Generally here in the Pacific NW west side, we have pretty heavy soils. You may want to mix in some sand for better drainage. Also, be sure the boxes have some kind of drainage--which you probably figured out already!

You might want to check with your local County USDA Agricultural Extension people, who should have some literature on the subject that will be geared to your local conditions.
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May your trails be crooked, winding, lonesome, dangerous, leading to the most amazing view--E. Abbey