Chilli Grow Guide - Plant Nutrients

A brief understanding of which nutrients are necessary to the healthy growth of chilli plants can be very helpful if you want to grow healthy plants that crop well. Most novice growers come to this section of the guide when there is a problem developing but averting the deficiency or overload of nutrients in the first place is much more effective than trying to fix a problem.

The key with providing your plants with the right nutrients is most obviously in the feeding. A general rule if thumb is to feed seedlings with a seaweed feed or general food plant fertiliser at a quarter strength on a weekly basis. Increase to a half strength feed once plants are potted into their 9cm pots and full strength once they are in their final positions.

However, be aware of what nutrients already exist in your growing medium. For example coir is not only completely devoid of nutrients, it also locks away some nutrients, such as calcium. For that reason, a mix of coir and compost is a good idea. If you grow directly into coir, a weekly half strength foliar feed would be advisable.

Nutrient Mobility


When deficient in a nutrient, a plant will always try to move the element from older growth to sustain new growth. However, whilst some elements are quite mobile, others cannot be unlocked from old growth at all. Deficiency of mobile nutrients will always show up in older leaves whereas deficiency of non-mobile nutrients will be displayed in new growth. Understanding nutrient mobility is key in diagnosing nutrient deficiency.

Common mobile nutrients (i.e. deficiency will show in older growth); nitrogen, magnesium, phosphorus
Nutrients with intermediate mobility (i.e. deficiency will show in old and middle growth); potassium, sulphur, zinc, copper, manganese.
Non-mobile nutrients (i.e. deficiency will show in new growth); iron, calcium, boron.
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  • micronutrients

    Manganese
    Manganese is important for the actions of enzymes within plants, amongst other things. Deficiency is characterised by yellow spots in the young growth, which may become necrotic and/or form elongated holes. Foliar feed with a half strength solution of manganese sulphate, available at most garden centres. Garden manure added to pots also works well, but is much slower acting.

    Boron
    Boron is needed in tiny amounts but plays a role in seed production, cell division and keeping stems healthy. Boron deficiency displays at the growing tips first.; sometimes these die off completely. Newer growths can curl up or become spotted like a strawberry. Stems become brittle, sometimes hollow. Secondary roots are swollen and short. Check for signs of potassium deficiency as plants need potassium to absorb boron, so the two can often go hand in hand. To re-introduce boron to your plants, foliar spray with a solution of ¼ teaspoon Borax, or Boric Acid, into 1 litre water. Manure and bonemeal are good slower ways of introducing boron.


    Copper
    Copper is essential for healthy stems and new growths. Where deficient, plant growth will be stunted. Growing tips may die back completely and/or new growth wilts and may become chlorotic. Stems loose their strength and become bendy. Copper deficiency often goes hand in hand with nitrogen deficiency so check for signs of this. Foliar feed with a half strength solution of Copper Sulphate.


    Zinc

    Zinc is needed in small quantities but plays a minor role in a large number of processes in the plant. It is important in the production of chlorophyll, the absorption of water and the growth of leaf and stem. Plants that are zinc deficient will display yellow or grey patches between the veins of new growth. End leaves may form a rosette or appear narrow and distorted. Treat with a half strength foliar spray of zinc oxide or chelated zinc or some people even suggest burying a galvanized nail in the soil.

  • sulphur

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    Sulphur plays an important role in root growth and supply of chlorophyll. Middle growth is affected first, becoming yellow fairly evenly across the whole leaf, including the veins. Often the chlorosis starts at the stem-side of the leaf, travelling outwards. If unchecked, the whole plant can become affected.

    Excessive heat hampers the uptake of sulphur so check the temperature of your greenhouse and ventilate as necessary. Foliar spray with Epsom Salts solution (Magnesium Sulphate) at a rate of 2 teaspoons per litre.

  • Magnesium

    Magnesium helps support healthy leaf tissue, in particular leaf veins. It aids photosynthesis and is needed in quite large quantities. Deficiency is characterised by chlorosis of lower leaves, from the leaf tips moving inwards, sometimes just around the leaf margins. The affected areas can become pale yellow with leaf veins remaining green. Plant growth is stunted.

