Answer: Tomatoes like rich, moist soil that's has plenty of organic matter and drains well. It sounds as if you are providing that. They also need consistent moisture for fruit to size up. I'm not sure what it means that the garden is included in the sprinkler system, but tomatoes should be water slowly, deeply and evenly, maintaining consistent moisture.
Tomatoes need nitrogen at the start of their growing for green healthy leaves. Try fish emulsion for an organic source of nitrogen. Phosphorous is needed to help flower and set fruit. Put it in the planting hole so it's ready for the roots to uptake. Bone meal is a good organic source of phosphorous. Or, you could use a product such as Miracle-Gro, which is 15-30-15.
As for varieties, you might want to try a tomato that matures quickly and is a smaller size, since you have a fairly short growing season. The larger beefsteak tomatoes are often harder to grow in short, hot seasons. I like Early Girl because it matures within 59 days. I also like Yellow Pear even though it takes 75 days because it puts on jillions of fruit and they don't have to be on the vine long. I hope this info helps.
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