Should You Use Soil as a Hydroponic Growing Medium? Here’s Why Not
Hydroponics uses different growing mediums to act as support for plant growth. Just like you, I was once curious if I can also use soil since it is readily available. I tried it a few times. Some worked, some did not. Today, I will discuss my findings in this article.
Generally, soil like fine sand, clay and silt are not recommended as hydroponics growing medium due to their fine particles. On the other hand, medium and coarse sand can be used as a hydroponic medium due to their high porosity and big particle sizes.
Clay, silt, sand, are the three known soil types. But which one is recommended for hydroponic use? This will be revealed as we move forward.
Can You Use Outdoor Soil as Hydroponic Growing Medium?
In general, it is not recommended to use garden soil as a hydroponic growing medium. Considering that hydroponics is a system that is highly operating with water, it is not practical to use soil as a growing medium. It might be messy, and worse, it may contaminate your hydroponic nutrient solution.
Hydroponics Growing Mediums: Definition and Purpose
Hydroponic growing mediums hold the plant upright during its growing period. Since their main purpose is to provide anchorage, most of the hydroponic growing mediums are not enriched with essential minerals.
Maybe you are wondering: Are all soil types not recommended to use as a hydroponic growing medium?
The quick answer is no.
There is a soil type that can be used as a hydroponic growing medium. We will discuss this in the next section.
Learn more about growing mediums in our article what is a growing medium in plants.
The Best Soil Type as Growing Medium
In general, coarse sand is the only good soil type that can be used as a hydroponic medium. This is because it would not easily dissociate into the hydroponic nutrient solution.
Among the four soil classifications (clay, silt, sand), sand is the most appropriate soil to use as a hydroponic growing medium.
Sand is further divided into different types—fine, medium, and coarse. Beach sand, for instance, is sieved according to its size and classified further.
Fine sand particle size ranges from 0.42 mm to 0.074 mm. Medium sand particle size ranges from 0.42 mm to 2.0 mm. Meanwhile, coarse sand particle size ranges from 2.0 mm to 4.76 mm.
Pros and Cons of Using Sandy Soil as Hydroponic Growing Medium
Sand is an easy hydroponic growing medium to work with. However, there is a huge tendency that it might mix with the nutrient solution, which can affect sanitation.
Among the advantages of using sandy soil as hydroponic growing medium are:
- It is an accessible choice.
- It is easy to work with since it works just like soil.
- Coarse sand can tightly hold the plant in a deep water culture (DWC) hydroponic system.
On the other hand, it also has disadvantages such as:
- Risk of hydroponic nutrient water contamination.
- There is an additional step before using sand, that is, sterilization.
- It can scatter down the nutrient water reservoir, which is messy.
Now that you already know that sand is the most qualified soil type as a hydroponic growing medium, let us dig deeper on what makes it a better choice among other soil types.
3 Desirable Properties for Hydroponic Growing Mediums
Some desirable properties for hydroponic growing mediums are 1) high water-holding capacity, 2) high porosity, and 3) big particle size.
1. High Water-Holding Capacity
Generally, a good hydroponic growing medium must be able to hold water for a long time. With this property, plants have longer access to water. Soil types such as silt, clay, and loam have higher water-holding capacity compared to sand.
The growing medium’s ability to hold water is useful for active hydroponic systems such as the nutrient film technique (NFT). In this system, one can (or usually) set a timer for water circulation.
For most cases, the pump is active for 30 minutes, and off for another 30 minutes. This is the reason why, a good growing medium, must be able to hold water—to give the plants water access even if the air pump is off. This is also helpful in cases of long power interruptions.
At this point, you may be wondering. Why is sand recommended for hydroponic use even though it has a low water-holding capacity?
Well, water-holding capacity is not the only factor to be considered. Later, we will discuss a very important aspect that can affect your plant’s growth and health.
2. High Porosity
A good growing medium allows gas exchange. Sand is a more porous soil type compared to silt, clay.
Did you know that your plants also breathe? Science calls it respiration. Just like us, plants also need space to breathe, or else, they will also die.
Porosity is a soil property that accounts for the spaces between each soil particle. In general, the smaller the soil particle, the lower its porosity. Whereas, bigger soil particles have higher porosity.
Air is essential for the roots of any hydroponic plant. With this, using a growing medium with high porosity gives your plant enough space to manage its air and water resources.
3. Big Particle Size
An ideal hydroponic growing medium has a large particle size (<2mm). Among the four soil types, sand has the biggest particle size.
Clay has a soil particle lower than 0.002 mm; silt particles range from 0.002 to 0.05 mm; while the size of sand particles range from 0.05 to 2 mm. Furthermore, loam is a combination of the three soil types.
So, why do I recommend sand for hydroponic use considering this?
Imagine this. Which will dissolve faster in water, refined sugar or rock salt? If you answered refined sugar, you are right!
