Kratky Hydroponics

Take a 4-10 litre container that light cannot get through, or wrap a clear one. Light in the reservoir grows algae, and algae compete for the nutrients and foul the solution.

Cut a hole in the lid so a net pot sits in it with its rim supported and its base hanging below. The lid holds the plant and keeps light out at the same time.

Start lettuce, basil or another fast leafy crop in a rockwool or coir plug until the roots show through. Leafy crops suit this method; fruiting crops drink far more than one filling can supply.

Make up hydroponic nutrient at the stated strength. Plants in water depend entirely on you for every element — this is not fertiliser as a supplement, it is the whole diet.

Test the solution and bring it to about 5.5-6.5. Outside that band, nutrients are chemically present but the plant cannot take them up — deficiency symptoms in hydroponics are usually a pH problem, not a nutrient shortage.

Fill until the liquid touches the bottom of the net pot — no higher. Submerging the whole plug drowns the young roots before they have anything above water to breathe with.

Mark the level and the date on the container. Every measurement afterwards is taken against this line.

Give it strong light — a sunny windowsill or a grow light — and a steady temperature. A cold reservoir stalls growth; a hot one loses dissolved oxygen and encourages root rot.

Do not top the reservoir up. The falling level is the mechanism — refilling it drowns the air roots the plant has just built and defeats the entire method.

Record the water level and the plant's height or leaf count on the same days. You are measuring transpiration directly, and the two curves rise and fall together.

Look into the container as the gap opens. Roots below the line stay fine and pale; roots in the air gap thicken and often develop root hairs. This visible division of labour is the thing worth seeing — one plant, two root environments.

Set up a second container filled to the brim and topped up whenever it drops. It will grow poorly and may rot at the crown. That failure is the proof of what the air gap does.

Harvest a leafy crop before the solution is gone entirely. A plant that drains the reservoir wilts fast because there is nothing left to draw on.

Weigh the harvest and divide by the litres consumed. Grams of food per litre of water is the number that matters when comparing growing methods, and now you have measured it yourself.

Where it comes from. The method is named after Bernard Kratky of the University of Hawai'i, who published the non-circulating approach in the horticultural literature. The observation behind it came during a sabbatical in Taiwan in 1985, where he saw farmers growing healthy vegetables in waterlogged fields — the crops survived because part of the root system stayed above the water in raised beds, breathing air. The suspended-pot system is that observation turned into a design.

The air gap is the aeration. Roots need oxygen. Conventional hydroponics supplies it by bubbling air through the solution or by circulating a thin film, both of which need a pump and electricity. Kratky lets the plant open its own air space by drinking, so the upper roots sit in humid air with all the oxygen they need. No pump, no power, no moving parts to fail.

Where it stops working. One filling has to last the whole crop, so this suits fast leafy plants — lettuce, herbs, greens — and not tomatoes or cucumbers, which drink far more over a longer season. Large-scale versions exist, but the constraint is real and it is set by total water demand rather than by the technique.

The honest environmental accounting. Hydroponics uses dramatically less water than field growing because nothing drains away or evaporates from soil, and Kratky adds no electricity at all. Against that: it depends on manufactured nutrient salts, which are mined and shipped, and on plastic containers. It is a genuinely efficient way to grow greens in a small space with no power — and calling it self-sufficient would be wrong, because the fertiliser comes from somewhere.

Why it is a good teaching system. Every variable is visible. You can see the water level fall, see the root systems differentiate, and measure transpiration with a ruler. Very little else in plant science is this legible without instruments.