Algae, Slime, and White Crust: Buildup in Your Olla Explained

01 · IDENTIFY IT FIRST

Green, slimy, or white: what is actually in your olla

Buildup in an olla falls into three distinct categories with three different causes, and treating the wrong one wastes effort. Look at colour, texture and where it sits before you reach for a brush.

The quick field test for white deposits: a drop of vinegar on calcium carbonate fizzes visibly, while root residue and plain soil do not. If it fizzes, it is scale and it belongs in the descaling routine below. If it peels off in a fibrous sheet, it is roots — covered in our guide to troubleshooting a slow olla.

02 · ALGAE

Green algae in an olla means light is getting in

Algae are photosynthetic. They need light, water and a little dissolved nutrient, and if any one of those is missing they cannot establish. Water and nutrients are unavoidable inside an olla; light is not. So green growth in a buried clay pot is a direct diagnostic: daylight is reaching the water surface or the upper interior wall^[8].

In practice that means one of a short list of causes: the lid is off, the lid is cracked or sitting crooked, the neck stands unusually proud so light spills in around a loose cap, or the olla has been standing filled and uncovered while you set up the bed. It is common in the first weeks of a season and rare in a properly capped pot that has been running for months.

Is it harmful? Mostly it is unsightly. A thin film of algae does not poison the plant and it is not the reason your olla output has dropped. But it can contribute to fouling: dead algal material adds organic solids to the water, those solids settle at the base and against the wall, and a layer of organic sludge does reduce the effective permeability of the clay over time. It also feeds the general biofilm slick and can make the water smell stale. Cleared early it is a five-minute job; ignored for a whole season it becomes part of the layer you have to scrub off at the end.

Prevention is one line long: keep the cap on. A well-seated lid excludes light, keeps out debris and insects, and stops the small amount of open-surface evaporation that occurs at the neck. It is the same single action that answers most olla maintenance questions.

03 · MINERAL SCALE

White crust: calcium carbonate and why it matters

Hard water carries dissolved calcium and magnesium bicarbonates. When that water evaporates or passes through a porous medium, the dissolved solids do not go with it — they are left behind as carbonate deposits. This is the same chemistry that furs a kettle, whitens a shower screen, and rings the rim of a terracotta flowerpot^[9][11].

In an olla the deposits accumulate in three places. Inside, as a chalky ring at the fill line and a gritty layer on the lower wall. Within the wall itself, as crystals precipitating in the pore network as water works its way through. And on the outside, as a pale bloom on the exposed neck where seepage reaches the surface and evaporates.

The middle one is the consequential case. An olla's output depends on a connected network of fine pores through which water is drawn by soil suction; the finer and more continuous that network, the more predictable the release^[1][15]. Mineral crystals growing inside those pores narrow them. The effect is gradual — you will not notice it in a month — but over several seasons of very hard water it shows up as an olla that empties more slowly each year despite being clean on the surface. The same logic applies to any dissolved salt you introduce, which is the main reason liquid fertiliser does not belong in an olla; we cover that separately.

Salts left in the surrounding soil are a second consideration. In arid regions with hard, mineral-rich irrigation water, salt accumulation in the root zone is a recognised concern for any localised irrigation system, and periodic leaching with low-salinity water is standard advice^[3][4]. A heavy seasonal rain does that job for free in most climates.

04 · THE CLEANING ROUTINE

How to clean algae and mineral buildup out of an olla

Do a full clean once a year, ideally at the end of the growing season when the pots come out anyway, plus a quick brush-out mid-season if you see green at the neck.

