Yellowing orchid leaves are related to fertilisation methods

Fertiliser yellow

When too much fertiliser is applied, especially too much nitrogen fertiliser combined with a lack of phosphorus and potassium fertiliser, it easily causes the leaves to turn yellow, showing as new leaves that are thick and fleshy with mostly uneven surfaces, while old leaves gradually turn yellow and drop off; in severe cases the leaf tips dry out and even the entire leaf becomes scorched yellow. When this happens, fertiliser should be applied immediately, the amount of watering increased so that the fertiliser flows out through the drainage holes, or the pot should be turned out immediately, part of the old soil removed, the root ball washed with water, replaced with some new growing medium, and the plant repotted into the pot.

Starvation yellow

When fertiliser is insufficient, especially when nitrogen fertiliser has not been applied for a long time or the potting soil has not been changed, the soil lacks nitrogen nutrition, causing the branches and leaves to become thin and weak, with thin, yellow leaves. The pot should be turned out promptly, the plant removed, and it should be repotted into a pot one size larger filled with fresh, loose, fertile growing medium, and planted firmly again. After watering thoroughly, place it in semi-shade to recover. Once it resumes growth, apply a dilute liquid fertiliser or compound chemical fertiliser.

Alkaline yellow

When acid-loving flowers are planted in the north, such as camellia, orchid, banana shrub, gardenia and osmanthus, because the soil in most northern regions contains more salt and alkali, after the above flowers and trees have been grown for a period of time they gradually lose their green colour and turn yellow due to a lack of soluble iron that the flowers can absorb. At this time, a 0.2% ferrous sulfate solution can be watered on at intervals, while alternately watering on a 0.1% potassium dihydrogen phosphate solution, so that "alkaline yellow" is cured; at the same time, potassium dihydrogen phosphate also contains phosphorus and potassium, which helps flower bud differentiation and improves flower quality.

Acid yellow

The southern regions of our country have red loam soil, and the soil is acidic or slightly acidic, so magnesium in the soil is easily lost; due to magnesium deficiency the leaves often show yellowing first between the veins of old leaves. Calcium magnesium phosphate fertiliser can be applied, or a 0.1% to 0.3% magnesium sulfate solution sprayed on. For the improvement of acidic soil, attention should be paid to applying more decomposed organic fertiliser and applying lime; applying a small amount of lime can not only neutralise soil acidity but also promote the decomposition of organic fertiliser.