Yellowing of the lower leaves can be caused by a few different things (and these are different things than if it starts with the upper leaves). How the leaves are yellowing can give you clues as to what is wrong.
If the whole leaf is yellowing, nitrogen deficiency is often the cause. If just the edges of the lower leaves are yellowing, potassium deficiency is often the cause. Too much potassium may cause lower leaf yellowing because it can inhibit magnesium uptake. Magnesium deficiency can cause yellowing of older leaves, but the veins may remain green.
Too much boron may also make the edges of the leaves yellow.
Large amounts of calcium may inhibit potassium, and too much magnesium will do the same in at least water plants. I guess this explains why giving my plants basalt rockdust seemed to give them potassium deficiency, with the edges of the leaves yellowing (rockdust is high in calcium).
I've heard a rumor that too much nitrogen inhibits potassium uptake, and perhaps vice versa (so, if true, too much of either can potentially cause some form of yellowing). I have not, however, found a reliable source to verify either of these claims. My personal experience makes the rumor seem as if it might be true, but not enough to be certain. I used to propagate this rumor, thinking it was something a lot of people knew (because I heard it more than once), but now that I've actually looked into it more, it may just be a rumor, after all. Too much nitrogen may inhibit roots, however (and potassium is supposed to help the roots), and, again, too much potassium can cause yellowing due to inhibiting magnesium.
In my experience, if you have lots of plants close together in a warm, humid environment, this may also cause yellowing of lower leaves, particularly the edges (probably due to some fungus in my area). It looks like potassium deficiency, but I've had it happen on plants that I had already given plenty of potassium previously. I'm not sure why this happens, but yellow discolored circles or patches, or bumps on the leaves can result, too (and usually do instead of it mimicking potassium deficiency). Giving the plants a cooler, dryer environment with more space has easily fixed this problem, for me. Lots of light is also important, but not as important as temperature, dryness and space. This happens much less on plants with enough potassium, even though it still may happen.