About 3 weeks ago I planted some marigolds from seeds. They are now 2 to 5inches tall. The smaller ones are looking yellowish. I water them at least 3 times a week and they get enough sun. What could it be?
With regard to nutrient defficiencies; yellow leaves is definately a lack of Nitrogen. Though, I would warn against just throwing some salt based fertilizer with a high N value on your flowers. Get some liquid seaweed fertilizer or even better some liquid fish fertilizer.