Use large 2-4mm polymer. 15 gal use 1 cup . Dig hole at least three times the size of the container and incorporate polymer through out the area. After planting, create a dam around the tree or if on a slope leave the topside open to collect water thus allowing a water reservoir to be readily absorbed by polymer. Adding additional smaller polymer also helps capture water faster.
Application can be either by hand or a lawn spreader.
Never use more than 5 pounds per 1,000 square feet tilled only one inch as prolonged sponginess may occur. After first rain hydrated polymer may appear and the sod may seem loose, however the polymer will dissolve and after the roots grow the looseness will dissipate. Adding additional smaller polymer also helps capture water faster. For established lawns in severe droughts, apply polymer to the dry grass and cover with about 1 inch of soil, then soak thoroughly with water. Use less for drier plants and more for moisture loving plants.