I used to grow annuals on my deck in large containers filled with soilless mix. The soilless mix was essential for drainage in spring and early summer when rains were plentiful, but when the rain stopped falling in July and August, I had to water every day. So I tried a combination of soil and soilless mix in my containers: a core of good clay loam surrounded by soilless mix.
I chose a bucket that was narrower at the top than the bottom of my container and 2 inches shorter. I filled the bucket with moist soil taken from a well-drained area and packed it down lightly. I turned the bucket upside down inside the container and lifted the bucket off. Then I filled the surrounding space with soilless mix. Before adding the soil, I put a thin layer of gravel in the bottom of my pot to cover the center drainage hole. The gravel directs excess water sideways to the drainage hole. (You can skip the gravel if you drill several holes around the bottom edge of the container.)
The technique worked. Instead of watering daily, I watered the plants on my deck only once a week. The ability of the soil to hold nutrients, along with the reduction in the frequency of watering, allowed me to reduce my fertilizing to a single application of slow-release fertilizer in the spring.