Lime in your borehole water? Managing hard water in Namibian hydroponics

If you grow hydroponically in Namibia, sooner or later you meet lime.

Most of our water here comes up out of the ground. We have dams, but on a real farming operation you are usually pumping borehole or dam water, and borehole water, clean as it is, tends to carry lime. The place name gives it away sometimes. I was at a client's plot near Kalkfeld once. Kalk is lime in both Afrikaans and German, so nobody was surprised when his borehole read an EC of 1.15 before we had added a single gram of fertiliser.

That number is the whole problem in miniature. This post is about what lime does to your water, why it is both a headache and a quiet helper, and how to grow good crops in it, lettuce, tomatoes, peppers and the rest, without buying a machine that costs more than your greenhouse.

First, borehole water is not the enemy

Water pulled from deep underground is usually free of the suspended grit and silt that comes with surface water, so you are not forever flushing muck out of your lines. That part is a real advantage.

The catch is what you cannot see. Borehole water in much of Namibia is loaded with dissolved lime, and dissolved does not mean harmless. As the water warms, sits and evaporates, that lime drops back out as scale, the same white crust that furs up a kettle, and it will slowly block your drippers and emitters from the inside. Lime hands you two problems at once: it scales up your hardware, and it changes how you have to feed your plants.

What lime actually does to your water

In hydroponics we add fertiliser to water. One of the big, important ones is calcium, and plants take up a lot of it. Now here is the useful part: lime in your water largely converts into two things, and one of them is calcium.

So high lime often means you already have a good amount of calcium sitting in your source water. Instead of paying to add all of it yourself, you can take advantage of what is already there. In many cases that saves you money.

The rest of the lime shows up mostly as bicarbonate, and bicarbonate is where it gets interesting, because it is both a problem and a gift.

The salt problem

Bicarbonate is a salt, and it pushes up your EC (the electrical conductivity reading that tells you roughly how much dissolved salt is in the water). Some crops shrug it off: fruiting plants like tomatoes and peppers will happily run at a higher EC. Leafy greens are the fussy ones, and lettuce in particular dislikes a salty root zone, which is why hard water bites your salad crops first. Lettuce is the sharpest example, so it is the one I will use to work through the numbers, but the same water logic applies to everything you grow.

Go back to that Kalkfeld borehole at 1.15 EC. A healthy lettuce solution usually runs somewhere around 1.2 EC, nutrients and all. That borehole was already sitting at 1.15 before a single gram of fertiliser went in. Add your feed on top of that and you shoot straight past the mark, to 1.6, 1.8, even 2.0 EC, which is too much for lettuce and hard to manage.

It gets worse in recirculating systems like deep water culture and NFT, where salts build up as water is used. You want to start low enough to leave yourself buffer room for that build-up. If your source water is already high and your feed starts at 2.0 EC, you have no headroom left before the plants suffer.

The bicarbonate gift: pH buffering

The good side of bicarbonate is that it buffers pH: it steadies the wild swings that plague clean-water systems.

Here is why it works. Bicarbonate is one half of a chemical pair (carbonic acid and bicarbonate) that can either mop up or release hydrogen ions, the ions that are pH. When something tries to push your pH down, the bicarbonate soaks up the extra hydrogen ions. When pH climbs, the pair releases them back. The result is water that resists sudden change, which is exactly what you want.

Your pH will try to wander. Young plants love nitrogen and ammonium and take it up fast, which pushes pH up, sometimes all the way to 8. Fruiting plants draining potassium can drag it the other way. Very clean water, with nothing to buffer it, jumps around the most. A little bicarbonate quietly absorbs a lot of that drama.

So the target is not to strip bicarbonate to zero. It is to knock it down to a level where the salt load is manageable but you keep some buffering capacity in hand.

Two ways to bring hard water down

There are two honest approaches: a mechanical one and a chemical one. Neither is magic, and the right answer depends on your water and your budget.

Reverse osmosis

RO is what most water-treatment people in Namibia will point you to, and it works. But for a serious operation the machine alone starts around N$300,000, and it is pricey to run: the membranes wear out and are not cheap to replace, especially if you are pushing a lot of water or pulling out a lot of salt. It also throws away a portion of your water as waste.

RO has one more quiet downside: it takes out everything, including the calcium you were happy to have for free.

If you do go the RO route, blend rather than treating all your water. Run only part of your supply, say half, through the RO, and mix it back with raw water. You keep some of the free calcium, you waste less water, you can often buy a smaller machine, and you go easy on the membranes because they are doing half the work.

The chemical route: acid

The other option is to knock the bicarbonate down with acid. When acid meets bicarbonate, the reaction breaks the bicarbonate into water and carbon dioxide, which simply bubbles off. What is left behind is whatever came attached to the acid, and that is where the acids differ:

• Nitric acid is the workhorse. It leaves behind nitrate, which is nitrogen your plants use, so the by-product is food rather than a problem. This is the one to reach for when you are treating your water.
• Phosphoric acid leaves behind phosphate, also a nutrient the plant wants, and a sensible choice where you could use a little extra phosphorus.
• Citric acid is best left out of this job. It leaves citrate, which is not a mineral nutrient, and microbes feed on it, so the pH creeps back up soon after you have dosed. Fine for a small one-off tweak; poor for treating a water supply.

