Fertilization part 1


A difficult topic for many growers, the difference between life or death for many plants. The following advices are general guidelines and plant behavior is always subject to local circumstances. Growers should always follow up on local legislation and consult a professional agronomist when in doubt. Syngenta does not accept any liability in any business damage resulting from the advice.

Author: Ben Geijtenbeek – Senior Crop Technical Specialist – Syngenta Flowers
Ben Geijtenbeek has forty years of experience as a technical expert in floriculture. He is specialized in educating growers in various cultures with new techniques and technologies. This way he revives the potential of Syngenta’s genetics, from the corporate philosophy “Bringing plant potential to life”.

Food for plants

A plant exists for more than 90% of water. The rest is organic and inorganic substance, partly made by the plant itself and partly taken up from the environment. At school we all learn that plants make organic matter, through photosynthesis. Sugars, carbohydrates and proteins are created from H2O and CO2 in the green parts of plants. The assimilation process of making organic matter is influenced by temperature and light. Plant roots can take up all other required materials from the soil to build and feed the plant. That’s why there must be enough stock of all required inorganic materials in the substrate. It’s an important part of the growing process were the grower can (and must) influence the growth and development of his crop.

We divide the nutrient elements into two groups. Major elements are required in big quantities by plants. Minor elements are needed in small amounts. But these minor elements are strictly indispensable. You can compare this with petrol and oil in your car. The distance you can drive is depending on the availbale amount of petrol, but you always need oil. Even if you only start the engine. So, the minor elements are as important as the major elements. 

The major elements are:Importance if you look to the ratio inside of plants:
N Nitrogen 1.000.000
K Potassium 250.000
Ca Calcium 125.000
P Phosphorus 60.000
Mg Magnesium 60.000
S Sulfur 30.000
​The minor elements are:Importance if you look to the ratio inside of plants:
Fe Iron 2000
B Borium2000
Mn Manganese1000
Zn Zinc300
Cu Copper 100
Mo  Molybdenum1

All these elements play a different role in the whole process, but you need all of them. The question for the grower is how much he must give, when, and how. This is comparable a mother feeding her baby. The amount and type of food depends on the age and the size of the baby. The first days, babies only drink milk. And when they grow bigger they need more food, like fruit, potatoes, vegetables, bread, milk, meat and so on. Realize that the breast milk contains all the elements that a baby needs during its first days, like Calcium, Iron, Proteins, Vitamins etc. Knowing this, there are two questions to answer. First: what is the ratio between the elements? Second: which amount should I give for a good growth? When we translated these questions to plants, a few other questions come up, namely:  

  • What is the optimum quantity per element that you are looking for?
  • What is already available in the soil?
  • Which elements do you have available in the water you use?
If you have an answer on all these questions, it is possible to make the optimum mixture of fertilizers, which will make your plants happy. And only a happy plant will grow as you want it to grow. 
The question of fertilization is as old as mankind, and comparable with all living organisms. Just looking at the plants gives you already a good idea about the controllable/uncontrollable aspects of fertilization. The first thing you could do is to think about this and to train yourself in assessing your crop. Like babies, who are unable to talk, have effective possibilities to make clear that they are hungry. Your plants will also tell you when they have a problem with a lack or excess of fertilizer.

How much?

In general, the bigger the plant, the more fertilizer it needs (in absolute quantities). The fresh substrate in which the young plants are transplanted contains a reasonable amount of fertilizer for making new roots. From the moment of transplanting and watering, the amount of available fertilizer is decreasing. Therefore, you should always put fertilizer in the water to keep the nutrition level at least the same as it was before planting. The first time after transplanting you could apply plain water to make the roots connect better with the substrate. From that moment on, apply fertilizer with around 0.8 to 1.0 EC. Liquid fertilizer is preferred, to maintain the stock of food in the soil. But also, to avoid big changes in EC level in the pot. Plants do not like mayor changes in nutrition.

Depending on the development of the crop, the EC should be increased every week with small steps. Start with around 1.0 EC and increase with 0.2 - 0.5 EC, step by step. For instance, 1.0, 1.2, 1.6, 2.0, 2.5. Inspect your crop regularly. Pay attention to the color of the youngest leaves. Also, the color of the oldest leaves can give an indication if the fertilization is correct. Last but not least, the development and the color of the roots indicate clearly whether the plant is satisfied. The best way is to administrate your fertilization scheme, inspect your crop development, and make a soil analysis several times to check the balance and stock of fertilizer. Soil analysis can improve especially crops like Cyclamen, Primula acaulis, Pansy or Gerbera, because these have a long crop time.

