When and how to use fertilisers

Tips and tricks
Content
1. what is fertiliser? 2. Types of fertiliser 3. Signs that a plant needs fertiliser 4. When to use fertiliser 5. How to use fertiliser

Plants have two basic needs: light and water. However, the process that allows them to synthesise their food through photosynthesis actually requires the intervention of a series of micronutrients, which we take for granted because they are usually found naturally in the substrate. But the soil does not spontaneously generate these nutrients, which are eventually depleted at some point, which is why the use of fertiliser is essential. In short, fertilisers are responsible for providing a series of nutrients to the soil so that they can be absorbed by the plants and contribute to their development, growth and splendour.

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Before you go into a frenzy of fertilising all your plants to make them lush and end up killing them through over-fertilisation, it is imperative that you understand when and how to use fertiliser. Don't worry, it's less complex than you think and you'll learn all the tricks to feed your plants at the right time. With these tips you can be sure that your plants will become the envy of the neighbourhood.

what is fertiliser?

According to the FAO, the Food and Agriculture Organization of the United Nations, a fertiliser is any material that contains at least 5% of the basic nutrients needed by plants. These nutrients are Nitrogen (N), Phosphorus ℗ and Potassium (K). This material returns to the soil all the nutrients that have been absorbed by the plants, but also supports the development, growth and formation of fruits and flowers.

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You need to know that fertiliser is not a quick cure for all your plants' problems. It will not bring your plants back to life or make them grow faster or faster. Fertilisers only have a function and usefulness depending on the specific need of each plant. If your plant is on the verge of death due to a cause other than lack of nutrients (lack of water or too much sun, for example), fertiliser will not help it at all and may accelerate its path to the other plane.

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Also, not all fertilisers are suitable for all types of plants. Some species have very specific needs, while others prefer poor soils. Learning these requirements and respecting them is the key to plentiful plants and abundant harvests.

Types of fertiliser

Simple fertilisers

We could say that there are two types of fertilisers: simple and multi-nutrient. Simple fertilisers are those that contain only one of the main nutrients (nitrogen, phosphorus or potassium). These types of fertilisers are usually used in agriculture or on a large scale, as it is essential to know the crop and type of plant very well in order to find the perfect balance of the nutrients to be used. Among the simple fertilisers we find

  • Nitrogenous fertilisers

As the name suggests, they provide nitrogen to the plant. This nutrient stimulates the growth of the plant, making it greener and more abundant. Some of these are urea, ammonia and ammonium nitrate among others. Urea is the fertiliser with the highest nitrogen content (46%) and is also quite cheap. Ammonium sulphate offers 21% nitrogen, while calcium ammonium nitrate has just over 27% nitrogen.

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  • Phosphates

They are responsible for providing phosphorus to plants. This micronutrient is linked to flowering, fruit production and root development. A phosphorus fertiliser can be single superphosphate (16 to 20% phosphorus) and triple superphosphate (46%).

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  • Potassium

This nutrient protects the plant from extreme temperatures, improves the fertility of the substrate and is responsible for the transport of nutrients in the plant. An example of these fertilisers is potassium chloride, which contains 60% potassium.

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Multi-nutrient fertilisers

Multi-nutrient fertilisers are fertilisers that combine several primary nutrients. They are also known as NPK or NP fertilisers, depending on the type of nutrient included. This is the type of fertiliser that we usually buy to treat plants at home, because they offer a balance of nutrients from the factory and are very easy to use. It is important to learn how to choose this type of fertiliser, as this will allow you to purchase one that is suitable for each type of plant.

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  • How to choose a multi-nutrient fertiliser

To choose the fertiliser you must read the label. You will find 3 numbers in the format x-x-x. These numbers indicate the percentage of nitrogen, phosphorus and potassium respectively. This means that in a 10kg bag of 20-15-30 fertiliser you will have 2kg of nitrogen, 1.5kg of phosphorus and 3kg of potassium. The rest of the ingredients will correspond to inert elements that act as a vehicle for the fertiliser.

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Usually the manufacturer will include on the label the directions for use of the fertiliser, as well as the plants that will do well with that ratio of nutrients; however, you can check our plant guides to see what ratio the ideal fertilisers should have for that particular plant species. As a general rule of thumb you can look for a fertiliser that has twice as much phosphorus as nitrogen and potassium, for example, 15-30-15 or 12-24-12.

Having this knowledge can even help you save money, as you will be able to choose the perfect option even if it has not been labelled by the brand. Let me explain: sometimes manufacturers put higher prices on a product with a nice label, but which is exactly the same as one with a simpler presentation. If you don't get the right ratio for the type of plant you have, you can apply a trick: read the label of the specialised fertiliser and take note of the numbers, which you will use to look for a generic one. For example, a special fertiliser for orchids has an NPK ratio of 30-10-10, so you can look for any fertiliser with these ratios for your orchids, even if the label doesn't specify it.

