs because there are few available ions or due to external factors. Cold temperatures, compact soil, use of certain pesticides, and poor microbial soil community are some of the most common elements that negatively affect phosphorus availability.
Symptoms of phosphorus deficiency
- Plants produce small, dark green leaves
- Leaves change color from dark green to purplish-red
- Slowed growth
- Prolonged maturation of plants
The symptoms are more pronounced in young, developing plants that require more P compared to mature plants. Phosphorus deficiency inhibits the growth of aerial parts of the plant more than the root. To compensate for the deficiency, the plant’s roots go through biochemical and morphological changes. These changes serve to increase the efficiency of phosphorus absorption. This usually manifests in roots increasing their biomass, penetration ability, biochemical activity, and lateral growth.
Deficient plants produce small leaves with high chlorophyll content, which is why they appear dark green. However, the photosynthetic activity of these leaves is significantly less intensive than in leaves with the optimal phosphorus content. Some plant species can also develop clusters of short, lateral roots specialized in phosphorus absorption to adapt to unfavorable conditions. P deficiency disrupts the process of carbohydrate storage, which usually results in carbohydrate buildup. The excessive amount of sugars accumulated in leaf tissues causes the darkening of the leaves and color change.
It can be easy to misidentify symptoms of P deficiency as N deficiency because both physiological disorders cause the reddening of young leaf veins.
Plants that are sensitive to low levels of available phosphorus are tomatoes, corn, wheat, oats, and other cereals, lucerne.
Poor fertilization practices are the most common causes of phosphorus toxicity. Adding too much mineral fertilizer is the most common cause of an excess of this element, as phosphorus is significantly less concentrated in natural fertilizers like manure and compost.
Symptoms of phosphorus toxicity:
- Plant metabolism speeds up
- Short vegetative phase
Phosphorus toxicity is rare and it occurs when its concentration in the plant exceeds 1%. This physiological disorder can cause zinc, iron, and copper deficiency, as well as increased manganese (Mn) uptake.
Fertilizing with phosphorus
Most commercial P fertilizers are made of pulverized phosphate rock. There are a lot of problems with this type of fertilizer. For starters, its production requires mining activities that often have severe negative effects on the natural environment. Mineral fertilizers are not readily available to plants, so they require microbial degradation to release the goods in a form plants can take up. This takes some time, and microbes only process a portion of the applied fertilizer. In the meantime, water from precipitation or irrigation carries away most of the fertilizer, providing a limited amount of available nutrients to plants. The “excess” leaches into the deeper layers of the soil, often ending up polluting groundwaters and surface waters.
Although the negative impact of mineral fertilizers is significantly lower in indoor plant cultivation, it is always better to opt for organic over mineral fertilizers. Aside from being sustainable sources of phosphorus, organic fertilizers contain readily available forms of P and additional ingredients that promote microbial activity, promote plant health, and increase yields.
Natural sources of P
Phosphorus is widespread in all living organisms, so their excrements, waste and remains present quite a good source of this element for plants. It may not be pretty, but it is the cycle of nature. Aside from the good ol’ manure, bat guano, and urine, you can use mushroom compost or bone meal to boost the phosphorus supply to your plants. Let’s cover the specificities of each.
Manure, slurry, sludge, bird and bat guano, and similar organic waste
Animal waste is a rich source of phosphorus and nitrogen, along with quite a few other nutrients. Depending on the animal species and its diet, the exact concentration of phosphorus and other elements in the waste can be highly variable. The general advice is to not get too wild with the application of this type of fertilizer, as it can change the microbial balance in the soil.
After learning about the discovery and isolation of phosphorus, this probably doesn’t surprise you. Nevertheless, the use of urine as fertilizer is not common for various, and some quite obvious reasons. However, you should have in mind that not all urine is equal in terms of plant fertilization. Generally, urine from carnivores is highly concentrated because they do not drink a lot of water. It is also more acidic due to their diet. The moral of the story is that letting your cat do the number one on your plants might not be a good idea. It won’t harm them immediately, but with repeated exposure symptoms of toxicity are likely to occur.
As the name says, this type of fertilizer is made of animal bones that usually come from the meat processing industry. Although the exact content can vary depending on the animal species, most commercial bone meal fertilizers have an NPK ratio of 3-15-0, and a considerable amount of calcium. Bone meal is a slow-release type of fertilizer that requires microbial activity or soil acidity to break down. Upon application, the bone meal fertilizer usually releases its nutrients over the course of 1 to 4 months.
Although it contains a pretty humble amount of phosphorus compared to the previously mentioned fertilizers, mushroom compost is an honorable mention. Its NPK ratio is usually around 1-1-1 or 2-1-1, but it also contains essential micronutrients that boost and ensure the proper growth of plants. Despite its seemingly low macronutrient content, mushroom compost can be a pretty helpful part of your natural fertilizer toolbox. It enhances microbial activity in the soil with its high organic matter content. The better microbial activity provides better absorption of other elements and indirect benefits to your plants.
- Ashley K., Cordell D., Mavinic D. (2011). A brief history of phosphorus: From the philosopher’s stone to nutrient recovery and reuse. Chemosphere pp.737–746.
- Functions of Phosphorus in Plants. Better Crops/Vol. 83 No. 1. (1999).
- Fidanza, M.A., Sanford, D.L., Beyer, D.M., Aurentz, D.J. (2010). Analysis of Fresh Mushroom Compost. Vol.20, No.2.
- Efficiency of soil and fertilizer phosphorus use. FAO (2008).