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Late autumn - i.e. mid to late October - is the right time to apply mineral fertilizers such as lime, magnesium, potassium or phosphate fertilizers if necessary. As a rule, however, this is only necessary if a soil test shows that the potassium or magnesium content or pH values are too low. In this case, slow-acting fertilizers such as potash magnesia (patent potash) and carbonated algae or dolomite lime are recommended for a lasting improvement.

Mineral fertilizers can be given to the garden until the end of October

Which mineral fertilizers are available and how they work

Mineral fertilizers are sometimes under general suspicion of being "artificial fertilizers" or even "bad chemistry". This is not correct, because most nutrients such as potassium or magnesium occur in nature mainly or only in mineral form, i.e. as a component of rocks. This is how most of the raw materials for mineral fertilizers are obtained in mining. If they are only crushed (e.g. ground), lime and potash fertilizers in particular only develop their effect very slowly, but all the more sustainably. For this reason, such mineral fertilizers should already be applied in autumn so that they can develop their full effect in the next season.


The fertilizer form of phosphorus is phosphate (P2O5). This nutrient is very important for flower and fruit formation as well as root growth and energy metabolism. A deficiency not only affects the development of fruit (and thus the harvest!): The plants often remain small, seem strangely rigid and the leaves turn dark to dirty green, sometimes even reddish. An excess of phosphorus, on the other hand, hinders the absorption of other nutrients such as nitrogen, iron and zinc and can severely pollute water bodies if they are washed out.


Potassium (K) is fertilized as a potash salt. It plays a very important role in water balance and mass transport, strengthens plant tissue and increases resistance to cold and pathogens. With potassium deficiency, the leaf tips and edges lighten and then turn brown, starting on the older leaves. In addition, the leaves often curl up and the plants appear limp and wilted. An excess of potassium in the soil, on the other hand, hinders the absorption of magnesium and calcium.


Magnesium (Mg) is an important building block of leafy greens, promotes the build-up of protein and other metabolic processes. In the event of a deficiency, the older leaves first turn yellow, later brownish; the leaf veins, on the other hand, remain green. An excess of magnesium in the soil is very rare. However, when it occurs, it can interfere with calcium absorption.


Calcium (Ca), the main component of lime, is important for the water balance and various metabolic processes in the plant. A direct calcium deficiency (where young leaves turn yellow and shoot tips snap off) is quite rare. However, many gardeners are familiar with blossom end rot on tomatoes and peppers, in which the fruit initially has a watery spot that later turns black-brown to gray at the top. The same can happen with zucchini and pumpkins. The main trigger for this is a poor supply of calcium - usually not due to a deficiency in the soil, but because an uneven water supply or excessive fertilization with other nutrients (especially nitrogen) impedes the transport of calcium to the fruit. In addition, calcium, especially in the form of lime, is important for soil pH and soil structure.


Although plants only need small amounts of trace nutrients such as boron, iron, copper, manganese, molybdenum and zinc, they are just as essential as the main nutrients.

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