In “Aquaponics’ Biggest Mistake“, we described how iAVs was usurped by the basic flood and drain system using media grow beds filled with gravel or expanded clay pebbles.
This article provides more detail around the proposition that sand is a much better ‘aquaponic’ growing media than gravel……or expanded clay pebbles.
GRAVEL > 
VS SAND > 
(click image to enlarge)
Sand is the better medium because of its:
- Much more effective mechanical filtration of all suspended solids – even microscopic particles – from the water column resulting in much cleaner water for the fish.
- Much greater specific surface area (SSA) for colonization by beneficial bacteria. E.g., Sand used in iAVs research had a minimum SSA 7,000 m2 m-3 (particle size – distribution method) well sorted (non-nested) without micro-surface irregularity factors and an effective – porosity > 0.3, aka void fraction of 30+%. The effective ‘non-sorted’ SSA using the BET adsorption method would probably approach 10,000 m2 m-3 or, approximately 200 times the SSA of 3/4″ (19 mm) gravel.1
- Vastly increased effective aeration of the media benefiting both soil bacteria/community activity and the plant roots’ assimilation rate – with 25,000 times (or more) greater concentration of molecular Oxygen (O2) than the maximum aqueous dissolved Oxygen (DO) saturation possible.
- Vastly expanded soil microorganism diversity, population density, and increased metabolic activity resulting in accelerated cycling of ALL plant essential elements.
- Maximal nutrient capture in the biofilter – virtually 100% – plus faster decomposition, mineralization, and plant assimilation resulting in increased system productivity and stability.
- Sand has a greater pore space per unit volume – porosity – than gravel (counter intuitively) and much different hydraulic conductivity characteristics as well as an increased water retention curve, bound water potential, et al. More on these and other related influential factors later.
Sand in process of being screened to size in a commercial quarry.
Also, with sand:
- There’s no need for plant nutrient supplementation – presuming a well- balanced fish diet is used. Vitamin-enriched and micro-nutrient supplemented feeds are not a requirement.2
- More efficient mechanical filtration and faster biochemical conversions of solid wastes permits higher feed input rates resulting in faster fish growth and higher yield.
- The higher feed input rate also provides for greater nutrient availability for the soil microbial communities which ultimately results in greater plant availability/uptake for improved vigor and yield.
- There is far greater cellular contact/interaction with the dissolved (soluble) nutrients in the water across the entire ( larger) biofilm surface area. Nutrient-rich solutes are not able to just flow past the microbes, out of reach, and not be ‘captured’ (adhere, adsorb, absorb) and metabolized.
- There is far greater availability of and effective micro-cellular contact/uptake of molecular Oxygen, which facilitates (energizes) all aerobic metabolic activity. This benefits both soil organisms and plant rhizosphere; due to:
- obligate aerobes require O2 for cellular respiration to oxidase substrates —e.g., amino and nucleic acids, ammoniacal-N, lipids, etc. – to obtain energy,
- increased Oxygen concentration ( – gravitationally facilitated suction replenishment of the soil atmosphere at 21% (210,000 ppm) Oxygen with each dewatering/drain interval),
- forced cellular (membrane) physical contact with Oxygen due to the individually smaller, yet greater composite surface area contact and more uniformly distributed pore volumes providing for a far greater colonized surface area in direct contact with Oxygen.
- Temporal retention of and direct microbial contact/interaction with plant root exudate contributes to a more diverse and effective soil ecology.
- Sterilization of potential pathogens (through fumigation and/or steam pasteurization) is possible if crop specific local conditions warrant. Treatment of the sand (when isolated from the water column) with a Chlorine solution and/or Hydrogen peroxide solution are other potential options. Sand is chemically inert.
- Inoculation with the full range of beneficial soil organisms is easy. Sand plus microbial communities plus ‘organic materials’ (substrate) + Oxygen (energy) = SOIL.
- Unlike expanded clay pebbles, sand never wears out or breaks down. The functional life of the biofilter sand is unknown at this stage ( w/ too many variables in ‘play’ for assumption), however, sand can be repeatedly washed for indefinite re-use.
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Other benefits of using sand include:
- It can be effectively analyzed (as a soil) for assessment of nutrient concentrations, forms and soil microbial populations.
- It’s easy on the hands and on plant roots and it’s easily worked with common garden tools.
