We often get asked what kind of water is best for coco. Should I run reverse osmosis? Is my tap water good enough? My well tested fine for drinking, so is that good enough for my plants?

RO is not automatically better and tap water is not automatically worse. Both depend entirely on what is dissolved in your water, and there is no way to know that without a test. Growers will spend months dialing in nutrient recipes, irrigation timing, and environmental setpoints, and never once look at the source water.

Why water quality matters more in coco than in soil

Coco coir is a clean, inert substrate. There is no mineral weathering happening in the root zone, no organic matter breaking down and releasing nutrients, no dependency on root exudates, and no soil chemistry interactions. Whatever is in your water arrives at the roots essentially unchanged.

This is why coco performs the way it does when your inputs are clean and understood. You get a predictable and repeatable root zone because the substrate is not adding variables of its own. It also means poor water chemistry expresses itself faster and more clearly in coco than it would in a living soil system.

Before you add a single gram of fertilizer, your water may already carry EC, alkalinity, sodium, and hardness. That background load reduces the working range available for your actual feed.

Alkalinity and bicarbonate

This is the most common well water issue we encounter, and growers usually misdiagnose it as a pH problem.

Water that has moved through limestone or carbonate rock picks up minerals and dissolved bicarbonates. Those bicarbonates resist changes in pH because they neutralize acid. This is why your pH will not stay where you put it, why you burn through pH down faster than you expect, and why your root zone pH creeps upward over the course of a run even though your feed pH reads perfectly.

pH and alkalinity are two different measurements. A well can read pH 7.1 and still be difficult to work with if the alkalinity behind it is high. pH tells you where the water sits right now. Alkalinity tells you how much it is buffered when you try to correct it.

Total alkalinity is reported as ppm of calcium carbonate. We consider 85 ppm or below to be good working water. Between 85 and 150 ppm, acid injection becomes a routine part of your fertigation program rather than an occasional adjustment. Above 150 ppm, RO treatment or blending is the more practical long term answer.

pH down also carries a nutrient contribution, usually phosphorus from phosphoric acid. With large constant corrections, you may be overfeeding phosphorus in your final solution.

Hardness

Hardness is the total sum of calcium and magnesium in your water, reported as a converted equivalent of ppm calcium carbonate. It says nothing about which of those two cations is actually present and available.

Hard water is not bad water. Calcium and magnesium are plant nutrients, and a well or tap source delivering moderate hardness is contributing real fertilizer value to your feed at no cost. Plenty of growers on RO are paying to add back exactly what they get for free from a well.

Hardness is also not automatically good, because the number is only a sum. Two wells can both report 130 ppm hardness and behave completely differently. One might be calcium dominant with very little magnesium. The other might be magnesium dominant with calcium in short supply. In production use, these two source waters will produce very different results in the final nutrient concentrations.

We generally want to see calcium in the range of 40 to 100 ppm and magnesium between 15 and 30 ppm, with the calcium to magnesium ratio between 3:1 and 4:1. A well heavily skewed toward magnesium will inhibit calcium uptake at the root, and you can end up chasing calcium deficiency symptoms while your water report shows plenty of hardness. Calcium heavy water can push you toward magnesium deficiency.

Read hardness as a starting point, then look at the two cations separately and adjust your feed accordingly. The operational downside of high hardness is scaling in your lines and emitters, and the fact that high hardness usually travels alongside high alkalinity. Address those as separate problems.

Sodium

Sodium is the one parameter on your report with no upside. It contributes to your EC without contributing anything useful, and it competes directly with potassium and calcium for the cation exchange sites in coco.

The practical concern is accumulation. Sodium does not flush as readily as growers assume, and repeated irrigation with sodium heavy water will gradually load those exchange sites over the course of a crop. Below 35 ppm is workable. Between 35 and 70 ppm, watch your leachate and maintain a healthy leaching fraction. Above 70 ppm, treatment or blending is worth the investment.

Our article on charging and buffering covers how those exchange sites function and why displacing sodium matters.

Chloride

Chloride usually travels with sodium, since sodium chloride is the most common soluble salt in groundwater. It provides no meaningful nutritional benefit at the concentrations found in most water supplies and adds to your background EC. At higher levels it competes with nitrate and phosphate for uptake, which can look like a nitrogen problem when it is a source water problem.

Under 50 ppm is comfortable. Above 100 ppm requires treatment. Chloride is sometimes left off basic water panels, so ask for it specifically.