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Signs that a plant needs fertiliser

  • The first step is to rule out other problems: watering, lighting and humidity.
  • The plant has small, yellow leaves. The lower leaves turn yellow and the newer leaves turn pale green. This happens when there is too little nitrogen, which is what gives them their green colour.
  • The plant does not grow and has no new leaves. If it also has yellow leaves, we may be dealing with a potassium deficiency.
  • Older leaves are a very dark but dull green colour which then changes to a reddish or purplish hue, then dries out. This indicates phosphorus deficiency.
  • If the leaves look yellowish in the centre but the edges and veins are still green, it is because they lack iron.
  • When the edges of the leaves look yellowish and dry, it indicates a lack of potassium. It may resemble the symptoms of magnesium deficiency, but this affects new leaves more.
  • Older leaves look yellow between the veins and edges, except for a green portion at the base. This is a sign of magnesium deficiency.
  • Check the lower, older shoots. If they have any discolouration or problems, this indicates a lack of macro-elements (NPK), but if the leaves with problems are the newer ones, which are located at the top, then it is the micro-elements that are lacking.
  • If you have used fertilisers and these problems persist, it is because the high pH of the soil must be interfering with the absorption of the elements.

When to use fertiliser

Houseplants in pots require a little more attention when it comes to fertiliser than those planted in soil. This is because the roots are confined in a small space, they cannot spread out as far as possible to reach nutrients, but have a limited amount in the substrate that is contained in the pot. On the other hand, there are substrate mixes that actually have little soil, have more perlite and other materials that facilitate drainage but have no nutrients, moreover, many of these commercial substrates are sterilised, so they do not have the beneficial bacteria that can help the plant synthesise nutrients better.

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Fertilisers should be used at the right time. Fertilising a plant at the wrong time can burn its roots or affect its growth. While each species has its own preferences, the general rule of thumb is to do it during spring and/or summer, as this is the growing season for plants. Fertilising in winter is unnecessary and may disturb the balance of the plant, as this is the resting season.

Indoor plants can be fertilised once a month with liquid fertiliser during spring, summer and early autumn. Outdoor plants have their own particular needs, especially those that bear fruit and are planted directly in the ground. Try to apply the fertiliser during the coolest time of the day, so that the roots can better absorb the nutrients.

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How to use fertiliser

The use of fertilisers will depend on the type. With this there is only one valid tip: read the manufacturer's label and follow the instructions. Every brand is different, concentrations change. Not all liquid fertilisers are used in the same way, so the ideal is always to read the label and follow the instructions. Don't improvise because you can kill your plants. That said, it is important to know that fertilisers come in 3 different presentations:

  • Liquid fertiliser: They are mixed with the irrigation water in the watering can, following the manufacturer's instructions. Their effect is very fast, so they have a short effect and must be reapplied frequently; however because they are easy to use and so fast, they are very popular.

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  • Granular fertiliser: They are spread on the substrate and act with the irrigation water. They dissolve easily and should be applied a little less frequently than liquid fertilisers.

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  • Time-release fertilisers: These come in the form of solid sticks or tablets that are buried close to the roots of the plants. They dissolve very slowly with each watering, providing nutrients at a constant rate.

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In Spring you can start when the days start to get longer and temperatures start to rise, as these environmental changes bring them out of their dormant state. Try not to apply the fertiliser at the concentration indicated by the manufacturer, it is better to dilute it by half for the first 3 doses. This is because the plants are just waking up from winter, starting their growth cycle and do not require as much fertiliser.

In summer you can start fertilising on a regular basis. The frequency of application will depend on the type of fertiliser you are using. For example, liquid fertilisers should be applied at least every two weeks; while granular fertilisers are used once a month or once every two months. Time-release fertilisers last much longer, so depending on the manufacturer's instructions, they should be applied every 3 to 4 months.

In autumn, before temperatures drop, you should decrease the amount and frequency of fertiliser. You can use half the amount indicated by the manufacturer 3 or 4 times before the onset of winter. In winter you should not fertilise, unless you live in a place with mild temperatures or where there are not many temperature changes; however, at this time you should use half the fertiliser and reduce the frequency by half as well. If you live in the tropics, you should fertilise your plants as if it were always summer.

The keys to good fertilisation are to apply less fertiliser more frequently and to dilute the fertiliser very well, and never apply it to plants that are excessively dry, rather water beforehand. Also, avoid applying to plants that are under stress or have just been transplanted. It is better to apply a little less of the recommended fertiliser and adjust the doses from there, especially when it comes to liquid or granular fertilisers, as it is easy to overdo it and burn the plant.
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About the author
Ame Rodríguez

Dedicated to creating an army of cacti, succulents, poodles and cats to help me conquer the world. In the little free time I have left, I play, write and dance.

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