A customised fertilizing program keeps plants healthy and resistant to pathogens and pests and also cuts down on the use of chemical control agents. Proper fertilizing also ensures a good soil structure.

There is a choice of fertilizing agents:

  • »  Compost and manure. These are organic fertilizing agents. As described previously, they are also effective in improving the soil.
  • »  Organic supplements that provide a complementary balance to organic fertilizing agents.
  • »  Compound mineral fertilizers

The type of fertilizing agent chosen depends on the kind of planting and the time at which the agent can be applied.

- Annual flowering (for displays and cut flower production)
Flower bulbs have already stored enough nutrients in their bulbs for the first flowering season, so providing them with fertilizer for a one-year flowering is unnecessary.

- Perennialising
Flower bulbs and other plants being used for perennialising will need supplementary fertilizing (a slow release fertilizer). After all, they will be extracting a lot more nutrients from the soil. In landscaping practice, existing borders are usually fertilized by applying compound mineral fertilizers.

Compound mineral fertilizers contain concentrated nutrients that plants need such as nitrogen, potassium, phosphorus, magnesium and calcium. These fertilisers contain exactly the right ratios and concentrations of nutrients. The quantities of nutrients in manure and compost can vary. Compound mineral fertilisers, however, do nothing to improve the structure of the soil because they contain no organic material.

Because compound mineral fertilisers usually dissolve easily in water, their ingredients are readily accessible to plants. This is why they should be used during the growing season. If applied outside of the growing season, they can be washed out of the soil before plants can benefit from them, which will also pollute groundwater and surface water.

Packages containing fertilizers usually provide the composition of the ingredients in the form of three numbers such as 10 + 5 + 3. In this case, these would mean 10 percent nitrogen, 5 percent phosphorus and 3 percent potassium. If a fourth number were given, it would stand for the percentage of magnesium. Over-fertilising can result in an excessively rapid growth of plants that will be weak and susceptible to diseases and pests. The use of quick-release fertiliser granules can “burn” plants (symptoms: yellowed leaves and wilting).

If the soil structure has to be improved, too, it will be essential to use organic fertilizers, possibly supplemented by specific supplementary mineral fertilizers to increase the level of a certain nutrient.

- Naturalising
When flower bulbs and other plants increase their numbers entirely on their own, this is evidence that they are being provided with a habitat that simulates their natural habitat. In these situations, nature is in balance: the type of soil, its structure, and its drainage perfectly accommodate the needs of the plants. In such a balanced situation, the addition of a fertilising agent is not usually needed.

Sometimes, however, plants display certain symptoms (often visible in their leaves) that indicate a deficiency of a certain nutrient. This is when it would be advisable to apply organic supplements. Because these are organic, they are more suitable for the natural environment in which these plants are located.

Organic supplements correct specific deficiencies in plant nutrition supplied by organic fertilisers; examples of organic supplements are a phosphorus fertilizer and vinasse (a waste product from the food processing industry that is very high in potassium). Finally, there are fertilisers that contain calcium such as marl (coral-algae calcium) that regulate the pH of the soil. If this type of supplement is used for flower bulbs, it should be applied immediately after flowering.

- In the grass
To be completely assured of a beautiful floral display year after year, the soil should be enriched once a year with fertilizers. Fertilizing in the spring is not advised since it would be better if the grass would not grow too quickly just at the time the foliage produced by the flower bulbs is still green. It would thus be better to fertilise in the autumn when both the grass and the bulbs can benefit from a single application.

When applying a compound mineral fertilizer, a total quantity of 2 kg / 4.4 pound (12-10-18/100 m² or 120 square yards) is usually distributed over three applications. Instead of a compound mineral fertilizer, organic fertilizers such as a mixture of blood meal, bone meal, compost or dried cow manure in granulated form can be applied. The best time to scatter this fertilizer is immediately before or during a rain shower; this way, the nutrients are dissolved straightaway.

- In pots and containers
An important characteristic of potting soil is that it retains water for a long time. This is important for keeping flower bulbs from drying out during their growth and flowering. If the bulbs are being planted for just a single flowering, fresh potting soil will last one growing season.

When using flower bulbs as tub plants, it is important to enrich the potting soil with fertilizer. This fertilizer is usually applied in the form of tablets that contain all the necessary nutrients for all your houseplants as well as your terrace, balcony and garden plants. The tablets are easy to use: simply poke a hole in the soil and slip the tablet in. The plant will then be supplied with nutrients for at least four weeks. Because the tablet dissolves slowly, the nutrients are released gradually for absorption by the root system. This eliminates the risk of “root burn”. Another option is to replace the potting soil with fresh potting soil every year before putting the pots and containers out.