In gardening, pH refers to the acidity or alkalinity of the soil, which controls how well the plants can absorb the nutrients. All plants have different needs, and pH meters are used to measure the amount of pH in the soil. Use the pH meter by inserting the probes directly into the soil and reading the resulting measurement on the needle of the device. Electrical pH meters read the pH by measuring the electrical current between the two probes, and work best in moist soil.