An important soil characteristic, the percolation rate, measures how long it takes water to drop one inch in a saturated hole dug in soil. Fast: 1 inch in 3 minutes (sandy soil). Slow: 1 inch in 48 minutes (clay soil). If it takes less than 5 minutes for the water to drop 1 inch in a saturated hole, the effluent will move too rapidly to be treated properly, such as in sandy soil.