That's too early for a sativa to be yellowing.
Get your pH down in the 6.5-6.8 range and mix in a very small (like 1/4 the recommended dosage) amount of fish emulsion into your next feeding to slow down the yellowing. Or a half-strength 'grow' fert for one of two waterings. You want to see the leaves turning yellow about 2-3 weeks before harvest at teh earliest; the last week only is much better.
stinkyattic Reviewed by stinkyattic on . sativa is turning yellow Here is some pictures of my Sativa its in four weeks into flowering , the bottom leafs are turning yellow rapidly working it way up an falling off. Their getting plenty of potassium ,phosphorus .ph level is 7. Rating: 5