The point is not to acidify all the way to nothing. Leave some bicarbonate in reserve for its buffering. As a rough working figure, up to about 0.5 mmol/litre of bicarbonate is a safe amount to hold in the water while keeping your pH stable.

Don't guess, test your water first

Here is the honest limit of all of this: you cannot see what is in your water, and you cannot dose what you have not measured. You know it is salty and you know it is 1.15 EC, but you do not know how much is calcium, how much is bicarbonate, or what else is riding along.

You cannot work that out at home. Send a sample to a water-testing lab. They will tell you exactly what is in it, and only then can you build a proper recipe. In Namibia, ask for an irrigation water test; it costs somewhere around R3,000, and it is the cheapest insurance you will buy all season.

Build your recipe: the nutrient calculator

Once you know what is in your water, you know what to add on top of it. This is where Habitat's free advanced nutrient calculator earns its keep.

On the website you will find a basic version at the top and an advanced version below it; you want the advanced one. Pick your crop (let's stay with lettuce), expand the source-water menu, and enter the figures from your lab test. The calculator works out exactly what to add to bring the water to where it should be, counting the calcium you already have so you are not paying for it twice.

Most of the nutrients on that recipe are available here in Namibia, and we deliberately stock them in small quantities: 100 g, 250 g, 500 g, on up to 10 and 25 kg. Some micro-elements you only ever need in traces: it makes no sense to buy a kilo of boron when 100 g will outlast your system, so we sell it by the 100 g.

You have two options from here. Take the recipe and buy the fertilisers wherever you like (we would rather it was from us, but the calculator is yours to use either way). Or let us mix it for you. Getting a recipe right needs an accurate scale, which we have, so if you send us your recipe and quantities we will make up ready A and B mixes: two bags to dissolve into two tanks, instead of a shelf full of little tubs. That mixing service is not always listed on the site, so email us if you want it.

Keeping it stable day to day

Say your source water is sorted. You are not done, because plants transpire: they pull water up and breathe it out through their leaves, which is how they move nutrients around in the first place. As that water leaves, your reservoir level drops and the salts left behind concentrate, so your EC climbs.

Topping up water and nutrients by hand, every day, gets old fast. The fix is an auto-doser that watches EC and pH and replenishes for you. We stock the Bluelab range, which is well built and reliable. It connects to Wi-Fi, so you can read your system and adjust the dosing from your phone whether you are on the plot or not.

For dosing you make up the same A and B mixes, just far more concentrated: stock solutions, often around a thousand times strength. Keep the two tanks roughly balanced by weight of fertiliser, or, if your recipe is lopsided, set the two dosing pumps to run at different speeds. The doser also carries a pH-up or pH-down pump, so it holds your pH steady too.

Even then, expect pH to wander while you learn your system, usually up, occasionally down, especially early on when there is still ammonium in the water for the plants to grab. That is normal. It is a learning process, and the bicarbonate you kept in reserve is doing quiet work in the background. Your job is to keep pH as stable as you can, in the range below.

The one thing to take away

If you remember nothing else: water management is the number-one thing in hydroponics. Get the water right and everything else follows. You want to make money from your crop, and you cannot do that with small, stalling, dying plants, which almost always traces back to the water. Don't take shortcuts on the basics.

Start with the free advanced nutrient calculator, and if your water has you stuck, our consulting service usually costs less than one failed crop.

Frequently asked questions

Is borehole water good for hydroponics? Yes, with a caveat. Borehole water is usually free of the suspended grit that clogs drippers, which is a plus. The catch is dissolved lime: you cannot see it, but over time it precipitates as scale that furs up drippers and emitters, and it raises your EC, which can push a salt-sensitive crop like lettuce over the edge. Test it, and treat it if the EC or the lime is high.

What EC is too high for lettuce? Lettuce runs comfortably at around 1.2 EC and up to roughly 1.8 as the solution concentrates; you can start as low as 0.8 to leave headroom. If your raw borehole water is already near 1.0 to 1.2 EC before you add any fertiliser, you have very little room left and should look at treating the water.

How do I lower the lime in my water without an RO machine? Bring the bicarbonate down with acid. Nitric or phosphoric acid are the sensible choices because the leftover (nitrate or phosphate) is a nutrient your plants use. Citric acid works chemically but rebounds; microbes consume the citrate and pH climbs again, so it is a poor choice for treating a water supply. Don't strip the bicarbonate completely; leave a little for pH buffering.

Why do hydroponic nutrients come as separate A and B parts? Because calcium will not share a concentrated tank with sulphates and phosphates: together they form solid calcium sulphate and calcium phosphate, precipitate out and clog your system. Keeping calcium in one tank and the sulphates and phosphates in the other, then diluting both into your reservoir, keeps everything in solution.

Do I really need a lab test? If you are serious, yes. You cannot dose accurately for water you have not measured. An irrigation water test in Namibia runs around R3,000 and tells you exactly what you are working with, far cheaper than guessing wrong across a whole season.

Further reading

• Best hydroponic kits for beginners: where to start if this is all new.
• Deep water culture buckets: setup and what to buy: the recirculating system where salt build-up bites hardest.
• NFT hydroponic systems: a setup guide: the other recirculating setup and how to run it.
• Bluelab Guardian monitor: review and setup: watching pH and EC so your water never surprises you.