The best hydroponic medium you have never heard of

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One of the most important choices in a soilless crop is the medium. Ideally, the media in a hydroponic crop should provide no nutrition but just act as support material for the plant. However, common media choices, such as coco coir and peat moss, are far from inert and their usage requires special modifications to the nutrient solutions in order to account for their specific chemical properties. In this post, I am going to talk about a great hydroponic medium choice that is fairly common in South American countries but rarely used in the United States or Canada.

Rice hulls, a key component of my favorite medium for soilless culture

Issues with existing media

The most commonly used hydroponic media types in the US are perlite, peat moss, coco coir, and rockwool. Peat moss tends to have higher than desirable water retention and acidifies strongly through time. For this reason, it is usually amended with perlite – to increase aeration – and with dolomite/limestone in order to buffer the constant increase in pH within the root zone. To maximize its potential, you need to account for these amendments and the natural evolution of peat moss through time in your nutrient solution or you will tend to have calcium, magnesium, and nitrogen uptake issues. All of which are commonly observed by peat moss growers.

Coco coir has other problems. It contains large amounts of chloride, sodium and potassium. It also decomposes through time and, in doing so, exposes cation exchange sites that strongly bind elements like calcium, magnesium and manganese. For this reason, you often need to either pretreat the coir with calcium containing solutions or adjust your nutrient solution chemistry to account for the evolution of the potassium release and calcium capture through the crop cycle. The concentrations and ratios of heavy metals also need to be changed to account for the affinity of the cation exchange sites for these ions.

Rockwool has better chemical and physical stability but the environmental impact of its production is high (1). It is also hard to reuse and its physical properties are hard to tune since it is hard to mix with other media effectively. Perlite, another rocky medium, is easy to reuse and has low environmental impact, but it dries back too quickly, which increases the need for energy for irrigation and dramatically increases the amount of waste generated in open (drain-to-waste) hydroponic systems.

Rice hulls, the first component of a better medium

Over the past 40 years, rice hull – also known as rice husk – has become a medium of choice in many countries due to its wide availability as an agricultural waste product. It is made primarily of silica structures supported by organic material, decomposes very slowly through time, and has very benign chemical properties. Rice hulls will not change pH through time, will slowly release bio-available silicon, and can be reused several times before they degrade. However, they usually contain insects and some rice, reason why sterilization of the media with hot water is usually required in order to avoid pest propagation and seedling death due to seed fermentation.

Another issue of rice hulls is their incredibly weak moisture retention. Rice husks are even worse than perlite at retaining water, reason why rice husks are commonly used as an amendment to increase aeration. A hydroponic crop using only rice husks as a medium is possible, provided that the crop is constantly irrigated to compensate for the very fast dry back period of the medium. This constant irrigation is achieved through drip systems.

Washed river sand, the perfect compliment

Given that rice hull is primarily made of silica and has excessively fast dry back, it would be ideally paired with a medium with similar chemical properties but opposite physical properties. River sand, which has exactly opposite physical properties and is also made primarily of silica, perfectly fits the bill. River sand has a very slow dry back. It is therefore hard to use on its own in hydroponics due to its tendency to cause waterlogging. However, when used in combination with rice husks, a medium with exceedingly tunable physical properties and very benign chemical properties appears.

Buy SPS River Sand 2kg Online at Low Prices in India - Amazon.in
River sand is chemically inert and provides a perfect compliment to rice hulls poor water retention properties

To prepare this media, mix 50% rice hulls by volume with 50% river sand. Rice hulls can be purchased for a very low cost, a 20 USD bag will be enough to prepare 400L of the medium. River sand is even cheaper and can be bought at around 50 USD per ton retail but can be bought wholesale at much lower prices. The density of river sand is around 1587 kg/m3, meaning that it will take around 317 kg to get 200L of sand. This means that the cost per 400L of final medium will be around 16 USD, taking the total cost of 400L of medium to 46 USD. This can be more cost effective than either peat moss, perlite, rockwool, or coco coir. Especially if you take into account that the media can be reused across several crop cycles.

Treating the medium before use

This medium needs to be treated before use, as rice hulls can contain some amount of rice that can be detrimental to seedlings. To treat it, water it with tap or RO water 3 days before use. This will ferment any of the remaining rice and the increase in temperature caused by this process will help get rid of insects and any pathogens present within the mix. Note that rice hulls are often parboiled, which means they have already been heated in boiling water, which will reduce the issue of pests.

Once this treatment is complete, you are ready to use the medium. You can also adjust the percentage of rice hulls and river sand in order to fit the particular dry back conditions you desire. More river sand will make the medium dry back slower, while more rice hulls will make the media dry back faster. This is similar to what happens when you mix perlite and coco or peat moss, with the advantage that river sand and rice hulls are much more chemically inert than these commonly used media types.

