At the start of this year I bought some large wooden planter boxes (lined on the inside with root cloth), filled them up with generic purpose potting soil and got to planting some homegrown vegetables. While the harvest (peas, rocket, purslane, parsley, coriander, monks cress) wasn't half bad, the plants all seemed to have trouble with drought resistance, with the peas prematurely succumbing to mildew.
Indeed, when removing the plants after harvest, I found that the roots of all plants were very superficial, barely going deeper than a few cm. I did not give any additional fertilizer (as I thought there was plenty of fresh soil to go about) and only watered occasionally, so I would expect the plants to form deeper roots to find more water and nutrients.
How can I improve the soil to encourage deeper roots and hopefully in turn improve drought resistance?