Use expanded clay & clay granules correctly

What is expanded clay and how does it differ from powdered clay and plant granulate? What is the function of expanded clay in hydroponics? We reveal what different properties the three clay materials have and how you can use them correctly.

Expanded clay is the basis of hydroponics for indoor plants [Photo: Firn / Shutterstock.com]

If you deal with hydroponics and suitable substrates, the subject of expanded clay and powdered clay or plant granules quickly comes up. But what exactly is that? We will show you which material is suitable for hydroponics and how to properly create and maintain it. We will also tell you how clay flour, clay granulate or plant granulate and expanded clay differ and how the clay products can be used.

How do expanded clay, powdered clay and plant granulate differ?

Expanded clay, plant granules and powdered clay are often used as synonyms. But that is not correct. In order to properly apply the three useful materials, it is important to know their properties:

Properties and uses of expanded clay

The basic substance of expanded clay is, as the name suggests, clay. This is ground, granulated and then burned at 1200 ° C. Only finely ground limestone is added as a release agent. The organic substances in the clay burn during the firing process and the balls expand due to the released carbon dioxide. This gives the expanded clay its typical round shape and the many gas inclusions. The grain sizes 2 – 4 mm, 4 – 8 mm and 8 – 16 mm are produced. The pores are only located inside the spheres, the surface, on the other hand, is closed-pore.
For this reason, the beads can only absorb, store and release very little water and nutrients. However, capillary forces are active on the rough outer layer and carry water upwards. This effect is used in hydroponics. Smaller grain sizes have a larger surface for a given pot volume, which means that more water rises, so the root area is more humid. The opposite applies to coarse grains.

A well-known problem with expanded clay is the high salt content: Many inexpensive products have too high a sodium, fluoride and chloride content. Salt damage can occur when used in hydroponics. It is therefore important, on the one hand, to use material that is specially suitable for hydroponics. On the other hand, you should definitely rinse the balls with clean water before using them for the first time. Expanded clay for other purposes also usually has a pH value that is completely unsuitable for plants.

Expanded clay are globules of different sizes with a closed surface [Photo: ivan_kislitsin / Shutterstock.com]

Why is expanded clay so popular and versatile? The advantages of expanded clay are the purely natural origin, that it does not go moldy and that it does not collapse, change or degrade even after a long time. This makes it possible to use expanded clay both indoors in hydroponics and as an aggregate in planters or in the garden. Mixed in substrates and soils, it ensures a loose structure, good soil ventilation and counteracts compaction.

By the way: Broken expanded clay is also available. By breaking open the spheres, the porous inner workings are not only visible, but also usable: Broken expanded clay can absorb much more water than whole expanded clay.

Properties and uses of plant granules

Like expanded clay, plant granules (such as Seramis®, Lechuza Pon® or BiLa-Ton®) are made from the natural product clay. The natural clay is liquefied with water and then mixed with air, so that an extremely high pore volume of up to 80% is created. The material is then dried, then broken into granules and sieved to the desired grain size. Here, too, the clay is burned at the end, but not quite as hot as in the production of expanded clay.

This process is used to make the surface of the fragments open-pored. This gives the plant granulate a very high water absorption capacity and good nutrient storage. Incidentally, the red color of plant granules is not achieved with dyes. It is iron that was oxidized when the clay was burned.

Due to its properties – high water and nutrient storage capacity – plant granules such as Seramis® and Co. cannot be called a hydroponic substrate. Rather, the plant granulate tries to imitate the properties of a soil: water and nutrients are in the substrate, not exclusively in a water reservoir. In order to be usable for as many plants as possible, plant granules have an almost neutral pH value (6.2 to 7.5). Suitable plant granules are also available for plants with special needs in terms of nutrients and pH value: For example, particularly coarse sieves for orchids or slightly acidic granules for palms.

Plant granules can absorb a lot of water and release it to the root ball [Photo: Ralf Geithe / Shutterstock.com]

Properties and uses of clay powder

Clay flours such as bentonite are soil additives that can be used as additives in substrates but also on bed areas. Clay powder is dried and then coarsely ground clay. It enables the positive properties of clay minerals to be distributed in powder form. These positive properties include the ability to store and supply nutrients, stabilize the pH value, and store water by swelling the clay minerals. Another advantage is the formation of so-called clay-humus complexes, which ensure a more stable humus content in the soil. Clay powder is particularly suitable for improving sandy soils, as it facilitates the build-up of soil humus. So it accelerates the increase in fertility through humus-promoting cultivation.