Conclusion

While not common in the US, mixes of rice hulls and river sand have been successfully used in hydroponic settings during the past 50 years in a wide variety of countries, especially South American ones. I have personally used them in both small and commercial-scale projects to grow from leafy greens to large flowering plants, with amazing results. This medium is chemically inert, very easy to tune, and has a low price point.

Had you heard of a mix of rice hulls and river sand as medium? Would this be cheaper than your current media choice? Let us know in the comments below!

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14 Comments

  • Jane
    April 15, 2021 @ 4:45 pm

    The rice hulls you link to are parboiled. The same type cannabis growers in the US have been using for at least 10 years. Why do you feel the need to pasturize them? To sterilize them you can’t just pour boiling water onto them.

    An why not just mix some DE into them, to kill insects like weavels? Instead of suggesting a soaking method that doesn’t scale well?

    Thank you

    • admin
      April 15, 2021 @ 7:18 pm

      Thanks for your comment! You’re right, they are parboiled, I didn’t notice! In this case it doesn’t make sense to try to sterilize them. Generally we never get them parboiled in South America, so I wasn’t expecting that. If it is parboiled it also likely doesn’t contain any insects, so that might not be necessary either way. About the DE, it is also quite expensive on a large scale, so I don’t know how practical it might be as a way to deal with insects, especially if you’re growing something like tomatoes or strawberries.

      • Jane
        April 16, 2021 @ 2:03 pm

        Thanks for the reply, makes sense about the DE. I’ve used parboiled rice hulls as aeration amendment in peat and coco mixes in the past. It works well, but they seem to loose their structure over time, getting almost mushy. I always assumed it was because they’re parboiled.

  • Hydroboy
    April 16, 2021 @ 7:25 am

    Dr. Fernandez, just asking how about using carbonized rice hull as a medium too? Thanks

    • admin
      April 16, 2021 @ 8:17 am

      Thanks for commenting! I’ve never used carbonized rice hull, so I sadly cannot comment on it as a medium. This process should significantly change its physical properties so it cannot be used as a replacement for regular rice hulls in the setup I described within the post.

  • Art
    April 16, 2021 @ 3:23 pm

    Thank you for the information, this is very helpful because I always thought there should be something else stupidly cheap and obvious, but also neutral and easy to cure. Here in Russia rice hulls cost surprisingly nothing and river sand is like a cup of coffee per ton (roughly speaking).

    I have a question though. How would you use the mix of these two in any other hydroponics approach than dutch buckets? I mean, considering NFT — sand is quite crumbly. Tho for plants with large root system like tomatoes and cucumbers I would not use anything but dutch bucket anyway, tho.

    • admin
      April 17, 2021 @ 7:25 am

      Thanks for your comment! I have used this in several different hydroponic systems, including NFT. In these cases, I generally wet the media first, so that it has cohesion, then put the wet media into net pots or whatever receptacle you will be using. You cannot load the dry media into these pots – it’s too crumbly and flows too easily, as you’ve said – so adding water is important to be able to properly place it and transplant the seedlings. Note that if you drip water through this media you will dissolve a lot of the finer particles of sand, so having adequate filters in your system to capture them or having a system that can deal with them without clogging, is important, especially if you’re recirculating solution.

      • Art
        April 19, 2021 @ 9:11 am

        Thanks for the reply, Daniel!

        All makes perfect sense! I am definitely going to try this out soon.

        • admin
          April 19, 2021 @ 9:26 am

          I look forward to hearing about your experience!

  • crown cannabis tulsa
    July 29, 2021 @ 5:07 pm

    would you be able to share your results when this 50/50 mix was used at scale. Did pumps ever get clogged from sand. We currently use a 70/30 coco rice hull mix. always looking to cut cost

    • admin
      July 30, 2021 @ 10:06 am

      We haven’t had issues with pumps getting clogged, but we did add filters before the pumps to prevent this from happening. It is true that you will have sand going from the media to the tanks, especially when the media is new, so you do need filters to account for this and tanks do need to be cleaned to remove this residue once every crop cycle.

  • Senthil
    August 20, 2021 @ 8:36 am

    Hello, can I use Msand (manufactured sand) in place of river sand? I live in a place where river sand is prohibited by local government due to excessive mining from river beds.

    Does Msand have properties similar to river sand from a hydroponics perspective? Will it be as good as river sand?

    Thanks.

  • Arjan
    October 2, 2021 @ 8:01 pm

    I’ve read that rice like flax seed as a crop is secondarily being used as a phytoremediation tool, that is for the decontamination of severely heavy metal polluted soils. I’m not sure where these pollutants, in American rice especially arsenic from the Mississippi delta , ends up within the plants. Hopefully not in the husks. That might be something to look in too while considering if rice husks are a part of the ideal substrate blend.

    • admin
      October 3, 2021 @ 11:30 am

      If you’re concerned about this, you can get an analysis of the particular rice hulls you’re using for your crop. Personally I have never found high heavy metal levels in any rice hulls I’ve used for hydroponic growing.

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