Short answer: a Kratky solution that turns cloudy or smells sour or rotten usually has bacteria growing in it, most often on decaying roots or other organic material. Cloudiness straight after mixing is a different problem: it is usually nutrient settling out of the water. Algae make the water green rather than milky. Each one has a different fix.
A healthy Kratky container should smell of very little: perhaps a faint mineral or earthy smell, never sour, sulfurous or like a drain. The solution should look like slightly tinted water.
When it happens tells you a lot
Cloudy straight after mixing
If the water turns milky or leaves a sediment as soon as you add nutrient, the nutrient has formed a precipitate. With two-part nutrients the usual cause is mixing the two concentrates together, or adding the second part before the first has dispersed. The calcium in one part and the sulfates and phosphates in the other combine into solids that plants cannot use. This is why two-part nutrients come in separate bottles. A batch like this is best started again with fresh water.
Cloudy or smelly after a few weeks
Gradual cloudiness, slime on the roots or walls, and a sour or rotten smell point to bacteria. They feed on dead roots, decaying plant material that fell in, or organic fertilizer. As they grow they use oxygen the roots need, which is why smell and root problems so often arrive together.
Green rather than milky
Green water or a green film means algae, and algae mean light is reaching the solution. When algae die and decay they also use up oxygen and can encourage bad smells, so a green container can become a smelly one.
Check the roots
Whenever the solution smells, I lift a plant and look at the roots. Healthy roots are white or cream and firm. Penn State Extension describes Pythium-infected roots as having brown, dead tips, with brown tissue on the outside of the root that pulls off easily, leaving a strand of inner tissue exposed. It also notes that infected plants are stunted and can wilt at mid-day and recover at night. If the roots look like that, the smell is a symptom of root rot, not just dirty water. See root rot in Kratky.
Why still water is vulnerable
The Kratky method has no pump and no air stone. The roots get oxygen from the air gap that forms as the level drops (UC Master Gardeners). Anything that uses up oxygen in the water, whether bacteria, decaying algae or warm temperatures, hits a Kratky container harder than an aerated system. The U.S. Geological Survey notes that cold water can hold more dissolved oxygen than warm water, so a container sitting in the sun is at a double disadvantage.
What I do about it
- Smell and look first. A faint odor with white roots is a warning. A strong rotten smell or brown, slimy roots means the container needs to be started over.
- Start over properly. I move the plant, if it is worth saving, to a clean container with fresh solution, and I do not top up the old solution. Penn State Extension stresses cleaning and disinfecting reservoirs and covering them to keep debris out.
- Clean the container. Wash out biofilm and residue before reusing it, then rinse well so no cleaner ends up in the next batch.
- Fix the cause. Lightproof the container if algae were involved, move it out of direct sun if it ran warm, and keep the air gap if the level had been raised.
Prevention
- Mix two-part nutrients into the full volume of water one part at a time, stirring in between.
- Use a lightproof container and lid. Kratky's own trials used darkened containers (Kratky, 2009).
- Keep the container out of direct afternoon sun. Oklahoma State University Extension gives 72 to 75 °F as the optimal solution temperature.
- Do not refill to the top once roots have grown into the air gap.
- Remove fallen leaves and debris.
- Think twice before using organic fertilizer in a still container; it gives bacteria plenty to feed on.
Quick questions
Can I fix smelly solution with hydrogen peroxide?
I do not rely on it. The sources this site uses do not give a dose for Kratky containers, and the cause, usually decaying roots or warm water, is still there afterwards. Starting over with clean equipment is more dependable.
Is a little cloudiness on day one harmful?
If it is precipitate, some of the nutrient is now unavailable to the plant, so the solution is weaker than you think. I remix rather than guess.
Related guide: Kratky nutrient solution problems and fixes. Also see how to prevent algae in a Kratky container.
Sources
- Moorman, G.W. Penn State Extension. Pythium.
- UC Master Gardeners of Orange County. Hydroponics for the Home Gardener.
- U.S. Geological Survey Water Science School. Dissolved Oxygen and Water.
- Dunn, B. and Singh, H. Oklahoma State University Extension. Electrical Conductivity and pH Guide for Hydroponics (HLA-6722).
- Kratky, B.A. (2009). Three non-circulating hydroponic methods for growing lettuce. Acta Horticulturae 843: 65–72.