Find a hollow steel golf club shaft to use as your soil probe. Make sure the shaft is a little more than 3/8 inch in diameter and about 4 feet long. A long rod will make the probe easier to use. Cut the head of the golf club off using a tube cutter and throw it away. Make the cut 3 inches to 6 inches from the club's head.
Make a notch in the end of the steel bar, where the golf club head used to be, using a grinding wheel. Start grinding between 1/4 inch and 3/8 inch from the end of the rod, making sure this initial cut is parallel with the rod's end cut.
Grind so that the cut goes down to half the diameter of the rod, and continue grinding toward the golf club's grip area so that you have created a 3-inch slot. Taper back the end of the 3-inch slot that is closest to the grip area so it takes up the full rod diameter.
Clean out the 3-inch slot and the rod's end cut using a round metal file. Brush off any metal shavings.
Test your homemade soil probe. Push the rod's end cut down into the ground so it penetrates the soil. When you pull the probe out of the ground, you will have a soil plug -- or sample of soil -- in the 3-inch slot. Tap the soil probe and release the soil sample from the notch to use for moisture and pH testing.