Early flower is when to increase phosphorous and potassium signifigantly and maintain adequate nitrogen levels, before reducing nitrogen mid flower.
Often the lowest/oldest leaves going yellow at this stage can be atributed to a lack of light penetration down low.
After looking at you diary, I would suggest your previous "cover crop/companion plants" have stripped a lot of the nutrients from the soil, so yes, a general lack of N/P/K would be a valid reason for the yellowing.