Answer: Yellowing leaves can be a sign of many things, but since you are still getting healthy fruit, it may be as simple as nitrogen deficiency. You didn't mention whether you fertilized or not, so I'm going to include info below. If you don't fertilize, you may want to apply a side dressing once or twice during the growing season, as annual vegetables are heavy feeders. Another possibility is improper watering. Tomatoes are very moisture sensitive, so try to keep soil moisture consistent, not going too long or staying too wet.
Here?s some basic info on fertilizer and nutrients that plants require. The 3 numbers on a fertilizer bag refer to the percentage of N (nitrogen), P (phosphorous), and K (potassium) in the bag. There are different formulations for different purposes. In general terms, nitrogen produces lush green growth, phosphorous helps strengthen stems and produce flowers, and potassium keeps the root system healthy. If you're applying fertilizer to fruiting (e.g., tomatoes) or flowering plants, you're not as interested in the plant developing leaves as you are in it flowers and fruit, so you'd use a formulation lower in nitrogen and higher in phosphorous, such as Miracle-Gro's Plant Food at 15-30-15. Bone meal is an organic source of phosphorous.
Organic sources of nutrients:
Nitrogen: alfalfa meal, blood meal, coffee grounds, cottonseed meal, fish emulsion, seabird guano.
Phosphorous: bone meal, rock phosphate
Potassium: greensand, seaweed, kelp
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