Orchid‘s leaves turn yellow is mostly related to the fertilizer and volume, magnesium sulfate can release mg trace element loses in acid soil of South China, and ferrous sulfate can release the soluble ferrous deficiency in the North China.
Too much fertilizer especially N fertilizer and lack of P and K fertilizer will make yellow leaves, and then it will appear the thick new leaves, concave-convex blade surface, and leaf opex will dry rot even the whole leaf is sallow. Under this situation, fertilizing and increasing watering is needed to make fertilizer outflow from dales or fall and remove parts of soil and change new ones.
Insufficient fertilizer especially lack of N fertilizer will lead nutrient deficiency of soil, and thin branches and leaves, when this situation occurs, it needs to change another larger basin and put some new loosen and fertile soil. Put it in the semi-shade place after watering completely. Finally fertilize thin liquid fertilizer or compound fertilizer.
The soil is red loam in the South China, and is acid or particle acid, it’s easy to lose Mg element, which makes leaves yellow, at this moment, 0.1-0.3% magnesium sulfate solution or calcium magnesium phosphate fertilizer is needed. We also can improve the acid soil with thoroughly decomposed organic fertilizer and lime.
There is too much salinization in the soil of North China, so some plants like tea plant, orchid, osmanthus fragrans lour and so on will lack of soluble Fe, and turn yellow. For the time being, 0.2% ferrous sulfate and 0.1% MKP is needed to cure yellow leaves diseases.