Phosphorus (P): Importance for plants, addressing nutrient deficiencies and proper fertilisation
Phosphorus deficiency causes leaves to die, slows growth and inhibits flowering in plants. Learn to recognise the first symptoms and take action in time, before phosphorus deficiency affects your harvest.
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Phosphorus is an important nutrient for plants, which they need for photosynthesis, respiration, the transfer and storage of energy, sugar metabolism and cell division. Along with nitrogen, potassium, calcium, magnesium and sulphur, phosphorus is classified as a macronutrient, which plants consume in the greatest quantities. Phosphorus is particularly essential for plants during flowering, when its deficiency can cause a significant reduction in yields. Phosphorus supports the growth of roots and stems, affects the plant’s overall vitality and is also needed during the first days after germination.
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Phosphorus deficiency in plants
Phosphorus deficiency first appears on older leaves because this element is mobile in plants. The leaves become limp and first change colour to dark green, followed later by purple and blue shades. The tips of the leaves curl downwards into a claw-like shape, and eventually the leaves begin to dry out from the edges. In the event of phosphorus deficiency, the leaf petioles or stems may also turn purple. However, some varieties have this colouration of the petioles and stems encoded in their genes, so their turning purple is not an indication of phosphorus deficiency.
- dark green, claw-shaped leaves
- leaves change colour and dry out from the edges
- twisted or deformed leaves
- slow growth and flower development
- purple stems and leaf petioles
Phosphorus deficiency is most commonly encountered during the transition to flowering and throughout the flowering period. During this time, plants’ phosphorus consumption rises sharply. For this reason, many growers use PK boosters during flowering, which are nutrient supplements containing phosphorus and potassium. When used correctly, they can significantly increase plant yields. However, they must be used with care. Excess phosphorus is toxic to plants and manifests itself through dark colouration, chlorosis and slowed leaf development. The flowers of plants over-fertilised with phosphorus may have an unpleasant “metallic” aftertaste.
Phosphorus deficiency is most common in plants grown hydroponically and in coco or inert substrates. It is usually caused by an incorrect pH of the substrate or nutrient solution, a low solution EC or an unsuitable type of fertiliser for the given growth stage. When grown outdoors, plants usually have sufficient phosphorus in the soil. However, the uptake of this element is affected by temperature, and roots may have difficulty absorbing it in cold weather. Nutrient problems may likewise be caused by soil pH that is too high or too low.
How to provide plants with phosphorus
The quickest way to provide plants with phosphorus is to use water-soluble fertilisers containing mineral phosphate salts. The PK boosters mentioned above, as well as flower fertilisers, contain large amounts of readily available phosphorus.
Remember that the choice of fertiliser should correspond to your growing style and substrate. Mineral fertilisers should only be used for indoor growing, in pots and in hydroponic systems. In open soil, such as garden beds, they should only be used when necessary because mineral salts damage soil microorganisms and disrupt natural processes. For outdoor and organic growing, bat guano is an excellent organic option for supplementing phosphorus in the soil. Calcareous dolomite acts somewhat more slowly, but has the advantage of increasing the pH of acidic soils when necessary.
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Remember that nutrient deficiencies can have many causes, and resolving them correctly requires at least a basic understanding of the principles of plant nutrition. You can find further advice and guides for outdoor and indoor growers on the higarden blog.