Which substrate should you use in your greenhouse?

Substrates have developed significantly over the last decade. The major shift has been toward organic substrates, which follow the market trend towards the desire for more organic produce and sustainable agriculture practices. It’s true that organic substrates might be more sustainable and water efficient. They might even lead to a higher profit margin because consumers have a natural preference for products grown on organic bases.

This doesn’t mean you should toss your inorganic substrates into the trash. Read on to learn about the differences between organic and inorganic substrates and how to decide what is right for your greenhouse.

Substrate types and advantages

At a high level, there are inorganic and organic substrates. There are also multiple types in each category. Here are the most popular substrate types.

There are advantages and disadvantages for each type of substrate. For example, inorganic substrates are generally easier to manage, while organic substrates are more unpredictable. Organic matter will compost itself throughout the season, requiring that you pay very close attention to ensure it maintains a healthy pH and nutrient balance for your plants. There are several factors to consider when choosing between organic and inorganic substrates. Here are a few:

• Stability: Organic matter can fluctuate, so it might be less stable than inorganic matter, which will not change shape or form.
• Nutrient and pH management: Organic matter might absorb nutrients itself and fluctuate in acidity levels, so it requires closer management.
• Root environment: Inorganic substrates tend to provide a drier root environment than organic. Wetter environments might lead to higher nutrient absorption in some plants.
• Fertilizer intake: Roots absorb nutrients differently in different environments. Generally, organic allows for better uptake; however, some chemicals might bind with organic matter and not reach crop roots.
• Buffer capacity: Organic substrates are more forgiving and resilient. If there is a disruption in water or nutrient supply, crops grown in organic substrates recover more easily. Their higher buffer capacity can make them a bit harder to steer precisely, but it also provides an added layer of safety. One exception to this is barque, which is an organic substrate, but has a lower buffer capacity for water and   nutrients.
• Climate: Your climate will also influence your choice. Organic substrates, with their bigger buffer capacity, are more popular in warmer and sunny climates. If you live in a colder or darker climate, inorganic might be more ideal.

Let’s organize the advantages and disadvantages of the substrate types in a table.

There are advantages and disadvantages to both types, so this doesn’t provide a clear picture of which to use. Let’s look at the plants.

What substrates do crops prefer?

Different crops might thrive in different environments. For example, orchids are known for thriving in barque because they prefer a drier root environment. Here are some of the top greenhouse crops and what they might prefer:

• Peppers: Peppers are notorious for their root problems. Pepper plant roots need humidity and hate too dry environments. This means they might thrive more in a humid environment, which is easier to achieve with organic substrates.
• Cucumbers: Cucumbers grow very fast and thrive in a more stable environment, meaning inorganic substrates might be preferable for them.
• Tomatoes: Tomatoes often thrive in wetter and oxygen-rich environments.

This isn’t to say that you cannot grow these crops in any substrate. You can choose to grow cucumbers in organic substrates and tomatoes and peppers in inorganic substrates. If you do, you will just need to develop an optimal irrigation strategy.

Substrates are like sponges with different capacities. You can manage the wetness and dryness level of any substrate by choosing the length and interval of watering throughout the day. You might choose longer shots of water with larger intervals or shorter shots with shorter intervals, depending on how much you want to flush out the substrate environment.

The substrate microbiome

We haven’t yet discussed one of the most important aspects of a substrate: the microbiome. Every substrate is an ecosystem with its own populations of bacteria, enzymes, fungi, and other microorganisms. It is a living environment that hugely affects plant health, nutrient uptake, disease resistance, and overall greenhouse productivity. The benefits of a stable microbiome cannot be underestimated.

Microorganisms in the microbiome can help break down nutrients in the soil to make them easier to absorb by plant roots. They might also stimulate growth and inhibit pathogens from spreading through the substrate.

Microbiomes are naturally part of organic materials. Organic substrates already have a thriving microbiome before you even bring them into your greenhouse. Inorganic substrates do not have a natural microbiome. A microbiome might emerge once you introduce them to your greenhouse, and add crops, fertilizers, and water. The microbiome in inorganic substrates is much simpler and more sensitive to changes in fertilizer and water content. If you use an inorganic substrate, you might need to nurture your microbiome if you want your plants to benefit.

Substrate volume per plant

Once you’ve chosen your substrate type, the next question is: how much do you need? This depends on your greenhouse size and the size and number of plants you want to grow. Drier substrates might also require a bigger volume per plant to provide enough water content per plant.

Seven to ten liters per square meter seems to be an optimal amount of substrates in a greenhouse. This allows for adequate plant spacing while making the most of your greenhouse area.

You might choose to use a smaller volume, but this is a risky choice. Less than seven liters per square meter means plant roots won’t have as much space to grow. It will require more irrigation to maintain your environment; however, shorter irrigation intervals may not give plants enough time to absorb nutrients between water shots.

If you choose to try to do more with less, you will need to closely monitor your plants and substrates to ensure they are getting what they need.

Recirculating water with substrates

Recirculating water through substrates is a great way to minimize pollution, waste, and your water bill. You can reuse water that drains from your substrate by recycling it through your irrigation system. However, if you do this, there are a few things to remember:

1. Your plants chose not to absorb the water that drains from your substrate. That means the water probably doesn’t include the nutrients your plants need, and it probably has an abundance of nutrients your plants do not need. You might need to rebalance the nutrient content in the water before recycling it.
2. Drainage may contain pathogens. If there are any diseases, fungi, or pathogens in your substrate, the water drainage is likely contaminated. Before you reuse it and spread the pathogens throughout your greenhouse, you need to disinfect the water.

There are several methods to disinfect water, depending on which substrate type you use. You might use UV technology, ozone, microfiltration, or heating. UV technology works best with inorganic substrates because they are less dense and lighter in color. Heating might be good for organic; however, too much heat might affect the microbiome in the substrate. Because of the complex microbiome in organic substrates, you might need to invest more in disinfecting the drainage before reusing.

Bottom line: Which substrate to use?

Every substrate has its own advantages and possibilities when it comes to your greenhouse. You should use whatever will help you optimize your crops. If you’ve been using the same substrate type for ages for no other reason than it is what you’re used to, it might be a good time to reexamine if it is your best choice. You might try something new and be pleasantly surprised by the results.