• Empty and brush first, always. Tip out the water, then work the inside with a stiff-bristled bottle brush and clean water. Mechanical removal handles algae, biofilm and loose scale, and it is the only step many ollas ever need. Do the outside with a bristle brush too — never a wire brush or metal scraper, which gouge the clay and damage the pore structure.
• Soak in dilute vinegar for carbonate scale. If chalky deposits resist brushing, fill the olla with a solution of roughly one part household white vinegar to three or four parts water and leave it for a few hours. Acetic acid dissolves calcium carbonate; you may see fine bubbling. For heavy external crust, sit the pot in a tub of the same solution. Then brush again.
• Rinse very thoroughly afterwards. This step is not optional. Porous terracotta absorbs whatever it soaks in, so flush the vessel several times with clean water and then let it stand full of plain water for a few hours and empty it once more before it goes back in the bed. You are removing residual acid from the wall, not just the surface.
• Never use soap, detergent or bleach. Surfactants and detergent residues lodge in the pore network of unglazed clay and are extremely difficult to flush out; they can alter how water wets the wall and reduce output. Bleach is likewise unnecessary near an edible bed. Brush plus vinegar plus rinse covers everything an olla realistically needs.

One optional extra for a badly clogged pot: after cleaning, let it dry completely for several days before reinstalling. Drying can help loosen residual deposits and gives you a chance to check the wall for the hairline cracks and spalling that indicate frost damage. Our full olla cleaning and maintenance guide walks through the whole seasonal routine.

05 · WATER QUALITY

Rainwater versus hard tap water for filling ollas

If your kettle furs up quickly, your ollas will scale quickly too. Water hardness varies enormously by region, and in limestone catchments it is high enough that the difference is visible within a single season.

Rainwater is naturally very low in dissolved calcium and magnesium, which is why it is the standard recommendation for lime-sensitive plants and for any application where scale is a nuisance^[13]. Filling ollas from a rain butt means essentially no carbonate deposition, no gradual pore narrowing, and no white bloom on the neck. It also costs nothing. If you go this route, screen the barrel inlet with fine mesh and keep the lid closed — an open rain barrel is a genuine mosquito breeding site in a way a capped olla is not^[12].

If tap water is your only option, that is perfectly workable — ollas have been run on whatever water was available for millennia. Just build the annual descaling soak into your autumn routine rather than treating it as an emergency repair, and expect to do it every year rather than every third year. Avoid using softened water from an ion-exchange softener, which replaces calcium with sodium and adds a salt you actively do not want in the root zone^[4].

06 · IS OUTPUT ACTUALLY DOWN?

Telling buildup apart from normal olla behaviour

Before you blame scale for a slow olla, rule out the far more common explanations. An olla releases water in response to soil suction, so its rate is supposed to vary. Wet soil after rain, cool weather, low crop demand and a young planting all slow it legitimately — a drier soil draws more, a wetter soil draws less^[1][15]. An olla that empties slowly in a wet fortnight is behaving correctly.

Poor soil contact is the next suspect. If the vessel was backfilled loosely, air gaps break the capillary path and the pot will barely empty regardless of how clean it is. Burial depth matters too — see how deep to bury an olla pot. Root felt on the outside and organic sludge in the base come next.

Genuine mineral clogging shows a specific pattern: a slow decline in output year on year, in hard-water areas, on a pot that is clean, well seated and in dry soil, with visible chalky deposits that fizz under vinegar. That is the case for a descaling soak. Everything else is either normal regulation or an installation issue.

07 · PREVENTION

Keeping an olla clean with almost no effort

Almost all olla maintenance reduces to four habits. Keep the lid seated so no light gets in and no debris falls in. Refill with rainwater where your tap water is hard. Put nothing but plain water in the chamber — no liquid feed, no fish emulsion, no additives, because every dissolved solid you add is another deposit left behind in the pore network. And do one thorough brush-and-rinse a year, adding a dilute vinegar soak only when there is scale that actually resists the brush.

Done that way, buildup never becomes the problem people worry about. A well-made unglazed vessel that is capped, filled with clean water, and brushed out each autumn will hold its output for many seasons — which is the real argument for buying a properly fired olla in the first place rather than improvising. Our complete guide to olla clay pots covers what to look for.