Short answer: the best nutrient for Kratky is a complete hydroponic nutrient that supplies calcium and micronutrients, mixed accurately once at the start. For most home growers that means one of four options: MasterBlend 4-18-38 with calcium nitrate and Epsom salt, a lettuce formula such as the one Dr. Kratky used in his University of Hawaii research, the General Hydroponics FloraSeries, or a complete one-part hydroponic fertilizer for a single jar.
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What makes a nutrient right for Kratky
A Kratky container is filled once and left. Dr. Kratky describes the method as needing no further watering or fertilizing after planting, so whatever goes in on day one has to carry the crop. That rules out a few things and favors others:
- Complete: nitrogen, phosphorus, potassium, calcium, magnesium, sulfur and the micronutrients. Garden fertilizers often leave out calcium and magnesium; see hydroponic nutrients vs regular fertilizer.
- Fully soluble: anything that does not dissolve is not available to the roots and can feed microbes in still water.
- Easy to measure at your batch size: a gram scale for powders, a syringe for liquids.
1. MasterBlend 4-18-38 with calcium nitrate and Epsom salt
Three dry powders. Masterblend's mixing sheet gives 2.4 g of 4-18-38, 1.5 g of magnesium sulfate and 2.4 g of calcium nitrate per gallon, dissolved in that order, at half rate for seedlings. The 4-18-38 bag lists chelated copper, iron, manganese and zinc, plus boron and molybdenum.
- Good for: growers filling many jars or totes, where dry powder goes a long way
- Watch out for: you need a scale, and all three parts every time
Full recipe and jar sizes: MasterBlend recipe for Kratky. MasterBlend 4-18-38 kits on Amazon.
2. A lettuce formula (8-15-36) with calcium nitrate and magnesium sulfate
This is the type of fertilizer Dr. Kratky used for lettuce. His University of Hawaii stock-solution sheet uses Chem-Gro 8-15-36 from Hydro-Gardens with magnesium sulfate in one concentrate and soluble calcium nitrate in another, mixed in equal volumes to an EC of 1.5 mS/cm. Without a meter, the sheet says to add 1 tablespoon of each stock per gallon of water. Masterblend also sells an 8-15-36 Lettuce Formula.
- Good for: lettuce and other leafy greens, the crops Kratky is best known for
- Watch out for: you are making two concentrates, which is more setup than a home grower with three jars needs
8-15-36 lettuce formulas on Amazon.
3. General Hydroponics FloraSeries
Three liquid concentrates: FloraMicro, FloraGro (2-1-6) and FloraBloom. General Hydroponics describes FloraMicro as the core source of nitrogen, calcium and chelated micronutrients, and says to add it to fresh water first. More FloraGro suits leafy growth; more FloraBloom suits flowering and fruiting. Doses come from General Hydroponics' feed charts, which list milliliters per gallon by growth stage.
- Good for: small batches measured with a syringe, and growers who want one system for greens and fruiting crops
- Watch out for: three bottles to measure, and doses that change with the growth stage
General Hydroponics FloraSeries on Amazon.
4. A complete one-part hydroponic fertilizer
For a single bottle, Dr. Kratky's simplest method used 5 grams of a complete granular hydroponic fertilizer (Chem-Gro 10-8-22) in a 4-liter bottle filled to within 4 cm of the top. One product, one measurement. The label must say it contains calcium and micronutrients and is made for hydroponics.
- Good for: one or two jars, classrooms and first crops
- Watch out for: check that the label lists calcium and magnesium, not just N-P-K
How I would choose
| Your setup | What I would pick |
|---|---|
| One to three jars of lettuce or herbs | A complete one-part hydroponic nutrient, or FloraSeries with a syringe |
| Many jars or totes of greens | MasterBlend 4-18-38 kit or an 8-15-36 lettuce formula, with a 0.01 g scale |
| Tomatoes, peppers or cucumbers | MasterBlend 4-18-38 (it is sold as a tomato formula) or FloraSeries with more Bloom |
| No scale and no syringe | Buy one or the other first; guessing doses is the most common mistake |
Whichever you choose, I would stick with it for a few crops. Learning how one product behaves in your water teaches more than switching every batch.
What I would avoid
- All-purpose garden plant food without calcium
- Organic liquid feeds in still water; see organic fertilizer in a Kratky system
- Products that list only N-P-K and no micronutrients
- Mixing two brands in one container
Quick questions
Is MasterBlend better than General Hydroponics?
Neither is better for every grower; they suit different batch sizes. The comparison is in MasterBlend vs General Hydroponics Flora for Kratky.
Do I need a bloom formula?
Not for greens. A bloom formula shifts the balance toward phosphorus and potassium for flowering and fruiting crops.
Related guide: choosing a nutrient for Kratky hydroponics.
Sources
- Kratky, B.A. 2009. Three Non-Circulating Hydroponic Methods for Growing Lettuce. Acta Horticulturae 843:65-72.
- Kratky, B.A. University of Hawaii CTAHR. Fertilizer Stock Solutions for Lettuce.
- Masterblend International. 4-18-38 Tomato Formula: Masterblend Mixing Instructions.
- Masterblend International. Tomato & Vegetable Formula 4-18-38.
- Masterblend International. Hydroponics.
- General Hydroponics. FloraMicro Calcium and Chelated Micronutrient Base.
- General Hydroponics. FloraGro Vegetative Growth Base Nutrient.
- General Hydroponics. FloraBloom Flowering and Fruiting Base Nutrient.
- University of Florida IFAS Extension. Set It and Forget It Hydroponics (HS1488).