Yellowing can be caused by lots of things, but you're right to suspect lack of nutrients (nitrogen in particular), because that's the most common culprit. If you REALLY want to know what's going on, send a soil test off to a qualified lab. Pragmatically speaking though, you might be better off giving everything a shot of nitrogen fertilizer and then observing what happens next.

Fish emulsion and quail manure are both high in nitrogen, so they should do the trick if nitrogen deficiency is the problem. Be advised that technically the quail manure should be composted prior to use, as it can contain pathogens. Composting it destroys those pathogens. I'd go the fish emulsion route for now. Mix it up according to directions (probably 2 TBSP per gallon), and give everything a good drink. Do this twice per week for about 3 weeks and see if things pick up.

-Diggity

Anna, it might surprise you, but nearly every plant on earth has very similar nutrient requirements. When we grow plants in the ground we typically count on the soil to store a good amount of nutrients and so we can often just add some organic matter and maybe a soluble fertilizer every now and then and grow terrific, healthy plants.

Growing in containers is a completely different world.

The organic stuff you are using in containers can work, but it's iffy. The reason is that soil-less potting mixes and containers in general just don't and can't support the rich biodiversity in soil life that the earth does.

You will be much better served using synthetic, immediately available, water soluble fertilizers in containers.

There is one fertilizer on the market I recommend above any other I have found and that is called Foliage Pro and is made by Dyna-Gro. It contains all plant nutrients, is immediately available to plants and the NPK ratio along with the ratio of secondary and minor nutrients is appropriate for nearly any plant.

I will refer you to a thread on the container gardening forum for more info.