Source water EC

EC measures total dissolved salts. It tells you how close you already are to your ceiling before fertilizer is added, but it does not tell you what those salts are or how much room you realistically have on top of them. A source water at 0.6 mS/cm driven by calcium and magnesium is a different situation than the same reading driven by sodium and chloride.

We want to see source water below 0.5 mS/cm. Between 0.5 and 1.0 is workable with adjustment, depending on composition. Above 1.0 you can expect salt accumulation, runaway root zone EC, pH swings, and nutrient deficiencies and antagonisms.

Iron, manganese, copper, and boron

These show up less often, but when they do they cause specific and identifiable problems.

In deep, oxygen poor well water, iron exists in the ferrous form, which is dissolved and invisible. Your water comes out of the tap looking perfectly clear. Once that water is exposed to air or oxygenated in a reservoir, the ferrous iron oxidizes into ferric iron, which precipitates out as rust colored sediment. Ferric iron is not plant available, so the iron in your source water is not contributing to your crop regardless of concentration. The iron your plants use comes from the chelated iron in your fertilizer. That precipitate also clogs drip emitters and filters gradually enough that growers often blame the emitters rather than the water. Keep iron below 0.3 ppm in the source water for any drip system, and lower if you can.

Manganese behaves similarly and causes the same clogging and staining issues, generally becoming a problem above 0.05 ppm.

Copper and boron are less common but should be screened for. We have seen well water come back with copper and boron at levels approaching phytotoxicity, particularly in mineralized geology and in arid regions. Copper above 0.2 ppm and boron above 0.5 ppm should be flagged, since these are micronutrients where the gap between sufficiency and toxicity is narrow. Elevated copper can also indicate corrosion in your own plumbing rather than anything in the aquifer.

Getting the right water test

Ask your lab for an irrigation suitability panel. Terminology varies, and most water testing labs are set up primarily for potability and drinking water compliance. A drinking water panel reports on bacteria, nitrate, and regulated contaminants. It will not give you alkalinity, hardness broken out by cation, sodium, chloride, and the micronutrients you need. Some labs call it an agricultural or irrigation water panel.

Your state agricultural extension service is usually the most economical route, and most commercial ag labs offer the same panel.

On your lab report, mg/L and ppm are the same number.

Well, municipal, and RO

Well water is the most variable of the three and the most likely to surprise you, but it is also the most likely to be excellent water that costs you nothing beyond pumping. Bicarbonate alkalinity is the usual culprit when a well is difficult. Well chemistry shifts with the water table, so a report from three years ago is not a current description of your water. Retest annually, or quarterly if budget allows, and retest after any significant change in draw or drought conditions.

Municipal water offers consistency, which has real operational value. Your utility publishes an annual water quality report and the composition rarely moves much across the year. Many municipalities have moved from chlorine to chloramine as a disinfectant, and chloramine does not off gas the way chlorine does. If you have been letting water sit in an open tank overnight and assuming you have handled it, chloramine will still be there in the morning. Removing it requires carbon filtration or chemical treatment. Municipal water is also not automatically clean water. Supplies in the arid west in particular can carry meaningful sodium and EC straight from the source.

RO gives you the most control and the cleanest baseline, and it is the right answer for genuinely problematic water. It is a tool with a cost, both in capital and in rejected water volume, and it is not automatically the correct choice.

The mistake we see most often with RO is treating it as a finished product. Pure RO water is not ready to use in coco. Water stripped of all dissolved solids will pull calcium and magnesium directly off the exchange sites you spent time and money establishing. RO works when your fertilizer program is built to account for it, meaning you are supplying all of your calcium and magnesium deliberately rather than relying on the water to contribute any.

Blending is the option most growers overlook. You do not always need to treat all of your water. Cutting problem water with RO permeate at a one to one ratio cuts every parameter in half. If your well comes back at 90 ppm sodium and 160 ppm alkalinity, a fifty fifty blend puts you at 45 ppm sodium and 80 ppm alkalinity, which moves both parameters into acceptable range. That is often achievable with a much smaller RO system than you would need to treat your full volume, at a fraction of the capital cost and with a fraction of the reject water.

Summary

Test your water and ask for an irrigation suitability panel. Look at alkalinity and sodium first, because they drive the most decisions. Read hardness as two separate nutrients rather than one number. Treat proportionally to the problem you actually have instead of defaulting to RO. Retest often, particularly if you are on a well.

HortGrow coco is engineered to deliver a predictable, uniform root zone with consistent hydration and repeatable dryback behavior. That consistency shows up fully when the water going into it is clean and understood. The substrate removes one large variable from your production system. Knowing your water removes another.