Raw organic substrates, where Rhododendrons are usually cultivated, are very poor on nutrients. It is necessary to enrich them with nitrogen since it can comes out easily. Composted manure is considered very suitable natural fertilizer. Dose of 10 kg on 1m3 is enough for several successive years. It needs to be fetched into the soil or into a hole before planting takes place. Of classic mineral fertilizers, Cererit is preferable. Fertilizers containing calcium carbonate are extremely unsuitable. From purely nitrogenous fertilizers, ammonia sulphate can be used. Rhododendrony demand also sufficient phosphorus and potassium. Of common fertilizers, potassic sulphate is suitable as well as potassium salt or superphosphate. Nitrogenous fertilizers shall be used only in the spring, with May the latest. Later fertilization will cause growing numbers of green sprouts that are late to mature before winter, freezing out. If we want to support abundant inflorescence, we can use irrigation or fine potassium sulphate spreading.
In connection with Rhododendron fertilization, we have to mention the need of very good irrigation; it is very important in the spring time so the plants can well-unroll the flower buds; then also in June and July, when the plants release new sprouts, and also in the autumn, just before the frost. Thanks to the substrate which has problems taking up water after drying up, most Rhododendrons do not freeze over during winter, instead they dry up. Rainwater is ideal for irrigation. On the contrary, calciferous water, or very cold water from a well in summer, are not suitable.