The same principle goes with using soil in hydroponics. If you use a growing medium with a small particle size, the tendency is that it will just mix with the hydroponic water. And, I am telling you, we do not want that!
Remember that soil is also rich in microorganisms. So if they mix with your hydroponic water, there could be contamination!
3 Soil Types That Are Not Recommended as Growing Mediums
Clay has the smallest particle size (<0.002 mm) among the established soil types. When exposed to water, it easily transforms into mud. This is the reason why it is not recommended for hydroponic use.
Silt has the second smallest particle size (0.002-0.05 mm) among the soil types. It resembles dust that each particle is difficult to see with the naked eye. With this, it also tends to dissolve in water, making it not viable as a hydroponic growing medium.
Loam is a combination of the three basic soil types, clay, silt, and sand. Although it contains sand, the fact that it still has clay and silt—that may dissolve in water—does not make it fit as a hydroponic growing medium.
12 Conventional Hydroponic Growing Mediums
1. Coco Coir
Coco coir is made from the outer husk of the coconut. It is an organic hydroponic growing medium that allows enough air space for optimal root structure.
2. Clay Pebbles
Clay pebbles are circular bits of expanded clay. They can retain water and also have high porosity. This growing medium is best for deep water culture (DWC) hydroponics.
Gravel rocks are basically free since they can be found anywhere. They are durable and reusable, as long as they are sterilized. They are ideal for deep water culture (DWC) systems and ebb and flow systems.
Growstones are made from recycled glass fragments which are crushed, melted, and solidified again. They are commonly mixed with calcium carbonates for safer use. They can retain water longer than rice hulls, peat, and perlite.
5. Peat Moss
Peat moss is a compact growing medium known for keeping nutrients and water accessible for the plant. The best way to use peat moss is to mix it with perlite or vermiculite to balance out its compact air space.
Perlite is a mineral, commonly used as a mix for too compact hydroponic growing mediums. Due to its high porosity, it allows the plant roots to get proper water and oxygen. This growing medium is most compatible with wick systems.
Like perlite, pumice is a mined mineral. It is a porous rock that can be found in volcanoes. Its good water-holding capacity and large particle size make it an appropriate hydroponic growing medium.
8. Rice Hull
Rice hulls are considered an agricultural leftover. It has a relevant silicon content that is also beneficial for the plants in hydroponics. One of its advantages is its good water resistance. Boiling rice hull before using it is a good practice to ensure that it is clean and disease-free.
Rockwool has a sponge-like texture and is made from granite and limestone rocks. This is a popular option for hydroponic growers as it holds moisture effectively, maintains oxygen efficiently, and decreases chances of overwatering.
Vermiculite is a low-cost choice for hydroponic gardeners. It is a mineral which is lightweight and sterile. Aside from these, it also has good water retention properties and a pH close to neutral. With this, using vermiculite will not easily adjust the pH of the hydroponic solution.
11. Wood Chips
Using wood chips is another cheap option for hydroponic gardeners. They retain water efficiently while providing enough porous space. However, before using this, one should sterilize the wood chips first through boiling or dipping in food-grade hydrogen peroxide.
Zeolite is a unique growing medium that can adjust the pH of the hydroponic solution, as it is available in acidic and alkaline forms. When using this, one must be consistent in checking the pH of the nutrient water.
Explore how to choose which growing medium is best for you in our article on hydroponic growing mediums.
What is the cheapest growing medium for hydroponics?
Sand is the cheapest hydroponic growing medium. This growing medium is available in most garden stores and can even be acquired for free if you are near bodies of water. Sand is a good hydroponic growing medium to start with since it is easy to use and is pH neutral.
Can I use soil nutrients for hydroponics?
Yes, nutrients that are used for soil gardening can also be used in hydroponics. Nitrogen, phosphorus, and potassium dry fertilizer forms need only to be dissolved in water to be used in hydroponics. Learn more about this in our article on hydroponic nutrients.
What can I use as an alternative for clay pebbles in hydroponics?
Gravel is another popular option that can offer the same characteristics as clay pebbles. They both are fit for deep water culture (DWC) hydroponic systems. Using gravel is also cheap since you can find rocks everywhere!
Summary of Soil as Hydroponic Growing Medium
Generally, soil is not recommended for use as a hydroponic growing medium because it tends to dissolve in water. The soil types with this tendency are clay, silt, loam, and fine sand.
The key criteria in qualifying for hydroponic growing medium use are high water-holding capacity, high porosity, and big particle size. Considering this, only medium to coarse sand can be used as hydroponic growing medium since they have bigger particle sizes.
Apart from soil, a hydroponic gardener has an array of choices for hydroponic growing mediums. Coco coir, clay pebbles, gravel, growstones, peat moss, perlite, pumice, rice hull, rockwool, vermiculite, wood chips, and zeolite are among the conventional options.
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- “Improvement of Hydroponic Culture Medium by Adding Calcium-Zeolite” by Fukuyama, T in Acta Horticulturae
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