With this experience, based on your specific climate and the water quality, real growth improvements can be achieved. A good and helpful tool for a grower is an EC meter. When used right, this meter gives you a clear indication about the EC changes in the pot. This is helpful to decide the next drip EC level.


Give fertilizer at the moment of watering the plants. The amount of food a plant needs is strongly connected with the developmental stage. The amount of water you need to give, is highly depending on the evaporation. The amount of water a plant uses for evaporation increases with more sun and higher temperature. Water plays a crucial role for cooling the leaves, which avoids sunburn. Try to keep the EC in the pot at the same level, regardless the amount of water you give. For this reason, the EC level in the water should be changed just opposite to the amount of water you have to give. Calculate more in gram fertilizer per m2 per week. For example: at the moment you apply twice the amount of water, you should lower the EC theoretically with 50%. But because of draining out, or plant growth, in practice decreasing the drip EC with 30-40 % is sufficient, to keep the EC level in the substrate more or less equal.
Fertilization 1


A plant can only absorb fertilizers in a soluable form, ionized in water. If you would apply fertilizer as a dry substance mixed in the soil, it would take time to dissolve in the pot moisture, and the distribution through the pot is not uniform. Simply because you water your plants almost daily in your greenhouses, the best way is to combine watering with fertilization. Another possibility is the use of slow release fertilisers.


The next step is the question of which fertilizer. In general, for indoor horticulture there are more possibilities.
  • Ready for use mixtures (NPK)
  • Single element fertilizers
  • Leaf fertilizers
  • Slow release fertilizers
All of these are useful, and all of these have advantages and disadvantages.

Ready for use mixtures

  • Very good soluble
  • Available in many different ratio’s
  • Easy to use
  • Relative high price per kg
  • Too low amount of minor elements
They usually contain only Nitrogen, Phosphate and Potassium. Sometimes they also contain a little Calcium, Magnesium and Sulfate. Especially for long terms crops as Cyclamen or Pansy, you must add the other elements as well for a good plant development. With the water quality information, a reasonable balance of elements can be made. However, a mix of several brands are needed, many times supplemented with Calcium nitrate and depending on the water quality, acids as well.

Single element fertilizers

This group is rather good soluble too, and with a good schedule almost each ratio can be made. It gives a very good possibility to steer the growth per element, and the available water quality can be taken in account rather easily. A good way to bring the total fertilization situation in the pot in a good balance.
I advise this system for the more technical equipped grower, who goes for top quality. Especially when you grow several crops together, it gives you a bit more flexibility in the recipe. And it comes closer to the real plant needs. I advise this system especially for the long-term crops. Say, more than 10 weeks of culture, to stay better in line with plant need and seasonal differences. 

Leaf fertilizers

To make myself rather clear, I always say that this group is no fertilizer, but just flower paint. This is not true of course, but I would not advice this system as a standard fertilization method. One can make a very good-looking plant, but after a short time the color will turn yellow again. Because it doesn’t influence the root system and has almost no impact on a good balance of food in the pot. However, it is very useful at moments when you have problems with the root system, or during times where the plant need strong attention. For instance, with unrooted cuttings, or mother-stock. Or in case you have little to no option to water.

Slow release fertilizers

This group is useful for outdoor growing plants such as Perennials. And they can be used indoors if you want to add value for your end consumer. The advantage is that in all conditions, the plant has some stock of the major elements available. The disadvantage is that the availability of fertilizers is depending on the temperature. This can create difficulties to control EC level at that moment. We use mainly the varieties with a low start, or a high end, which means that most of the fertilizers become available after sales at the consumer. This is a big contributor to customer satisfaction, because the garden performance increases a lot without influence of the gardener. In combination with other fertilizers, this can work very well.

Working of slow release fertilizer

Water goes in
Water goes out
Water goes in via osmosis Water goes out again with fertilizer


Below the impact from the temperature on the release speed of the fertilizer. For instance, the blue line: it is a variety with 8-9 months activity at 21°C. But at 30°C it works only 4 months, and with 5°C around 15 months. The activity differs from climate to climate. So be aware that when your substrate temperature in a hot area is high, you have three options to control it.
  • This is the choice for a mix from shorter and longer activity time, for instance 50% 12-14 months and 50% 16-18 months.
  • Second option to change completely to a longer-term variety.
  • Third option is to avoid over-feeding by lowering the amount per m3.
To keep control on the EC level in the substrate with the use of slow release fertilizer, in combination with water soluble fertilizer, the weekly use of your own EC meter is a must.
Relation release speed

Relation release speed with temperature
Wrong fertilized 1
Good fertilized 1
Wrong fertilized 2
Good fertilized 2
Wrong fertilizedGood fertilized 
Average: 1.4 (26 votes)


Ben Geijtenbeek
Syngenta Flowers