What Size Gravel is Best for Aquaponics?

In the same way that hydroponics is a soilless means of growing, so is aquaponics. One of the key differences being the way the grow beds are constructed. While hydroponics systems tend to grow in pots in various systems, an aquaponic system most often uses flood and drain grow beds, which are full of growing media. Gravel being the most common.

What size gravel is best for aquaponics? There are many sizes available, and in theory, any size gravel can be used. Most hydroponic gardeners prefer pea gravel, which is between 1/8 of an inch up to 3/8 of an inch in size. Although, the smaller 1/8 sized pea gravel being the gravel size of choice. 

Considerations of Aquaponic Growing Media

When you first build any aquaponics system, there needs to be a few considerations when choosing the ideal growing media. Here you can find the things you need to think about, and you can see why pea gravel is the ideal media to use.

Inert: Any aquaponic grow media needs to be pH neutral. Similarly, as a hydroponic system, the pH level in an aquaponic system needs to be controlled.

Fish and plants have their preferred range of pH range, and ideally, you need to aim for a range of pH 7. Many media can change the pH level, although pea gravel isn’t one of them.

All you need to do is thoroughly cycle your system before adding fish or plants to clean away any dust.

Grow Bed Depth: Any grow bed in aquaponics should be a minimum of 12-inches deep for most crops.

Weight: Any grow media in use in a hydroponic system should be too heavy to handle. It should be light to medium weight, so you can easily dig your holes when you plant your seedlings. On the flip side, if they are too light, they can float in the grow bed and won’t offer support.

Easy to Work With: Grow media can comprise any size or shape, although sharp edges are best avoided. If you use sharp-edged stones, these can be hard on the hands and damage any grow beds that are not constructed from solid materials.

Non-Decomposing: No aquaponic grow media should break down because it can work its way through your system and clog your water pump and reduce dissolved oxygen in the water for your fish.

Pea Gravel for Aquaponics

Pea gravel fits all the criteria and then some. The smaller pea gravel has a large surface area that bacteria can thrive on. With this, the nitrogen cycle is complete and, in turn, delivers robust plant growth.

From the fish waste, the broken-down compounds create ammonia, which passes around your system. Once it flows through your smaller sized pea gravel, it is converted into nitrites by two bacteria strains (Nitrosomonas bacteria).

From here, these nitrites are consumed by the second strain of bacteria (Nitrobacter bacteria) and converted to nitrates. Your crops then use the nitrates as fertilizer.

As there is more surface area on the gravel, a larger number of nitrates are being produced than if larger gravel was being used.

Pea Gravel with Limestone

Generally, your pea gravel won’t contain anything to change your aquaponic system’s pH level radically. However, if there are any traces of limestone, the results could be different.

Before filling your grow beds, you can carry out a test to check whether your bags of small pea gravel.

Take a handful of gravel, place it in a container, and then cover it with some white vinegar. You can see the vinegar fizzles if there are traces of limestone.

You need to avoid limestone as this will keep your pH above 8, which can harm your plants and your fish.

You can place some gravel in distilled water and check the pH after a couple of days, although it is better not to purchase any gravel you think may be suspect.

One thing to note is that the name pea gravel isn’t a type of gravel, it is because of its size. One dealer can sell something different to another.

Grow Bed Depth and Pea Gravel

One thing any gardener needs to understand when using pea gravel is the depth of the grow bed. The recommended depth is 12 inches, and for a good reason. When building a system, you may be tempted to go for a shallower bed.

Even while pea gravel is the cheapest media you can purchase, the amount you need could be considerable. However, there are reasons why you should opt for a grow bed of this depth, if possible.

Here you can see why avoiding the shallower bed sizes can be beneficial, and it is advisable to scale your system to support the larger bed and the increase in the size of the fish tank you will require.

You will face limitations if you decide to go for a shallow grow bed filled with pea gravel.

Eco-System & Dead Zones

In deep aquaponics grow beds, you naturally create a layered eco-system. One significant benefit of this is that you will prevent any ‘dead zones’ dotted around your grow beds.

Besides this, as you have a thriving environment packed full of beneficial bacteria, worms, and lots of space for roots to grow. You don’t have to clean out your grow bed.

These bacteria and worm communities, along with your plants, do all the cleaning for you. Anything less than 12 inches, and you won’t be able to reap these benefits of a stable eco-system.

Plant Limitations

If you are growing nothing but crops similar to lettuce, you may get away with a grow bed shallower than 12-inches.

However, if you grow indeterminate tomatoes, cucumbers, and corn among the many other crops that can benefit from aquaponics, these shallow beds can’t support them. You won’t receive the same base nutrients and beneficial bacteria for these longer living crops.

One thing not to forget is your aquaponic system can be highly configurable, and you can mix grow beds and use raft systems for short rooting crops and larger grow beds or Dutch becket systems for deep-rooted crops.

Three Zones of Pea Gravel Grow Bed

Zone 1 – Surface Zone

The top 2-inches serve two purposes. It allows for light penetration and acts as the dry zone to limit evaporation. The second reason for this dry zone in your pea gravel is to prevent collar rot and stop any algae forming as the nutrient-rich solution faces light exposure. Also, the infection from powdery mildew is minimized.

Zone 2 – Root Zone

It is in the second or middle layer where the plant activity occurs. Making up this layer is around 6 to 8 inches in depth. As you drain after flooding your grow bed, water drains away completely and presents highly efficient oxygen delivery to the roots, microbes in the soil, worms, and beneficial bacteria.

When you are in the flood cycle, incoming water will distribute all the fish waste particles containing nutrients through the entire grow zone. Worms are highly active in this middle zone, where they break down solid matter to release minerals and add to the underlying worm tea. On each successive flood and drain, this beneficial tea is spread around all your crops.

Zone 3 – Solids & Mineralization

Once you reach the bottom 2-inches, you find all the fish waste solids and worm castings are gathered together. By the time it reaches here, it will have been reduced by around 60%.

In each of your flood and drain cycles, this zone is kept fresh from the effective delivery of oxygen-rich water.

Conclusion

Once you see the benefits of using 1/8-inch pea gravel, you can understand why it is among the most popular media.

It is highly affordable, and any gardener is advised to lean toward the deeper grow beds when possible. Besides this, there is nothing you need to do with pea gravel, and it is one of the easiest materials you can work with.

There is nothing but benefits you can get from using this smaller sized pea gravel than if you used a larger gravel, or an alternative growing media.