    A foliar spraying of Magnesium Sulphate (Epsom Salts) at a solution of 2 teaspoons per litre is recommended. This is the fastest way to get magnesium into plants and also avoids the build up of salts that soil feeding can produce. Check your plant feed, magnesium lockout can be caused by excessive potassium, in which case Epsom Salts application may have little or no effect.

  • phosphorus

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    A major element, Phosphorus is associated with rigorous growth of the whole plant, in particular the root growth and reproduction. Stunted, weak growth is evident when Phosphorus is deficient. Older leaves become dark green, flushing to purple and plants will be unlikely to flower. Problems can extend across the whole plant if deficiency is severe.

    Chilli plants find it difficult to take on phosphorus in low temperatures, similarly if the soil ph is too low. Excessive iron can also hamper the uptake of phosphorus, although this is unlikely unless you have been applying it. Use any fertiliser with a high ratio of phosphorus, like tomato feed or slower-acting bonemeal.

  • potassium

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    Potassium plays a major role in plant health, helping to create sturdy stems, aiding photosynthesis and aiding disease resistance. Potassium deficiency is characterised by scorched leaf tips and edges in the middle of the plant, that can turn brown and die. Older leaves may display a mottled pattern and stems become brittle.

    Potassium deficiency can be caused by excessive perspiration of plants, so check humidity in your greenhouse and mist with water if too dry. Potassium can also be displaced by excessive calcium or sodium so check your feed and soil. Poor quality coir, for example, can add salt to compost. A high potassium, or potash, feed such a a general tomato feed, should rebalance this deficiency.

  • nitrogen

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    Of all the elements, nitrogen plays the biggest role in plant growth. It is responsible for the production of chlorophyll and amino acids. Nitrogen deficiency can have a serious impact on the growth and general health of chilli plants. Nitrogen is the most mobile of elements so the lower leaves become yellow first, the deficiency working its way up the plant.

    In severe cases leaves can form brown patches or flush with purple and the plant may drop these leaves. Plant growth becomes completely stunted and it can take a long time for plants to recover. Affected leaves will not recover.

    It is unlikely that nitrogen deficiency is caused by an actual absence of nitrogen, more likely the plant is unable to access it. The most common cause cause is overwatering. A film of water around roots makes it difficult to take on nitrogen. Do not try to add extra fertiliser as excessive nitrogen will have the effect of locking out other elements, such as calcium.

    Let soil dry out and adjust watering. Foliar spray with a solution of Epsom Salts (Magnesium Sulphate) at a rate of 2 teaspoons per litre of water. This will aid the production of chlorophyll and speed up recovery.

  • Calcium

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    Calcium is important for cell division and its deficiency is characterised by distorted new growth and weakened stems. New leaves have a twisted, bubbled appearance and edges can become necrotic. Old growth is not affected as calcium is the least mobile of elements. If calcium is deficient during fruiting, plants may develop Blossom End Rot, with sunken, necrotic patches developing at the blossom end of pods.

    An application of CalMag (Calcium Magnesium feed) can help but it would be unusual for there not to be sufficient Calcium in garden compost, it is more commonly a problem of nutrient lockout. Excessive nitrogen and/or potassium hinders the plants ability to take on calcium, so check you are not feeding excessively.

    Calcium deficiency can also be the result of low transpiration. If your pots are continually dry, this could be contributing to the problem and you should alter your watering regime.

  • Iron

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    Iron plays an important function in the production of chlorophyll and enzymes. Deficiency causes stunted growth in chilli plants and is characterized by interveinal chlorosis, or the yellowing of leaves whilst leaf veins remain green. Affects the new growth. Chlorotic patches appear first but these can grow to cover the whole leaf. In extreme cases, leaf tissue becomes completely white.

    There is usually sufficient iron in compost or soil for your plants needs. Iron deficiency is more commonly caused by a lock out of nutrients With iron, this could be high soil ph, high temperatures or excessive phosphorus in feed. Check and adjust as necessary.

    Iron deficiency can be treated with chelated iron, commonly available in garden centres. Foliar spray at half the recommended dose. You will see the green start to flush back into leaves within a few days and, apart from any completely white patches, leaf tissue will completely recover. Be careful not to overfeed with iron as this can in turn can lock out phosphorus.

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