What does phosphorus do for plants? The P in NPK

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What is Phosphorus

Part of our series on NPK Plant Nutrients

Read part 2 here
Read part 3 here

Phosphorus is one of the three plant macronutrients that are essential to plant life. The other two are potassium and nitrogen. 

Plant fertilizers contain macronutrients in different proportions. Different ratios of the three macronutrients are expressed in the NPK ratio which you find on bottles of fertilizer. NPK stands for nitrogen, phosphorus, and potassium respectively. These are plant macronutrients - the three nutrients needed in the largest amounts by plants. The NPK ratio found on fertilizers, such as 15:15:15 or 5:20:20, indicates the percentage of these nutrients inside the fertilizer. 5:15:5 would indicate a phosphorus fertilizer.

Different NPK ratios serve different types of plants and support different developmental processes. Fertilizers that contain a higher proportion of phosphorus are intended for use on flowering plants. 

Like other mineral plant nutrients, it is found in tiny amounts in plants. Normally, between 0.1 - 1 % of plant tissue is phosphorus. Phosphorus is essential but plants need less of it than the other two macronutrients.  

History of phosphorus fertilizer

Since ancient times, animal biomass and animal bones had been applied to agricultural soils to improve crop yields. However, for most of human history, it was unclear why these substances were beneficial to growth. Only in the late eighteenth-century was it recognized that the active ingredient was phosphorus.


Bones are an excellent source of phosphorus. Animal bones were the major source of phosphorus agricultural fertilizer before mined rock phosphorus https://www.pxfuel.com/en/free-photo-iupuj 

One of the first people to systematically experiment with phosphorus fertilizer was John Lawes. He organized a field experiment at his estate where he applied phosphorus sources to the soil. Recognizing that rock phosphorus was more abundant than animal sources of phosphorus in bones, he patented the first acid-treated rock phosphate in 1842. The mineral phosphorus fertilizer industry was born and most of the world’s phosphorus fertilizer comes from rocks rather than organic sources.

What does phosphorus do for plants?

Depending on the pH of the soil, plants take up phosphorus either in the form of H2PO4  (Dihydrogen phosphate) or HPO42- (Hydrogen Phosphate). These are phosphates, oxidized forms of phosphorus. 

At the level of the whole plant, phosphorus is essential for root growth, fruit, stem and seed development, and disease resistance. 

At a molecular level, plants use phosphates to create and store energy. Phosphates are found in the most important energy-creating and storing molecules: adenosine diphosphate and triphosphate (ADP and ATP). When plants need energy, a phosphate is detached from ATP. This turns the ATP molecule into an ADP molecule and releases energy which the plant uses to perform cellular processes.  Any energy left over from these processes is stored by reattaching a free phosphate molecule to an ADP molecule, turning it back into ATP. 

Phosphates are also the building blocks of plant tissue: phospholipids (thee are important in cell membrane structure), nucleic acids (which carry genetic material from cell to cell), nucleotides (structural units of RNA and DNA that play a role in metabolism), and phosphoproteins. Phosphorus is also found in large amounts in phytin, a chemical which makes seeds germinate. This means that phosphorus deficient seeds are less viable. Young cells need plenty of phosphorus while they divide and enlarge, so any deficiency will limit early initial growth in plants.


Without enough phosphorus, seeds cannot germinate U.S. Department of Agriculture commons.wikimedia.org/wiki/File:Seed_germination.png

Signs of phosphorus deficiency are mostly indistinguishable from signs of other nutritional deficiencies. Leaf drop may occur starting with the oldest leaves. However, unlike in nitrogen deficient plants where leaves go pale green or yellow, phosphorus deficient plants will turn a darker green, and perhaps even red or purple. The reason for the dark green coloration is because chlorophyll formation is not affected by phosphorus depletion, while cell and leaf expansion are. The concentration of chlorophyll increases while the ability of chlorophyll cells to conduct photosynthesis decreases. Seedlings will display reduced height, stem diameter, and leaf size. Phosphorous deficiency will also limit fruit size.

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