Text and photo: Christine Wiemann

It is not always an easy job to use fertiliser for the plants in the greenhouse. We tend to love our plants so much that we end up giving the plants too much fertiliser at the wrong time. Many of you may have pre-germinated tomatoes, chillis, cucumbers and flowers. Now, all these plants are ready to be transplanted to the greenhouse, but what do you do regarding fertiliser?

In this blog you will get an easy guide for how to use fertiliser in your greenhouse. This blog is based on the fertilisers that you will find at plant nurseries and garden centres.

Fertiliser theory (summarised)

In general, all plants need 17 forms of nutrients to live, grow and thrive. Carbon, oxygen and hydrogen the plant gets from carbon dioxide from air and water. The nutrients in a plant can be divided into macronutrients and micronutrients. Some nutrients are absorbed through the leaves and others from the roots. Each plant species has specific needs to be healthy and thrive. Therefore, you can find fertilisers for a specific plant. Also, plants only absorb nutrients when it is in the form of ions. For the nutrients to be in ion form, depends on the pH-value for water and soil.

Did you know?: Fertiliser for plants is just the same as what vitamins are for people. The nutrients are the same.

When to fertilise

In general, you only fertilise during the summer season and not in winter. Should you fertilise in winter, your plants will not benefit from this not even those inside. Did you know that most soils already have fertiliser added? This fertiliser must be used before you add anymore.

Types of fertiliser

Overall, there are two types of fertiliser. There are organic fertiliser and inorganic fertiliser. Both come in liquid form and in granules or tablets. Both types are ideal for greenhouse plants. Liquid fertiliser is great for capillary boxes and drip irrigations. Fertiliser tablets are used for plant beds and growbags.

Inorganic fertiliser

Inorganic fertiliser is also called chemical fertiliser and are different nutrients that are mixed together. The benefit of inorganic fertiliser is that they can be concentrated, which means that you have fertiliser for a long time. Always follow the recommended dosage.

Organic fertiliser

Organic fertiliser is made of organic matter and can for example be lupin, field beans and vinasse which is a by-product from the production of sugar made from sugar beets. Moreover, organic fertiliser can be made from seaweed, wool and chicken faeces. This type of fertiliser can even be made from insects. Organic fertiliser is usually not as concentrated as inorganic fertiliser, and you may be surprised of how much organic fertiliser that you need before it has an effect.

What is N-P-K?

Often on fertiliser packaging ‘N-P-K’ will be written followed by some numbers. The three letters are an abbreviation of three macronutrients. The numbers indicate the mixture proportion of nutrients in the fertiliser.

N = Nitrogen

P = Phosphorus

K = Potassium

Plant needs

• If the plant needs fertiliser, it will look pale, have light spots and be thin.
• If the plant has been overfertilised it will have dark curly leaves and thick.

When to fertilise

• Germinating seeds and small cotyledons need no fertiliser
• Fertilise two to three weeks after the young plants have been transplanted
• Tomatoes and chillis that have been transplanted in growbags need fertiliser two weeks after transplanting them
• Adult plants in the greenhouse need fertiliser a couple of times a week
• In case of heat waves, try not to fertilise too much and no more than once a week
• From October you do not have to fertilise anymore in the greenhouse nor in the garden or inside.

Tip 1

To make the most of your fertiliser, you must irrigate with tap water or a mixture of rainwater and tap water. If you only use rainwater, it will be too poor in nutritional value.

Tip 2

Prevent to overfertilise by shading your plants with shading blinds when there is a lot of sun and heat waves. Shading prevents accumulation of fertiliser in growbags and also in the tomato plant. Especially tomato plants benefit from this. By providing shades for your tomato plants, you also prevent blossom rot. Find some of the options for shading here.