This is what distinguishes expanded clay from plant granulate and powdered clay:

	Expanded clay	Plant granules	Clay powder
water-tank	Low	High	High
Nutrient storage	Low	High	High
use	Hydroponics, aggregate to increase aeration in potted substrates	Full-fledged indoor plant substrate, additive to increase nutrient and water storage	Additive to increase the water and nutrient storage in potting substrates and soils

Hydroponics with expanded clay in indoor plants

There are many arguments in favor of hydroponic indoor plants. Hydroponics is the right choice for anyone who is not very lucky with the cultivation of plants in soil, is often not at home for a long time or simply has fun doing something new.

What is hydroponics?

If plants are cultivated exclusively in an aqueous nutrient solution, this is called “hydroponics”. Some of it is already being used in the production of vegetable plants. The cultivation of indoor plants in a solid substrate such as expanded clay with an underlying nutrient solution is called hydroponics. This is exactly where expanded clay comes into play. The roots can hold onto the expanded clay balls and at the same time absorb water and the nutrients it contains. However, the roots do not hang in the water, otherwise they would start to rot due to the lack of oxygen. The expanded clay, on the other hand, is enclosed by a thin film of water, which also transports the nutrients contained in the nutrient solution. In this way, the plant roots get their essential nutritional elements with good ventilation.

Create a hydroponic for indoor plants with expanded clay

If you are interested in creating a hydroponic system, you have two options: You can either buy a plant that has already been planted in a hydroponic system from a specialist shop, or you can create a system yourself. To start with, it is advisable to buy a ready-made system. Anyone who has already gained some experience can transfer a plant from cultivation in soil to hydroponics. For this purpose, the root ball is carefully freed from the soil and the roots are washed out with water. Then you can put the bare roots in a special hydroponic pot. This consists of an inner pot, a waterproof outer pot and a water level indicator. You can then fill the inner pot with the expanded clay and add water to the outer pot. You can see the amount required from the water level indicator.

Tip: Even cacti (Cactaceae) and orchids (Orchidaceae) can be planted in hydroponics.

In the case of hydroponics, no soil is used [Photo: Firn / Shutterstock.com]

Properly water indoor plants in expanded clay

How much water does my plant need and when should it be watered? Watering is made very easy with hydroponics. The water level indicator indicates an area in which the indicator needle should be located. When it drops below the minimum, it's time to water. You should not water beyond the maximum mark to prevent rot. Of course, different plants have very different needs when it comes to water supply. Here it is worthwhile to use a houseplant guide to avoid giving too much or too little water.

Properly fertilize indoor plants in expanded clay

When it comes to hydroponics, we don't actually speak of water at all, but always of a nutrient solution: you have to provide the plants with the nutrients they need through the water. In contrast to earth, expanded clay does not provide any nutrients. To fill up the liquid in the planter, water is never used, but always a nutrient solution, i.e. water with fertilizer dissolved in it. Make sure to use special fertilizers for hydroponics, because only these contain the nutrients in the right proportion.

Organic and mineral fertilizers are available for hydroponics. However, organic hydroponic fertilizers have the disadvantage that the container has to be cleaned more frequently. Otherwise, at some point, foul-smelling biofilms will be deposited on the expanded clay. When using mineral fertilizers, salts can build up over time. You should therefore pay attention to the exact dosage of the fertilizer according to the instruction leaflet, as this is important for the success of the system.

The expanded clay pellets retain their shape even after a long time [Photo: Firn / Shutterstock.com]

Tip: With both types of hydroponic fertilizers, we advise you to take the plant with the inner pot out of the hydroponics and to clean the planter at least once a year. At the same time, you can rinse the plant in the inner pot under running water to remove salt or organic deposits.

Summary: hydroponics in indoor plants

• The cultivation of plants with bare roots without soil in expanded clay: an inner pot with expanded clay stands in an outer pot with nutrient solution.
• Expanded clay serves as a hold for the roots and transports a film of nutrient solution to the outside of the expanded clay balls via capillary forces.
• The amount of nutrient solution to be poured is determined by a water level indicator.
• Various houseplants can be cared for in hydroponics, but they differ in their water and nutrient requirements.

Cultivate plants in plant granules

The cultivation of plants in plant granules such as Seramis® or Lechuza-Pon® is not a hydroponic culture because a nutrient solution is not used. The plant is placed with the ball of the earth in a watertight pot that is about a third larger than the ball. Then the pot is filled with plant granules so that the roots are covered with a layer. Finally, a moisture meter is inserted into the ball of the earth.