- It’s far cheaper by volume than expanded clay pellets or other hydroponic aggregate.
To summarise:
- Sand benefits the fish with cleaner water, stripped of suspended solids and water soluble compounds,
- Benefits the biofilter/soil organisms by providing greater physical access to (contact with) both nutrients and Oxygen for efficient metabolism.
- Benefits the plants due to increased nutrient availability with increased Oxygen for effective ion exchange, respiration and root metabolism,
- which combine to promote and sustain a diverse rhizosphere ecology by which to ‘mineralize’ ALL plant essential elements.
The use of sand over gravel – or expanded clay – has a leveraging effect where the benefits are greater than the sum of their parts.
By the way, terrestrial plants evolved root systems over 100’s of millions of years to most efficiently assimilate nutrient from/in soils and are not optimized for being submerged (drowned). Certain plant species (mostly herbaceous dicots) are relatively more tolerant of such ‘abuse’ than are most other species (notably flowering, fruit-bearing spp.).
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End Notes:
1 At a uniform particle diameter 0.5 mm, smooth grains has SSA 44,000 m2 m-3. At 2mm uniform diameter, SSA 1,100 m2 m-3. Even higher values occur with ‘sharp’ (Rhombic icosahedron) shaped grains. According to Nate Storey, 3/8″ (9.5 mm) pea gravel is SSA 280 and 3/4″ (19.1 mm) gravel SSA 69 m2 m-3. Other reported values for these materials are 120 and 40 m2 m-3, respectively. When sand is SSA 10,000 and gravel is SSA 50, then sand has 200 times more surface area per unit volume.
2 Some commercial fish feeds may have too much metal supplements added, notably Copper and Zinc. Toxicity has not been observed, however the potential for accumulation(s) from ‘unbalanced’ (overloaded) input should be considered and monitored over time. Select the fish feed source (ingredients) with care. Elemental input quantities remain in the ecosystem, unless and until harvested. Therefore, inputs need to be approximately balanced with outputs on annual/biannual basis.
I never knew that there was so much you could do with sand and gravel. I think a lot of people prefer sand because it is finer and you don’t need very much of it. I am looking for both gravel and sand for the sandbox we are building. I want to have the bottom layer to be gravel and have sand from there to the top.
Gary, thanks for your feedback, definitely am factoring those things into my design. My original question was also about whether it’s a sustainable or worthwhile approach to try to source some of the fish feedback from worms or BSFL, which I would probably feed with free horse manure from local ranch or other organic waste.
Richard……While I’ve experimented with home-made fish diets, I’m still not satisfied with the results. Some fish will eat BSF and some won’t. Any fish will eat worms but worms are worth much more than fish food.
Most things (like duckweed, BSF larvae, worms, etc) are useful supplements to a commercial ration. Remember, what you feed the fish also has to provide the nutrients for your plants.
I’d encourage you to experiment….and to let us know how you went.
I have the same kind of inclination as Wendy. I am looking for a productive, easy to maintain, growing system that require inputs that is as environment friendly as possible. I was thinking reasonably large worm bed and BSFL bed. Get horse manure from local range for worms, get meat waste from butcher for BSFL, and harvest worm/BSFL to supplement fish feed? Having read some stuff in APN, I know Gary have tons of experience on microponic … is above approach going to be too much work for what it’s worth?
BTW, I’m so very excited to run into iAVs. Been reading this site feverishly during past 2 days. I have a ~5000 sqft front yard edible garden. But we’re in drought in California and I can’t sustain my garden with the water restrictions we have. Have been looking for an ideal solution that use less water, less nutrient, and less maintenance … I am in design/preliminary building phase of a front yard aquaponics / vermiponics system to test things out (1200 gal tank expandable to 3600), 5 x 50 sqft grow bed potentially double in size. Now I have to rethink my design. I will post questions under another more appropriate topic after I gather all my thoughts …
Richard Liu…..the most effective way to set up any integrated system is to design the workflow so that movement is minimised. Start with listing all of your system’s aspects and note the frequency with which you need to visit them…..for feeding, planting, harvesting and maintenance…..and ensure that those sub-systems that you visit most frequently are closest to your house. Try to eliminate all double handling.
I share your excitement around iAVs. The more I look at the system, the more conscious of its technical elegance I become.
