Question:
If you use a standard water softener (exchanging sodium for calcium), it willbe bad for plants.

Two alternatives are:- Demineralization. This uses both cation and anion exchange resins, to removeall ionic contaminants. The cation exchange resins are regenerated with anacid, such as HCl. The anion exchange resins are regenerated with a base, suchas NaOH. After running through both cation and anion exchange resins, theresulting water is very low in dissolved minerals. These systems are commonlyused on an industrial scale. I don't know if they are available or practical ona home scale.

- Reverse osmosis. This uses a membrane, which allows pure water through, butexcludes dissolved ionic solids. A pump is used to provide the pressure toovercome the osmotic gradient across the membrane. Since there aren't any ionexchange resins, regeneration isn't necessary. The RO unit produces a stream ofpurified water, and a waste stream of water with concentrated dissolvedsolids.This is a practical approach to desalination.

Answer: I'd like to see some "plant health" comparisons on this -- just for idlecuriosity since I'm fortunate to live in an area with naturally soft water& where the soil is naturally acidic. But it's my understanding thatreverse osmosis removes many of the essential & useful minerals that onlydistilled water would be purer -- & for aquarium keepers this is regardedas a way to doom one's pet fish since they won't thrive in such sterilewater. "Pure" can be a bad thing if it lacks residual nutrients for plantsor animals.

I've seen it argued (more by way of opinion than with evidence) that hardwater is better for plants due to the minerals in it. The more acidicground & rainwater of the Pacific Northwest certainly seems to suit ourplant species best so what might be true of plants native of one area maynot be true in others. And the opposite argument has been that excesses ofcalcum in hard water displaces or locks out other beneficial mineralscausing more harm than good -- whether that alleged harmfulness would bemore or less than with ionized water seems a point that would have to beexpressed statistically after controlled studies rather than by randomgardener suspicion.

Hard water does not well assist plants in obtaining nutrients from theearth the way soft water breaks down the nutrient content of soil to beaccessed by plant roots. Again gardener suspicions don't supplant solidstudies, & would the stress put on plants getting only calcium-magnesiumfrom hard water that retards their accessing other minerals even from thesoil -- is the magnitude of that problem less, more, or the same as anyproplem imposed by ionization to soften water??

And if the studied & proven fact is HARD water injures plants, and waterthat is not naturally soft but has been softened by ionization also harmsplants, & the excess purification of water by reverse osmosis starvesplants for certain essential nutrients, is it all just one big Catch 22 &the gardener should just up & move somwehere which just naturally hasbetter water for plants?? On a relative scale of bad hard water, badionized water, & bad sterile water, does any scientific evidence reallypoint to a lesser of three evils??

I've seen "doom" reports that prime American farmlands are building upsalts from fertilizers & watering systems so that in 20 years nothing willgrow in America & mass starvation will result. But I've already seen thatwarning repeated for 20 years too, so have my doubts.

Certainly more salts are introduced to a garden environment throughfertilizing methods than from water softening agents, though if plantsalinity tolerance rests somewhere around 500 ppm, the same maximum thathumans won't taste (& for common garden plants originating around theMediterranean tolerances are vastly higher than that), for some plants thetolerances are already being pushed by artificially fertilizing or usingother gardening chemicals & pesticides, then the minute sodium quantitiesfrom an ionization tank I can image being the straw that breaks the camelsback. But if salt build up is a threat, as it might well be, the quickestsmartest way to arrest this problem would be to stop using gardeningchemicals since that's how the lion's share of sodium gets introduced.

That was just some random thoughts -- & as I already said, I'm just gladthe rainy northwest gives my plants exactly what they seem to want withoutmy having to "adjust" anything for them or for me.