A photo of the whole plant would help. Position of the leaves at various stages of yellowing would give a better idea of what's happening. Just normal senescence could also be a possibility as plants will withdraw some minerals from older leaves.

Might be the pH of the soil. Acidic soils can block uptake of nitrogen and possibly other nutrients.

Too much water?

It looks like it's only the oldest leaves are affected. The new/younger leaves look quite good. Lack of Nitrogen seems to affect all the leaves so that there's yellowing to a greater or lesser degree over all. Magnesium shows up mainly in the older leaves leaving a bit of green in the centre or along the veins and yellowing further towards to edges. Lack of Iron is a bit similar to lack of Magnesium but it tends to affect the whole plant.