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So far, the recommendation was always for the care of citrus plants: low-lime irrigation water, acid soil and lots of iron fertilizer. Heinz-Dieter Molitor from the research institute Geisenheim has meanwhile proven with his scientific investigations that this procedure is fundamentally wrong.
The researcher took a closer look at the nursing plants of a wintering service and found that of around 50 citrus trees, only one third had green leaves. The remaining specimens showed the well-known yellow discolorations (chloroses), which are due to a lack of nutrients. The compositions and pH values of the soils and also their salt contents were so different that no connection could be established. However, after examining the leaves it was clear: The main reason for leaf discoloration in citrus plants is calcium deficiency!
The chlorosis is clearly evident in the yellow discoloration of the leaves
The requirement of the plants for calcium is so high that it can be covered neither by commercial liquid fertilizer nor by a direct Kalkung. For this reason, citrus plants should not be sprayed with lime-free rainwater, as is often the case, but with hard tap water (calcium content of at least 100 mg / l). This corresponds to at least 15 degrees German hardness or the former hardness range 3. The values should be obtained from the local water supplier. The nitrogen requirement of citrus plants is higher than previously thought, the phosphorus consumption, however, significantly lower.
The right fertilizer
The potted plants grow under favorable site conditions (for example in the conservatory) around the year and in such cases occasionally need fertilizer even in winter. In cool wintering (unheated room, bright garage) eliminates the fertilizer, watered is only sparingly. The first fertilizer should be given at the beginning of the sprouting in spring, either once or twice a week with a liquid fertilizer, or with a slow-release fertilizer.
In citrus care, the formula is: a lot of nitrogen, little phosphorus, a lot of calcium
For the optimal citrus fertilizer Molitor calls the following composition of nutrients (based on about one liter of fertilizer): 10 grams of nitrogen (N), 1 gram of phosphate (P205), 8 grams of potassium (K2O), 1 gram of magnesium (MgO) and 7 grams of calcium (CaO). You can cover the calcium needs of your citrus plant with calcium nitrate (available in the country), which is dissolved in water. You can combine this with a nitrogen-rich and low-phosphate liquid fertilizer with trace elements (such as green plant fertilizer).
Pour citrus properly
If the leaves drop in quantities in winter now, in the rarest cases light deficiency, fertilizer deficiency or waterlogging are to blame. Most problems stem from the fact that there are either too large gaps between the waterings and thus too large fluctuations between days-long wetness and drought. Or that there is not enough water flowing with each watering - or both. It is correct, never let the soil dry out completely and always wet to the bottom of the pot, so not just superficially wet. During the growing season from March / April to October this means daily watering in good weather! In winter, every two to three days, the soil moisture is controlled and pours, if necessary, not according to a fixed pattern such as "every Friday."