Every spring we bring in a pick up load of playground sand. there's no gravel under it. The nightcrawlers come out at night and crawl around and are constantly stirring it up. so by April every year it's time for a new,fresh load. It's pretty inexpensive here- $22.00 for a full size pickup truck. and that includes the heavy equippment scooping it up and dumping it in the truck. luckily we don't have to shovel it in ourselves, LOL. The sand keeps the run so clean. I don't like muddy birds !!!
