Folhas a amarelar! Duvida?

How it progresses and wher it starts helps diagnose visible symptoms.. Visible symptom are not discrete, so it's better if you have more information than just what you see. Track your fertilization choices and let that help inform you. This is interveinal chlorosis -- paling between the leaf veins. Also, starting at the top, so it isn't nitrogen deficiency with great confidence. Unfortunately, more than one things causes interveinal chlorosis. Because it's happening at the top and there's no necrotic spots, this looks like a trace element issue - Zn or Mn, but this would be very rare and more often caused by pH issues or providing too much of a particular nutrient that it locks out other nutrients. Too much p or K could be the cause if you recently amped that up as part of a "bloom phase" fertilizer. I think this is more about something out of balance than something lacking. This stuff doesn't always happen at a fast pace. It could be slightly out of balance and take a month or more to show a symptom, or if more extreme, less time to show a symptom, but i would consider recent changes to fertilization.