In a Kratky system the nutrient solution is the plant's only food. There is no soil to fall back on, so the mix has to supply everything, and a tomato's needs change as it moves from leaves to flowers to fruit.
Use a Hydroponic Nutrient
Hydroponic nutrients differ from soil fertilizer. Soil supplies trace elements such as iron, manganese and zinc on its own; water doesn't, so a hydroponic formula includes them. Use a nutrient made for hydroponics, ideally one with a tomato or fruiting-plant feed chart, and follow its label for mixing.
Two Stages: Grow, Then Bloom
| Stage | What the plant is doing | What to feed |
|---|---|---|
| Seedling and young plant | Building roots, stems and leaves | A standard (grow) hydroponic mix, at the label's seedling or reduced strength at first |
| First flowers onward | Flowering and setting fruit | A bloom mix, which has more potassium and phosphorus and less nitrogen |
Too much nitrogen late in the season gives a big leafy plant with few flowers. Switching to bloom when the first flower clusters open steers the plant toward fruit.
Measure Strength with EC or ppm
An EC (electrical conductivity) meter, also sold as a TDS or ppm meter, tells you how strong the solution is. Readings in ppm depend on the meter's scale (500 or 700), so compare like with like. As a common guide:
| Stage | EC (mS/cm) |
|---|---|
| Seedlings | About 1.0 to 1.5 |
| Fruiting plants | About 2.0 to 3.5 |
Your nutrient's own feed chart takes priority over these ranges.
The meter also tells you how the plant is drinking. If EC rises over a week, the plant is taking water faster than nutrients, so top up with plain water or a weaker mix. If EC falls, top up at full strength. That routine is covered in topping up a Kratky tomato reservoir.
Calcium and Magnesium
Fruiting tomatoes need steady calcium. Most complete hydroponic nutrients include it, and some growers add a calcium-magnesium supplement with soft or filtered water. Calcium in the solution is only half the story, though: it reaches the fruit with the water the plant draws up, so an uneven water supply can still cause blossom end rot.
Nutrient uptake also depends on pH. See Kratky tomato pH.
Back to the Kratky method for tomatoes.
Frequently Asked Questions
Can you use regular plant food for Kratky tomatoes?
Not on its own. Soil fertilizers assume the soil supplies trace elements, and in water it doesn't. Use a complete hydroponic nutrient.
When should you switch Kratky tomatoes to bloom nutrients?
When the first flower clusters open. From then on the plant needs more potassium and phosphorus and less nitrogen.
Do you change the solution in a Kratky tomato system?
No. In Kratky you don't drain and replace the solution; you top it up as the plant drinks, adjusting the strength by what an EC meter shows.