How do I manage sodium in a hydroponic solution?

Rick Donnan

Answer by Rick Donnan

Just wondering, what is the best way to lock up sodium? I’m having big sodium issues in my hydroponics.

Answer
You give no indication of what type of system you have. Some description of your system would allow me to make some specific suggestions, however, I usually also seek to widen my answers to make them applicable to a broader range of readers.

Other questions worth asking you are: what are your actual problems and why do you think that the cause is sodium? Assuming that you have accurately diagnosed the cause of your problems as too high a level of sodium ions in your nutrient solution, what can you do about it? (Sodium has the chemical symbol Na, from the Latin word natrium. Sodium ion is a cation, that is, has a positive charge. In this case it has a single charge, with the symbol Na+.)

My column before last (PH&G April 2014) went into some detail about managing the influence of raw water. I’ll now look only at sodium build up in your system.

First, we need to consider—where is the sodium coming from?

One possibility is your medium, especially if you are using cocopeat. Cocopeat comes from coconut husk, which already contains substantial amounts of sodium. Manufacturers treat it through a process called ‘retting’, where it is held in fresh water to leach out most of the salts, especially sodium. Quite often this does not remove most of the sodium.

Consequently, you should only buy from a reputable supplier and test every new batch you receive. Wet a sample of the medium (1 ½ volumes of distilled water to 1 volume of expanded cocopeat). If the test solution EC is higher than 3 mS/cm I’d advise a commercial grower to have the sample chemically analysed.

It is advisable to flush any cocopeat before use. Be aware that some unscrupulous operators ‘ret” with sea water instead of fresh water, resulting in the sodium levels being inflated high enough to kill anything planted directly into the unflushed media.

Another possible sodium source is fertiliser, however, this will not be the case with reputable greenhouse grade fertilisers.

The most likely ongoing source of sodium (and chloride) is raw water. If using a town water supply, the water authority should be able to give you an analysis. (Also check wether they have different sources of supply and whether their analyses differ significantly.)

Your possible management choices

Locking up nutrients
The ability of a medium to bind nutrients is known as ‘cation exchange capacity’ (CEC). In general, organic media, such as cocopeat, have relatively a high CEC; and most inorganic media, such as rockwool, have a very low CEC. Which is better is largely a matter of personal preference. However, it is important to keep the CEC in mind for your nutrient management, especially when first planting a crop.

When first being used, a high CEC medium will ‘load up’ individual cations until those retained within the medium reach their maximum. In a high CEC media-based, dripper-irrigated system, this maximum will be reached within one or two weeks, or earlier if the media is pre-conditioned.

Thereafter, the medium can’t lock up any more of that cation. In your case, if any extra sodium locks into the medium, this would only be for the short time until it was fully loaded. If you are using cocopeat, it is more likely that sodium would be released into your solution rather than taken out.

This is not a management option.

Choice of system management
By this I mean how a system is operated.

Any system which recirculates is most vulnerable to sodium build-up within the recirculating solution. Some relief may be obtained by bleeding from the recycle or discarding more frequently.

If the sodium levels are still too high, then you have two choices if you have a dripper-fed, media-based system with recycle. One choice is to convert from recovering and recycling to operating as a free drainage (open) system. The second choice is to remove the sodium (and all other dissolved solids) by installing reverse osmosis (RO) equipment. If you have a continuously recirculating system such as NFT (nutrient film technique), your only choice is to install RO.

When operating a free drainage system, increasing the percentage run-off may help, but at a financial and possibly (but hopefully not) environment cost. The only choice then left is to install RO.

    Table 1. Sensitivity of plants to sodium chloride.     Adapted from Sonneveld C, de Kreij C, van der Wees, "Normen voor waterkwaliteit in de glastuinbouw", Research Station for Floriculture and Glasshouse Vegetables, the Netherlands.

Adapted from Sonneveld C, de Kreij C, van der Wees, “Normen voor waterkwaliteit in de glastuinbouw”, Research Station for Floriculture and Glasshouse Vegetables, the Netherlands.

Choice of crop
Crops vary widely as to their sensitivity to sodium chloride. Table 1 shows the sensitivities of a range of species. A possible major choice you could make is to change to growing a less sensitive crop.

PH&G June 2014 / Issue 144


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