My plant's leaves are turning yellow. How do I tell overwatering from underwatering from a nutrient problem?

Yellowing is a symptom with a dozen causes, but the pattern narrows it down quickly. Look at which leaves are affected, whether the veins stay green, and what the texture is.

Overwatering shows first on the lowest, oldest leaves. They turn uniformly yellow, feel soft or limp rather than crisp, and often the stem base is slightly mushy. The soil is damp and may smell sour. Roots pulled from the pot are brown and slough their outer layer when pinched instead of being firm and pale.

Underwatering also hits older leaves first but the texture is the opposite: dry, crisp, often with brown edges before the yellow spreads. The whole pot is light, and the soil may have pulled away from the sides.

Nitrogen deficiency yellows the oldest leaves uniformly while the plant keeps pushing pale new growth, because nitrogen is mobile and the plant cannibalises old tissue to feed new. Iron deficiency does the reverse — it hits the newest leaves and leaves a green net of veins on a yellow background, and it is usually caused by soil pH being too high rather than by an actual shortage of iron.

Sudden yellowing of several leaves at once, regardless of age, points at shock: a recent repot, a cold draught, a move to much brighter light, or a large temperature swing.

One caveat worth knowing: a single old bottom leaf yellowing and dropping every few weeks on an otherwise healthy plant is normal senescence, not a problem to be solved. Plants shed old foliage to reallocate resources, and treating that as a disease leads to a lot of unnecessary intervention.