What Is In Your Water

The watering guide tells you not to buy reverse osmosis gear for grow one, and that is right. Tap water grows good plants nearly everywhere, and the people who buy RO first usually buy it to solve a problem they have not diagnosed.

This page is for after that: when feeding and pH are dialed and something still does not add up, or when you want to know what your tap is actually contributing before you adjust anything else.

Start with the report, not a meter#

Your municipal water supplier publishes an annual water quality report. It is free, it is online, and it tells you more than any device you can buy.

What to look for:

Hardness, usually as calcium carbonate in parts per million or as grains per gallon. Under 60 ppm is soft, 60 to 120 moderate, 120 to 180 hard, above 180 very hard.

Calcium and magnesium individually, if listed. This is the number that determines whether you need to add cal-mag at all.

Alkalinity, which is not the same as pH and matters more. Alkalinity is the water's resistance to pH change, and high alkalinity is why your pH keeps drifting back up after you adjust it.

Sodium, which nothing good comes from, and is the main argument for RO in some areas.

Chlorine or chloramine, and which one, because the handling differs completely.

A TDS or EC meter tells you the total dissolved solids and nothing about what they are. That is still useful as a baseline, because knowing your tap starts at 200 ppm means your nutrient solution is really at your target plus 200.

Hard water, which is the common case#

Most American tap water is moderately hard, and for soil growing that is usually fine or mildly helpful, because the calcium and magnesium are nutrients the plant wants.

Where it bites:

You are adding cal-mag you do not need. If your tap is already supplying 80 ppm of calcium, adding a full dose of cal-mag on top can push you into calcium excess, which locks out potassium and magnesium and presents as a deficiency of those. This is a genuinely common and genuinely confusing problem.

pH will not stay put. High alkalinity means you add pH Down, it reads correctly, and by the time it reaches the roots it has drifted back up. Growers respond by adding more acid each time, which adds up.

Scale builds in equipment. Cosmetic mostly, but it clogs drippers and emitters in automatic systems.

Soft water, the other direction#

Naturally soft water, under about 60 ppm, simply does not supply calcium or magnesium, so a cal-mag supplement goes from optional to necessary. If you are running soft water and seeing calcium deficiency symptoms on a correct feed schedule, that is the reason.

Softened water is a separate and worse problem, and the watering guide covers why you should not use it. The number worth adding here: a softener typically adds sodium in rough proportion to the hardness it removed, so the harder your supply, the more sodium the softened output carries. On very hard water a softener can put out enough to matter within a single run.

The two fixes are an outside tap that bypasses the softener, or RO.

Chloramine, in one line more than the guide#

The watering guide has the practical version: chloramine does not off-gas the way chlorine does, and at municipal levels it will not hurt a plant in soil.

The detail worth knowing once you are further along is that it does suppress soil biology, which matters if you are running living soil or a heavily amended mix where the microbes are doing the feeding. A carbon filter handles it. So does a small amount of ascorbic acid, which neutralizes it instantly and costs nothing. Boiling, sitting out and air stones all do not, and all three are still widely recommended for it.

When RO is actually justified#

Four cases, and otherwise it is equipment you do not need:

1. A water softener you cannot bypass. Sodium is the one contaminant worth building a system around.
2. Very hard water above roughly 200 ppm, where you are fighting alkalinity every watering and the dissolved solids leave little headroom for your actual feed.
3. High sodium or high sulfate in the report.
4. Hydroponics, where there is no medium to buffer anything and you want to control the solution precisely.

If you do run RO, remember you have removed everything, including the calcium and magnesium your schedule assumed. RO water needs cal-mag added back, every time, and growers who switch to RO and keep the same feed schedule immediately produce the deficiency they were trying to avoid.

The seasonal catch#

Municipal water is not a constant. Source blending changes through the year, so a tap that reads 120 ppm in March can read 180 in September, and alkalinity moves with it.

If you are tracking this at all, test the tap once a quarter rather than once. Water chemistry sits underneath feeding and pH both, and a feed schedule built on a spring reading can be wrong by autumn without anything on your end having changed.