Staked up foil is a BAD idea. When it is fresh and shiny, it makes your grow noticeable- to the point where someone passing by, or even flying over, might give a second look. Combined with pots and obvious disturbance, that screams COME BUST ME!!! When it is weathered, it is no longer shiny anyway, and can start to leach aluminum around your plants, whcih can lock out other nutrients.

Take that sandy layer and clayey layer and add a good helping of well rotted organic material- any type of old compost is great, especially rotted leaf humus. Composted manure is another good addition to a sandy soil to help it hold water better while providing nutrients to the plants. Mix everything up really well and let it sit through the winter rains if you can. Be VERY careful adding store-bought granular nutrients; read the instructions, and use less, or none, if you plan to give liquid feed too.