Nutrients for Kratky Potatoes

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What a complete nutrient contains

• The main nutrients: nitrogen (N) for leaves and stems, phosphorus (P) for roots, and potassium (K) for tubers and overall health.
• Secondary nutrients: calcium, magnesium and sulfur.
• Micronutrients: small amounts of iron, manganese, zinc, boron, copper and molybdenum.

Fertilizers made for soil often leave out the micronutrients, because the soil supplies them, and organic fertilizers depend on soil life to break them down. In a container of water there's no soil to fill the gaps, so use a nutrient made for hydroponics, which dissolves fully and contains everything.

Types of hydroponic nutrient

• One-part nutrients (powder or liquid) are the simplest: one product, one measure.
• Two- and three-part liquids, such as the General Hydroponics Flora Series, let you change the balance as the plant moves from leaves to tubers by changing how much of each part you use.
• Three-part dry mixes combine a hydroponic fertilizer, such as MasterBlend 4-18-38, with calcium nitrate and Epsom salt (magnesium sulfate). They're often sold together as a kit, and they're popular with Kratky growers because they cost little per batch.

How to mix a nutrient solution

• Follow the label for the amount per gallon or liter.
• Weigh dry nutrients on a kitchen scale rather than measuring by the spoonful.
• Add each part to the full volume of water separately, and stir well before adding the next.
• Never mix the concentrated parts together. Calcium reacts with sulfates and phosphates in concentrate and drops out of the solution.
• Check and adjust the pH last, after everything has dissolved.

Changing nutrients through the season

Early on, the plant is building leaves and roots, and a standard vegetable mix suits it. Tubers often start forming around the time the plant flowers. From then on, a mix with less nitrogen and more potassium puts the plant's energy into tubers instead of extra leaves. With a multi-part nutrient, follow the maker's chart for the flowering or fruiting stage.

In a Kratky container you can't drain the solution and start again without disturbing the roots, so make the change through your top-ups. Mix the top-up solution at the new balance, and the container shifts toward it each time you add some.

pH and EC

Potatoes like a slightly acidic solution, roughly 5.8 to 6.5. Check the pH after mixing and whenever you top up. When the pH drifts too far, some nutrients stay in the water but the roots can't take them up, so a plant can go short even in a well-fed container.

EC (electrical conductivity), often shown on meters as ppm, measures how strong the solution is. Take a reading after the first fill so you have a starting point. If the reading climbs well above it, usually in hot weather when the plant is drinking water faster than it takes up nutrients, top up with a weaker mix. If it falls, the plant is using nutrients faster, and a normal-strength top-up is fine.

Signs of a nutrient problem

• Older, lower leaves turning yellow: often a shortage of nitrogen.
• New leaves yellow between green veins: often iron, and frequently caused by pH that's too high.
• Brown, crisp leaf edges: often a solution that's too strong.
• Lush leaves and few tubers: too much nitrogen late in the season, or too much heat.

These signs overlap, so check the pH first: a pH problem can look like almost any shortage. For the rest of the setup, see how to grow potatoes without soil.