Why Do Plants Need Phosphorus? Every Important Information

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If you regularly reach for a fertilizer bag to feed your plants nutrients, you will have noticed that phosphorus is usually listed as one of the main ingredients on the bag to ensure that your plants can grow healthy. But why, exactly, do plants need phosphorus? 

Plants need phosphorus for proper growth and development. It’s one of the most essential plant nutrients as it helps absorb, store and convert the sun’s energy into vital biomolecules that facilitate growth in all stages. If plants have a deficiency of phosphorus, they won’t grow healthy.

In this article, I’ll explain what you need to know about why plants require phosphorus and how they can get more of this essential nutrient. I’ll also look at the science behind how phosphorus affects plants. 

Why Phosphorus Is an Essential Plant Mineral

Besides nitrogen, phosphorus is the other essential nutrient that plants can’t grow properly without. When your plants aren’t performing well, the reason is most likely a result of a shortage of phosphorus or nitrogen rather than a lack of any other element. 

This is because phosphorus plays an important role in all life processes, and it’s a part of every living cell, so without phosphorus, there wouldn’t be any normal or healthy life on earth. Phosphorus has many important functions in keeping plants healthy. 

How Does Phosphorus Work In Plants?

There are many important roles that phosphorus plays in ensuring that your plants grow healthy and as expected. This crucial nutrient works by regulating plant energy, forming plant genes, encouraging root development, assisting in cell division, promoting healthy leaf growth, and also promoting the movement of water. 

Let’s take a closer look at how phosphorus works in plants. 

Phosphorus Regulates Plant Energy 

Phosphorus is an essential component of sugar phosphates, so it contributes to plant respiration and energy transfer via a molecule known as adenosine triphosphate (ATP).  ATP is a molecule by which plants store energy so that they have it available for future processes when energy is needed by their cells. 

Phosphorus also regulates and controls the process of photosynthesis. During photosynthesis, plants use light energy to transfer water and carbon dioxide into simple sugars that they can use. 

Phosphorus is important because it captures, stores, and converts the sun’s energy so that it gets transferred into molecules like ATP. These regulate biochemical reactions such as photosynthesis. This energy is collected by the ATP, which then becomes energy for other reactions in the plant. The sugars also get used for other cells’ structural and storage components. 

Phosphorus Forms Plant Genes

Phosphorus is an essential structural element in a plant’s genetic molecules. Phosphorus forms part of both ribonucleic acid (RNA) and deoxyribonucleic acid (DNA). Since phosphorus is part of the nucleic acid structure of plants, it’s a nutrient that regulates protein synthesis.

If plants can’t synthesize proteins, they probably won’t grow as expected. Plant proteins assist the effectiveness of many processes that plants need to survive.  

  • Plant proteins help to boost the immune system of plants, making them more resistant to disease. Plant immune systems help to protect them against pathogen infections. 
  • Plant proteins assist in photosynthesis.
  • Plant proteins contribute to the transportation of nutrients throughout the plant. 

Since phosphorus is found in deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), both of which store information on how plants should perform functions, this mineral is responsible for functions such as:

  • Early root growth
  • Seed formation
  • The effective use of water 

Amazingly, phosphorus continues to benefit the soil after the plants in it have died. When plants die, the phosphorus that they contain enters the soil and can be taken up by other plants as well as microorganisms present in the soil to strengthen their growth and improve their health. 

Phosphorus Encourages Root Development

Roots are necessary to draw nutrients from the soil to help plants grow. They also support the plant so that it can tolerate harsh conditions like rain and wind, so the roots themselves need to be strong. Without enough phosphorus, plants will have stunted, spindly roots.

Phosphorus helps roots to grow and encourages moisture and mineral absorption throughout the entire root. Plants can draw two types of phosphorus from the soil: orthophosphate and organic phosphate. 

  • Orthophosphate. Usually, when plants pull phosphorus from the soil with their roots, phosphorus is absorbed in the form of orthophosphate. This is normal phosphorus that’s made up of one phosphate unit per molecule.
  • Organic phosphate. Plant roots can also absorb certain types of organic phosphorus, with examples of organic phosphorus including animal residue or microorganisms that are present in the soil. 

Phosphorus travels to the surface of the roots via diffusion. This process entails the movement of molecules from one area where the concentration is the highest to an area of lower concentration. 

Unfortunately, phosphorus can only travel a short distance via diffusion, which is why a large and active root system in the plant is essential. Besides diffusion, phosphorus is also supplied to the plant roots via root interception, where the roots grow into phosphorus-rich soil. 

