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 >  Dorband gravel barrel    VS  SAND >  R411 Tom3

(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  mm-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 mm-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 quarry

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.

tilapia school +Sand_from_Gobi_Desert=ripe tomatoes

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:

At a uniform particle diameter 0.5 mm, smooth grains has SSA 44,000 mm-3.  At 2mm uniform diameter, SSA 1,100 mm-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  mm-3.  Other reported values for these materials are 120 and 40 mm-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.

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