THIS is a phenomenally helpful answer, thank you both for taking the time to really consider the issue and share your insights.
I hear you on the the tradeoffs with spending all one’s finite time formulating feeds (something I’ve thought of as I prepare to raise coturnix quail).
I would never have known even half of what Mark wrote about the questionable make up of aquaponics-marketed feeds! I assume that the best direction to look is going to be in the aquaculture market. ?
In some ways maybe my worrying at this level of detail is misguided – I buy farmed fish (and farmed everything else) without analyzing feed down to this level… (and yeah, soy and cotton = heavy pesticide use and tilled soils, etc etc – EVERYTHING is screwed up!). Just working to raise my food responsibly is a good step.
Thanks, again, to both of you for your conscientiousness and hard work!
I’m not sure if this is the appropriate place to ask a fish question, but your emphasis on quality diet brings up one of aquaponics’ main drawbacks (as I see it, from my purely theoretical, non-praxis, total outsider viewpoint – oh dear, I see how terrible that sounds, but bear with me)… and Gary, feel free to tell me if I should be asking this at APN, it’s just that I’m less interested in AP than I am in iAVs. 🙂
I have great qualms about raising fish that require inputs from fragile/costly resources: namely, feed that comes from ocean fish. This is probably what will make or break the inclusion of aquaculture in my home system.
In your iAVs research (Mark) or aquaponics experience (Gary), have you come across viable fish species that don’t require ocean inputs? Catfish feed (http://www.thefishsite.com/articles/171/catfish-nutrition-feeds-and-feed-formulation/) seems to contain 2-4% fishmeal and I assume that’s mostly ocean-sourced… maybe that’s an acceptable amount, I don’t know.
My inclination is to attempt to design the most resilient system possible (I know, maybe it’s a fool’s errand, who knows what the future holds for soybean and cottonseed meal…), so I’m looking to find out if there are fish who can “eat locally” and don’t need major temperature controls for my area…
Anyway, to keep it relevant to this post, I think that’s a really important consideration that you bring up, to keep in mind that whatever you put in stays in until you take it out due to it being a relatively “closed loop” system.
Wendy….We share your concerns about the use of wildcatch species in fish feed. Having said that, the aquaculture feed manufacturers have made significant progress in recent years in reducing the proportion of fish oil and meal in aquaculture diets. Much of the seafood content of fish food now comes from krill.
Another point to consider is that soybean and cottonseed meals are probably no more sustainable when all things are considered.
In any situation where you relying on fish waste for plant nutrients, you are going to have to confront this issue. At this stage, I don’t know of any way that you can grow fish and plants at any sustainable rate without using a formulated fish ration.
For what it’s worth, when you consider the excellent feed conversion ratio of most freshwater species….and the subsequent fertiliser substitution…..fish feed is pretty efficient stuff.
Mark adds: If one is prepared to learn about fish nutrition and implement one’s own fish feed preparation, it is theoretically possible to eliminate ocean sourced oils and proteins. However, this is a challenging prospect and will be strongly influenced by the species one is attempting to grow. The practicality of formulating one’s own feed is IMO not particularly efficient assuming one assigns any value at all to one’s time and efforts. While on the topic of feed, I note that, from what I’ve seen of pricing on so-called ‘aquaponics’ related sites, their feeds cost from 2 to 4 times the price of purchasing from established feed producer outlets (not including shipping). I further note that both ’boutique’ (small scale niche hype) and the major commercial manufacturers do not formulate feed feeds to address/meet plant requirements from the ‘wastes’ generated. Many feeds are also very high in Sulphur, Copper and Zinc (et al.) and relatively low in Calcium and possibly other elements depending on the brand and type. Most producers also add substantial vitamin ‘supplements’ and some also add antibiotics, none of which are required nor desirable in iAVs applications. Furthermore, IMO, ANYTHING one purchases from the ‘aquaponics’ sources is vastly overpriced – everything! IMO, they are all cyber pirates predating on willfully gullible flocks of fools. I won’t name names here but will say that if the term ‘Aquaponics’ appears on the ad/site/page, then you can be assured that you are being ripped-off. Additionally, the equipment I’ve seen offered on such sites is inferior junk. There is no substitute for quality and no excuse for not shopping around for the best prices.