The porosity of your soil is essentially the rate at which your soil drains water. Often, when you read planting instructions, they state such things as "prefers well-draining soil," "tolerates dry conditions" or "does not like wet feet." Therefore, knowing the porosity of your soil is essential to a successful garden. Testing will guide you in the right direction as to what types of plants and trees to grow in your soil as well as help you determine if you need to amend your soil, such as by adding in drainage pipes or organic matter to help or slow drainage.
Dig a hole 1 square foot wide in diameter and 12 to 18 inches deep.
Fill it with water and wait until it completely drains.
Fill the hole with water again and time how long it takes for all of it to drain out. Soil that drains water well averages a water loss of 1 inch per hour. Slower than that and your soil does not drain water well and retains it instead. If it drains faster than an inch an hour, your soil is very porous and will quickly dry out.