Kratky Yellow Leaves: What the Pattern Tells You

Yellow leaves are a symptom, not a diagnosis. The useful clue is which leaves turn yellow and how. In a Kratky kit the cause is usually one of five things, and they can be checked in a sensible order.

Read the pattern first

Yellow leaf patterns in a Kratky kit and their usual causes

What you see	Usual cause	Check
Older, lower leaves turn evenly yellow first	Nitrogen shortage	Solution strength; is the reservoir nearly empty late in the crop?
Newest leaves yellow, veins stay green	Iron not available, often from high pH	pH of the solution
Whole plant pale and stretched	Too little light	Hours and strength of light
Yellowing with wilting, in solution	Root problem (air gap lost, rot)	Lift the lid and look at the roots
One or two bottom leaves yellow, rest healthy	Normal ageing or shading	Nothing; remove the leaf

The order to check

1. Roots. They decide whether anything else can be fixed. White, firm roots mean the problem is above the lid. Brown, slimy roots point to Kratky root rot.
2. Water level. Has the reservoir run nearly dry? Has it been refilled to the top, flooding the air roots?
3. pH. Test a sample. Most Kratky crops prefer about 5.5 to 6.5; the Kratky pH guide explains testing and adjusting.
4. Nutrients. Was the solution mixed at the strength on the label, with a hydroponic nutrient? See the Kratky nutrients guide.
5. Light. Is the plant getting enough hours and intensity? See the Kratky grow lights guide.

Working in this order avoids the most common mistake, which is adding more nutrient to a plant whose real problem is its roots or its pH.

Nitrogen shortage

Nitrogen moves within a plant. When there is not enough, the plant takes it from older leaves to feed new growth, so the lower leaves yellow first and evenly. In a Kratky kit this happens when the solution was mixed too weak, when a general-purpose fertilizer without a full hydroponic formula was used, or late in a crop when the reservoir is almost used up. A plant near harvest can usually just be harvested. Earlier in the crop, a small top-up of correctly mixed solution, added below the air roots, can carry it through.

High pH and iron

Iron does not move within a plant, so a shortage shows on the newest leaves: they turn pale yellow while the veins stay green. The solution may contain plenty of iron, but as pH rises above the preferred range it becomes harder for roots to take up. Correct the pH in small steps rather than adding more nutrient.

Too little light

A plant short of light grows pale, thin and stretched, with long gaps between leaves. This is common on north-facing windowsills and in winter. More hours, a closer light or a stronger light usually fixes it within a week or two of new growth; old pale leaves will not turn green again.

Root problems

Roots that cannot breathe cannot take up nutrients, so a plant with damaged roots can look short of everything at once: yellow, wilted and stalled. The fix is at the root zone, not in the nutrient bottle. The guide to Kratky plant wilting covers the air gap side of this.

Yellowing in different crops

Lettuce shows nitrogen shortage quickly on its outer leaves, and tipburn (brown edges on inner leaves) is often mistaken for a nutrient problem when heat and fast growth are the cause. Basil turns yellow and then black when it gets cold, so a reservoir on a winter windowsill can produce yellow basil with nothing wrong in the solution. Mint and other vigorous herbs fill the reservoir with roots and can run the solution down faster than expected, so lower leaves yellow as the level drops. The guide to the best plants for Kratky lists which crops are easiest.

Normal yellowing

The oldest leaves at the base of lettuce and herbs often yellow as the plant grows and shades them. If the rest of the plant is green and growing, pick off the yellow leaves so they do not fall into the reservoir.

Frequently asked questions