Root systems that are larger and have higher ratios of surface area to volume will be able to reach a larger quantity of soil, which will ensure that the plants can be exposed to more phosphorus. In addition to this, mycorrhizae fungi, which increase the volume of soil that plant roots access, also assist in the plants’ uptake of phosphorus.

Phosphorus Helps Cell Division 

There’s a cambium located inside the plant stem where cell division occurs so that the plant can grow. Cambium is a layer of dividing cells between plant tissues that encourages the secondary growth of roots and stems.

Phosphorous deficiency almost certainly affects the growth of plants. This is because phosphorus encourages cell division down to the ribonucleic and deoxyribonucleic acid levels. Without this process, the plant’s stems won’t manage to grow longer or become wider. 

Phosphorus is a basic component of membrane phospholipids, which means that its uptake is essential for the functional integrity of plant cell membranes. This is required for the plant cells to remain strong and healthy. 

Plants also need phosphorus to develop new cells and transfer genetic code from one cell to another. Since phosphorus is so essential for the development of seeds, it’s usually found in large quantities in fruits and seeds. 

Phosphorus Encourages Healthy Leaves 

Phosphorus is also crucial to the health of leaves, primarily due to its role in photosynthesis. This nutrient helps in the transfer of energy from the sun to plants, which is vital for the overall growth and health of plants. 

The hugely important mineral also plays a part in plant metabolism, more specifically in the production of carbohydrates. These are essential for providing plants with enough sources of energy and carbon skeletons (backbones of organic molecules that are composed of carbon-carbon atoms that produce organic compounds). 

Since leaves are involved in photosynthesis, they produce carbohydrates for the plant to use. Photosynthesis occurs in the leaf chloroplasts of the plant. And without adequate phosphorus, photosynthesis can’t happen properly! 

Phosphorus Encourages Movement of Water 

Vascular plants contain xylem tissue that enables nutrients and water to move through the plant, from the roots to its stems and leaves. This ensures that every part of the plant will get what it needs to grow and be healthy. 

Water has to be pushed up the xylem tissue, and this process is dependent on the presence of phosphorus. Essentially, a larger amount of water will be transported throughout the plant when phosphorus levels are high.

Proteins in the plant also work with phosphorus to encourage better water transportation throughout the plant. The proteins have structures that form gate-like channels in the plant’s membranes, and these open or close depending on if they’re phosphorylated (in other words, if phosphorus has been introduced to them). 

The gated channels regulate how water and minerals move through the plants, but this entire process won’t work effectively if phosphorus levels aren’t high enough. This can put the plant’s health at risk because it won’t get the water and nutrients it requires. 

Phosphorus Encourages Greater Nutrient Concentrations 

Plants need to draw nutrients from their environment to grow healthy. In order for nutrients to be transported around the plant, cell membranes require energy. High-energy phosphorus compounds and ATP are required for this process to occur. 

Phosphorus also affects the plant’s uptake of useful nutrients, such as magnesium and calcium. The movement of water and these other nutrients not only depends on phosphorus but how available ATP is – if there’s enough ATP, more phosphorus will be available to open the channels in the cells so that more nutrients, and water, can enter the plant. 

How Phosphorus Moves Through Plants 

Phosphorus first enters plants via their root hairs and root tips, as well as the outer layers of root cells. Mycorrhizal fungi that are present in the soil enhance phosphorus uptake, so they are beneficial. 

Once the phosphorus has entered the root of the plant, it can be stored there, where it facilitates root growth. The phosphorus can also be sent to the upper parts of the plant.

Chemical reactions enable the phosphorus to get incorporated into organic compounds, such as:

  • DNA and RNA nucleic acids
  • Phospholipids
  • Sugar phosphates
  • Enzymes
  • Compounds such as adenosine triphosphate (ATP) 

Why Plants Struggle To Draw Phosphorus From the Soil 

Phosphorus is a tricky mineral for plants to access in the soil. It’s not always easy for plants to draw phosphorus from their environment because most of this nutrient is located in rocks and sediments. There are other factors that make it difficult for plants to access phosphorus. 

  • Bacteria. These transform phosphate into organic forms that aren’t accessible to plants. 
  • Adsorption. When inorganic phosphate is adsorbed or chemically bound to soil particles, it becomes inaccessible and, therefore, impossible for plants to access. 
  • Soil pH: For inorganic phosphorus compounds to be drawn by plants, they need to be soluble. However, the soil has to have the correct pH in order for this to occur. If the soil pH is lower than 4.0 or higher than 8.0, the phosphorus becomes bound with other compounds, so plants can’t draw it from their environment. 

