How to Grow Potatoes Without Soil

You can grow potatoes without soil using the Kratky method. A seed potato sits in a basket of loose medium in the lid of a deep, dark container of nutrient solution. The roots drink as the level falls, and new tubers form in the medium under the lid. This guide covers the whole process.

What you need

An opaque container with a lid. Deep storage totes and 5-gallon buckets are common choices, and bigger is better for potatoes.
A net pot or basket deep enough to hold several inches of medium.
Perlite, coco coir or a mix of the two, to fill the basket and give tubers a place to form.
Certified seed potatoes.
A complete hydroponic nutrient, a pH test kit or pen, and pH down.
Full sun outdoors, or a grow light indoors.

Step 1: Choose and prepare seed potatoes

Use certified seed potatoes rather than supermarket potatoes. Seed potatoes are inspected for disease, while grocery potatoes may be treated to stop them sprouting. Let them sprout (chit) in a bright, cool spot until short, sturdy shoots appear. Small seed potatoes can go in whole. Larger ones can be cut into pieces with at least one or two eyes each. Let the cut faces dry for a day or two before planting.

Step 2: Set up the container

Fill the container with nutrient solution mixed to the label's strength, then adjust the pH. Set the basket in the lid so that its base just touches the solution. Place the seed potato in the medium with the sprouts pointing up and cover it with more medium. Check that no light gets in through the lid, the walls or the gaps around the basket.

Step 3: Let the air gap form

As roots grow down into the solution, the level drops. That gap is what makes the Kratky method work: the upper roots get oxygen from the air while the lower roots stay in the solution. Don't refill to the top. If you top up, add only enough to keep the lowest roots wet. Filling the gap drowns the roots that have adapted to air.

Step 4: Keep the tubers in the dark

As the plant grows, stolons spread through the medium and swell into tubers. In soil, gardeners hill earth up around the stems. In a Kratky container you do the same thing by adding medium around the stems as they grow, as long as the basket has room. Any tuber that sees light turns green.

Nutrients and pH

Potatoes like a slightly acidic solution. Aim for a pH of roughly 5.8 to 6.5 and check it whenever you top up. Use a complete hydroponic nutrient. Tubers often start forming around the time the plant flowers. From then on, a formula with less nitrogen and more potassium puts the plant's energy into tubers instead of extra leaves.

Light and temperature

Potatoes need strong light for the leaves and a cool root zone for the tubers. Tubers form best at roughly 60 to 70°F (15 to 21°C), and tuber growth slows when the root zone stays above about 80°F (27°C). Keep containers out of hot spots and away from walls that soak up the sun. Indoors, a full-spectrum LED grow light works.

Harvesting

For new potatoes, you can gently feel through the medium for small tubers a few weeks after flowering. For a full harvest, wait until the foliage yellows and dies back. Early varieties are usually ready in about 70 to 90 days, and late varieties take 110 days or more. Lift out the basket, tip out the medium and pick out the tubers. Let them dry out of the light for a few days, then store them somewhere cool and dark.

Common problems

Green tubers: light is reaching the root zone. Cover any gaps and add medium. Cut away green parts before eating, and throw away potatoes that are green all over.
Rotting seed potato: it's sitting in the solution. Raise it so that only the base of the medium touches the solution.
Lots of leaves, few tubers: too much nitrogen, too much heat, or a variety that needs a longer season.
Slimy brown roots: root rot, usually from warm solution or a missing air gap.
Algae in the solution: light is getting in. Make the container fully opaque.