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Phosphorus (P) Nutrient Deficiencies

Second only to nitrogen, phosphorus is the most essential nutrient to guarantee optimal growth, health, and function of our plants. It is one of the key elements required for just about every plant.

Phosphorus (P) Nutrient Deficiencies


Second only to nitrogen, phosphorus is the most essential nutrient to guarantee optimal growth, health, and function of our plants. It is one of the key elements required for just about every plant. It's a mobile element which means that if a plant is deficient, it will transfer phosphorus from other parts of the plant to areas it needs it in. Its one of the three main macro nutrients most often found in fertilizers and amendments. Without adequate amounts of phosphorus, your garden will fail.


Why Is Phosphorus Important?

Phosphorus is essential for plants to transport the energy they produce from light during photosynthesis throughout the plant. Phosphorus is one of the main components of plant DNA and plants generally have higher concentrations in root tips, growing shoots, and vascular tissues. Phosphorus is vital for hardy growth of the plant and its vital to healthy root development and plant cell activities. It also effects germination, and bloom production.




Identifying Phosphorus Deficiency In Plants

Phosphorus deficiency results in stunted growth similar to nitrogen deficiency. Plants remain a very poor size. Leaf color appears to change from normal to dull dark green, or bluish green, with a purplish color of the petiole (leaf stem) and veins on leaves. The older leaves, located near the bottom of the plant, will be the first to show signs of a phosphorus deficiency, gradually moving upwards to new growth. In fruit bearing plants and plants that bloom, flowers may still be produced, but any blooms will be smaller, and in less abundance. Leaves may also begin to curl downward, in extreme cases they will become necrotic and fall off.


  • Leaf and plant growth stunted
  • Dark green leaves sometimes accompanied by purplish leaves and stems
  • Hard or brittle stems
  • Stunted root growth
  • Spots may appear on leaves
  • Leaves may curl under


Examples Of Phosphorus Deficiency


How To Correct Phosphorus Deficiency

A phosphorus deficiency is compounded by cold conditions or excess iron in the grow medium. One of the first things to take a look at is pH balance, in both soil and hydroponic growing systems. A pH imbalance will block nutrient uptake through the plant’s roots. It is important to regularly check the pH, and to be sure to keep the pH within the appropriate range. As a general rule, if growing in soil, nutrients will be absorbed by your plant the best if it is at a pH level between 5.5 and 6.5. In hydroponic systems, a pH range between 5.8 and 6.2 has proven to be ideal. In this range, the nutrients present in the soil or water are soluble, and are easily taken up through the plant’s root system. When the pH level is outside of this range, even when the proper nutrients like phosphorus are present, they are not able to be absorbed by the plant. If your pH balance is at the appropriate level and you are still seeing a phosphorus deficiency, add plenty of fertilizer with high concentrations of phosphorus. A complementary approach, is to use a seaweed, kelp or fish extract spray, applied directly to the leaves, to stimulate the plant’s enzyme system and encourage phosphorus absorption in the short term, as you work long term to amend the soil or soilless growing system. Other sources of phosphorus supplementation include any products that contain worm castings, fish meal, and crab or lobster shell.



How Do I Know Which Fertilizers Or Amendments Contain Phosphorus

NPK numbers usually appear prominently on fertilizer indicating the amount of nitrogen, phosphorus, and potassium respectively that the fertilizer contains. In the example image to the right, the 12-0-0 indicates that the fertilizer is high in nitrogen (12) but contains no phosphorus (0) or potassium (0). Other sources of phosphorus are bat guano, bone meal, and barnyard manure. Compost teas can be used in conjunction with a regular feeding program of a fertilizer that contains adequate amounts of phosphorus suitable for the stage of plant growth. If you aren't sure be sure to consult the product MSDS sheet to find out what it contains or contact our helpful experts.




What To Do If You Apply Too Much Phosphorus?

First, stop adding phosphorus enriched nutrients. You should flush your plants with 3 gallons of water for every gallon of grow media to remove excess phosphorus from the soil and allow you to start again. In hydroponic systems, drain off some of the phosphorus rich water and replace with neutral fresh water in order to restore balance.


Want to learn more about nutrient deficiencies?

Read our entire nutrient deficiency series as well as learning about micro vs macronutrients.


Questions? Call 888-815-9763 or email [email protected].