What Happens When Plants Lack Phosphorus 

If your plants don’t get enough phosphorus, this will show up in a variety of ways. Here are some common symptoms to look out for:

  • Reduced leaf expansion and leaf surface area. 
  • Fewer leaves on the plant. 
  • Decreased root growth. 
  • Reduced shoot growth. 
  • Dark green leaves. This is a result of the reduced use of carbohydrates by the plant, even though carbohydrate production via photosynthesis continues. 
  • Purple leaves (such as in the case of corn and tomato plants). This occurs because sugars build up and cause anthocyanin pigments to develop. These pigments can appear purple, black, blue, or red, depending on their pH. 
  • Shallow root systems. This is something plants do to compensate for the lack of phosphorus. 

You’re likely to first see symptoms of a phosphorus deficiency in the older parts of the plant. Newer foliage will look healthy, but the leaves are usually small if there is a phosphorus deficiency.

How To Ensure Your Plants Get Enough Phosphorus 

If you can see that your plants are displaying symptoms of insufficient phosphorus, you might wonder how to test your soil and increase phosphorus levels to promote growth. It’s worth noting that healthy phosphate levels in the soil range from 25 to 50 ppm, and this is also the range to work with if you’re growing vegetable gardens.

You can test your soil for phosphorus and other important nutrients with a soil test kit. I’d recommend the Luster Leaf 1601 Rapitest Kit, which is available from Amazon. It tests soil pH, phosphorus, nitrogen, and potash with a user-friendly capsule system and color comparators to make it easy to view the results.

If your soil is healthy and has the right amount of phosphorus for plants to grow, you should give it a light application of phosphorus every year. However, if the soil has less than 25 ppm, a starter fertilizer is beneficial to the soil. 

Before adding fertilizer to your soil, you should read our guide on the Best Fertilizer Numbers for Spring, Summer, and Fall. This will help ensure you give your plants the nutrients they need without giving them too much of a specific nutrient that can be harmful. 

How To Add Phosphorus to Your Soil

Instead of adding chemicals to the soil, which can disrupt it, you can add phosphorus to your soil in a natural way. A great method is planting green manure crops. These crops have a high phosphorus content. By plowing after harvesting these high-phosphorus plants, you can increase the soil’s overall phosphorus content. Examples of high-phosphorus plants include clover, mustard, and oat. 

To revive plants that need a boost of phosphorus, you can also add compost to the soil around them. Compost is a good phosphorus source for your plants, and for best outcomes, it’s best to mix it into the top layer of soil.

Finally, bone meal can also help to boost the phosphorus content of your garden soil. I’d recommend a product such as Burpee Natural & Organic Bone Meal, which is available from Amazon. It contains OMRI-listed ingredients and provides a good blend of organic nitrogen, calcium, and phosphorus to encourage the growth of healthy plants with vibrant flowers. It gets released into the soil slowly, so it won’t harm your plants. 

Other Things To Know About Phosphorus 

Now that we’ve looked at why phosphorus is such an important nutrient for your plants and how you can boost your plants’ phosphorus levels, here are other things to know about it, especially since some types of plants require more phosphorus than others. 

  • Most annual plants need more phosphorus when they start to grow. Since annuals need to live through all their growth stages in one growing season, phosphorus helps them to produce seeds, bloom, and reach maturity.
  • Plants in cold conditions require more phosphorus. If plants are growing in cold weather conditions and have limited roots but quick top growth, they need more phosphorus. Plants require phosphorus during cold weather because it strengthens them against harsh conditions.
  • Established plants don’t require as much phosphorus as younger plants. In addition, if they’re grown in hot climates, they will require less phosphorus. 
  • You can give your plants too much phosphorus. Beware of giving your plants an abundance of phosphorus. Too much phosphorus decreases the plant’s ability to draw required micronutrients in the soil, such as zinc.
  • Some plants don’t require any phosphorus. A prime example is carnivorous plants, such as Venus Flytraps. These thrive in poor, acidic soil that doesn’t provide nutritional value because they get all the nutrients they require from the bugs that they eat.

Final Thoughts 

One of the most important minerals your plants need to grow is phosphorus. This is a common ingredient in plant fertilizers, but there are important things to know about phosphorus and the roles it plays in plant health. 

Some of the vital roles of phosphorus in plants include: 

  • Forming part of plant genes. 
  • Encouraging the healthy growth of plant roots.
  • Transporting water throughout the plant. 
  • Playing an essential role in photosynthesis. 
  • Encouraging the absorption of other important nutrients.
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