Overview of Salt in use Aquaponics
Salinity is used to expresses the total salts in a substance; the salinity of water can be measured using a handheld Seawater refractometer or a digital salinity meter. Salinity tolerances vary among aquatic species, though even most freshwater fish prefer some salinity. Salt in the water helps regulate osmatic functions in the cultured species. Salt also stimulates mucus production and reduces stress caused by transportation, handling, and environmental factors Salt can also be used to treat external parasites and improve over-all fish health
HOWEVER, salt should only be used in freshwater aquaponic systems under certain conditions; this is primarily because of the adverse effects of Sodium (Na) on plant health and development. Salinity tolerances of plants vary from species to species through most vegetables and fruits have low levels of salinity tolerance. Currently, research is being done on the use of salt-tolerant crops in the saltwater aquaponic system though published data is limited.
In regards to freshwater aquaponic producers, salt should be used under two conditions:
- In quarantine systems, which are isolated from your main system and are used to treat incoming or compromised fish to help to prevent the introduction of external parasites or pathogens into the system. Typical forms of salt include table salt, pickling salt, meat-curing salt, rock salt, pond salt mixes, and marine grade salt mixes. (see treatments options below)
- Seawater can be used as a micronutrient supplement if diluted down in a 1:30 ratio of seawater to freshwater, and applied as a foliar spray or added directly to the water in a 1:30 ratio of seawater to system water (equates to roughly 1.2 g/l or 0.12 % salinity). If seawater is not available, a mix can be made using a salt mineral mix, such as SEA-90 and applied at the recommended rate (a general rate is 1 g/L or 0.1% salinity) . NOTE: Not all salt mixes are equal as the ratios of the individual salts, particularly sodium, can differ between them, use one with a comparatively low sodium level.
Typically the seawater mix is used as a foliar spray on flowering and fruiting crops during their reproductive phases once every three weeks. In fruiting trees, it has also been shown to increase the sweetness or Brix content of the fruit when applied as a foliar spray two weeks before ripening. NOTE: if you see yellowing on leaves after application increase the rate of dilution before next application.
Because traditional aquaponic systems operate in a recirculating system over long periods, there may be the potential for Sodium concentrations to build up when added directly to the water; which can be monitored by periodic micro-macro testing. For easy testing at home, you can use an Electric Conductivity (EC) to monitor the overall concentrations of ions in the water; while this does not tell you how much Sodium (Na) in particular is in your system, it does give you an overall idea of the salts in your water. In general, this should be less than 2.0 dSm-1 for Leafy greens, less than 3.0 dSm-1 for tomatoes and less than 3.5 dSm-1 for cucumbers. You can find a link to the USDA’s research on salt tolerance of soil-based crops here. To convert EC of Saturated Soil Paste extract (ECe) to the EC irrigation water (ECw) use ECe=ECw*1.5