It could also be nutrient lock out due to calcium deficiency. A easy symptom to look for is twisted stunted new growth.
Looking at saros' picture, I'm not that sure about that. Calcium is not mobile, it does not travel easily inside of the plant, and symptoms for Ca defficiency appear more clearly in new growth. Calcium defficiency will twist new growth and necrosize young leaves, but it should not make the whole plant turn yellow.
I'd try first foliar N fertilizer, but it's your pick.
Regards,
Mandrake
The reason i said that was; if there are several plants in the same area and only one or two have problems it might be a nutrient uptake problem. If it is a uptake problem it would present its self in several different ways, n deficiency, mag deficiency etc. This could be caused by several things, calcium is not mobile but it does play a key part in nutrient uptake and distribution, lack of beneficial fungi in the soil, pests in the soil (mealy bugs on the root), a build of of fertilizer salts, too much water or a host of other things. Nutrients can not be directly absorbed by plants, there has to be something there to convert it for the plant.
I did jump the gun and did not provide a clear explanation with my other post., I apologize.
Mandrake is right start with a foliar spray, if that does not help start slowly eliminating other possibilities before changing the soil composition.