Tip: The moisture meter can easily be confused with the hydroponic water level indicator, but it works completely differently: The moisture meter contains a special paper that changes color depending on the humidity. It shows how moist the plant granulate is. According to the manufacturer, the moisture meters should be replaced every 12 to 18 months.

Cacti can also be cultivated well in clay granules [Photo: Patty Chan / Shutterstock.com]

Pour houseplants correctly in planting granulate

As already explained above, plant granules can store water very well and thus supply the root ball with water. Depending on the plant, this enables the watering interval to be extended. A moisture meter in the ball of the earth will help you find the right time to water. If it turns red, it's time to water. The indicator then turns blue within about two hours. However, you do not have to adhere to the watering indicator extremely closely: it only changes color when the substrate starts to dry out. However, some plants do not need water until much later or much earlier. With a little practice and experience, the watering indicator can safely be left out.

Correctly fertilize indoor plants in plant granules

To fertilize the plants in plant granulate, we recommend using special fertilizers that are specially tailored to this, because normal fertilizers are often too high in doses. When applying fertilizer, always pay attention to the instructions in the package insert. The phosphate supply of the plant is particularly important when cultivating in plant granules. The plant granulate binds phosphorus, which is why a high phosphate-nitrogen ratio should be ensured. The use of pure phosphate fertilizers such as superphosphate can also prevent deficiency over a longer period of time.

Summary: Cultivate plants in clay granules

• The plant is planted with the ball of the earth in planting granulate.
• The plant granulate stores a lot of water and nutrients and releases both of them to the root ball.
• The moisture indicator shows when the substrate is dry.
• You should fertilize regularly with special fertilizers to prevent deficiencies.

Other possible uses of expanded clay and plant granulate

Expanded clay in particular can be used in a wide variety of ways. But plant granules are also useful apart from their original purpose.

Expanded clay as drainage

Expanded clay is ideal as a drainage layer in planters and flower boxes. Just put a layered layer in the jar before pouring in the soil. Such a layer protects against waterlogging and promotes the drainage of excess water, because very few plants like waterlogging. This prevents the development of root diseases and rot. One advantage of expanded clay is that it does not dissolve, collapses and does not lose its effect as a drainage material over time: it is “structurally stable”. The expanded clay can be used for a long time by simply collecting and cleaning the pellets the next time you repot.

A layer of expanded clay on the bottom of the planter protects against waterlogging [Photo: Ostranitsa Stanislav / Shutterstock.com]

Tip: If you put an old cloth on the drainage layer and only then fill in the potting soil, you prevent the soil from mixing with the expanded clay and thus improve drainage.

Expanded clay: use in the home

Expanded clay can be used to locally increase the humidity of potted plants. But why can increasing the humidity be helpful? Pests such as spider mites or thrips feel particularly comfortable in dry air in living rooms or in the greenhouse. To increase the humidity, simply add some expanded clay to the planter or saucer of the potted plant. Then you can add just enough water so that the expanded clay is not completely covered with water. If you now put the planter back into the planter or saucer, the roots will not come into direct contact with the water. This prevents rot from occurring. Due to the large surface area of the expanded clay, the water evaporates and locally increases the humidity. In addition, the higher humidity ensures that the soil dries out less quickly.

Tip: Finer expanded clay (2 – 4 mm) is useful to protect against fungus gnats: Simply cover the surface of the ball of the earth with a layer of clay balls about 2 cm thick. Sciarid gnats are less interested in dry, coarse earth surfaces. This also prevents mold and unpleasant odors in the potting soil.

Expanded clay: use in the garden

Weeds can grow quickly, especially if you cultivate small trees in planters. Expanded clay can help suppress weeds. To do this, you simply put a 3 cm thick layer of expanded clay on the surface of the earth. In the case of heavy, loamy soils outdoors, you can dig the planting hole a little larger than you would normally when replanting and mix the substrate with expanded clay for filling. This gives the soil structure and loosens it up.

Plant granules and clay powder: Use in potting soil

Plant granules such as Seramis® and BiLa-Ton® store, as well as clay powder, moisture and nutrients. They can also be used as an additive for potting soil if it should stay moist longer and be able to store more nutrients. Some plants such as the Chinese hibiscus ( Hibiscus rosa sinensis ) or myrtle ( Myrtus communis ), sword fern ( Nephrolepis ) and nest fern ( Asplenium nidus ) need an always moist soil. You benefit greatly if the potting soil used is mixed with some planting granulate.

Exotic indoor plants go very well with a modern method of plant cultivation. We present some particularly exciting examples to you in our special article.