What is hydroponics? A beginner's guide

Hydroponics is growing plants without soil. Instead of rooting into the ground, the plant sits with its roots in water that carries dissolved mineral nutrients, or in an inert material like clay pebbles or rockwool kept wet with that same solution. The word comes from the Greek for water and labour.

To see why it works, it helps to know what soil does for a plant. Three things, mostly. It anchors the roots so the plant stands up. It holds a reserve of water and mineral nutrients for the roots to draw on. And, less obviously, it holds air in the gaps between its particles, which the roots need in order to breathe. A hydroponic system supplies all three another way, with no soil involved.

Why grow without soil?

So why bother, when soil is free and plants have managed in it for millions of years? A few reasons.

A plant in a well-run hydroponic system never has to hunt for food or water. Both are delivered to the roots at the right concentration, so more of the plant’s energy goes into growth. Lettuce and other leafy greens can be ready noticeably sooner than the same crop in soil.

It also uses far less water. A recirculating system reuses the same solution instead of letting it drain away into the ground, so it can grow a crop on a fraction of the water an open field needs. That counts for most where water is scarce.

And you can grow where there is no soil worth using. A flat roof, a balcony, a warehouse, a desert research station or the middle of a city all work, because you bring the growing medium with you. Indoors under lights, you can grow all year, whatever the season outside.

What the plant needs

Plants make their own sugars from light, air and water. What they cannot make are the mineral elements, the nitrogen, phosphorus, potassium, calcium, magnesium and the rest, that they use to build proteins, chlorophyll and cell walls. In soil these come from the ground. In hydroponics you supply every one of them, dissolved in the water, as a nutrient solution.

This is the part beginners underestimate. A hydroponic plant depends completely on what you put in the tank. You mainly watch solution strength and pH. Electrical conductivity gives you the strength, because dissolved nutrients carry a current and pure water does not. pH controls whether the roots can take those nutrients up at all. Most crops want a slightly acid solution, around pH 5.5 to 6.5. Let the pH drift and a plant can starve in a tank full of food, because the nutrients are present but locked in a form the roots cannot absorb.

The main systems, simplest first

Systems range from a jar on a windowsill to a computer-controlled greenhouse, but they all solve the same three problems in different ways. Start with the simple ones.

Wick systems are the kitchen-science version, with no pump at all. A wick runs from a reservoir of solution up into the pot, and the growing medium draws the liquid up it like a paper towel dipped in water. A wick can only move so much, so this suits herbs and small leafy plants rather than a thirsty tomato.

Deep water culture hangs the roots straight down into a tank of solution, with an air pump bubbling away to keep the water full of oxygen. It is cheap, simple and fast. For most home growers, this is the sensible first system. The passive version, often called the Kratky method, skips the pump and leaves an air gap above the water that the upper roots breathe from as the level drops.

Ebb and flow floods the roots with solution on a timer, then lets it drain back to the tank. The draining is half the point, because it pulls fresh air down around the roots between floods.

Nutrient film technique, or NFT, runs a thin film of solution down slightly sloping channels, so the lower roots sit in the flow while the rest stay in moist air above it. The channels of leafy greens in the photograph at the top are an NFT setup, and it is the workhorse of commercial salad growing.

Aeroponics goes furthest and mists the bare roots with solution in an enclosed chamber. It gives superb growth and uses the least water of all, but it is fiddly and unforgiving, so it is not where a beginner should begin.

Roots still need air

Remember the third job soil does, holding air for the roots? It is the one a beginner is most likely to get wrong. Roots need oxygen as much as leaves need light. Drop a plant’s roots into still, stagnant water and they suffocate and rot within days, the same root rot that kills an overwatered houseplant. Every system above gets air to the roots somehow. An air pump bubbles the tank, a thin film leaves the upper roots in the open, or a timed flood drains and lets air back in. If a system has gone still and smelly, oxygen is usually what it is short of.

What grows well, and what does not

Fast leafy crops are the easy win. Lettuce, spinach, basil, mint and other herbs grow quickly, stay small and are ready before much can go wrong. Watercress, which grows in moving water in the wild, takes to it with no trouble at all.

Fruiting crops like tomatoes, cucumbers and peppers work well too, but they are taller, hungrier and thirstier, so they need support, a stronger solution and more attention. Root crops such as carrots and potatoes are the poor fit, because there is no soil for the root to swell into and no simple way to hold it steady.

What it is good and bad at

So, is it worth it? That depends on what you want from it.

You get fast, clean, predictable growth in a small space, on very little water, in places where growing in soil is hard or impossible. That is why it supplies salad to cities, herbs to supermarkets, and fresh vegetables to research stations and, yes, the space station.

Against that, the system has to keep working, and you have to keep checking it. There is no reservoir of soil to buffer a mistake. Miss the pH for a week and growth stalls. Lose power to the air pump on a hot afternoon and a deep-water crop can be dead by evening. A disease in the tank reaches every plant through the shared water, which is why cleanliness matters more here than in a border. And running lights and pumps indoors uses electricity, which can cost more than the water it saves.

Getting started

If you want to try it, start small and passive. A single deep-water tub or a Kratky jar with a lettuce or some basil needs no pump and almost no kit, and it teaches you the two numbers you have to watch, pH and solution strength, on a plant you can afford to lose. Get one leafy crop to harvest before you spend money on channels, pumps and lights. Larger systems use the same principles, with more plumbing, monitoring and failure points.