Top, removing soil from the probe; middle, squeezing the sample by hand; bottom, hand showing wet outline of soil

Using stage of growth and weather conditions to schedule irrigation is a calculated guess at best. A better method is to evaluate the soil moisture condition by either sampling with a soil probe or short piece of thin-walled pipe or installing a soil-moisture-measuring instrument. The probe should be inserted into the bed 6 to 12 inches from the base of the plant, where roots are actively growing, or for closely spaced crops, in the plant row. To avoid cutting drip emitter lines, determine their locations before inserting the probe.

Nearly all garden crops grown under irrigation extract water from the top 2 feet of the soil profile. In fact, up to 95 percent of the roots are in the top 12 inches, so it isn't necessary to sample below a 12-inch depth when you use a soil probe.

You can estimate available soil water by appearance and touch. To estimate moisture content, remove a handful of soil from the probe and squeeze very firmly with your fingers. If the soil ball is squeezed immediately following an irrigation or heavy rain, water will drip from it. If this high level of moisture persists, plant health and performance could suffer, but if you're growing plants in raised beds, excessive moisture shouldn't be a problem. The ideal soil moisture content is indicated when, upon being squeezed, the soil ball emits no free water and only a wet outline of the ball remains on your hand. If there is no outline, the soil is dry and should be irrigated. Although measuring soil water by appearance and feel is not precise, with experience and judgment, you should be able to schedule irrigation with a reasonable degree of accuracy.

A more accurate method of measuring soil moisture to schedule irrigation involves the use of a tensiometer, a device that measures how tightly water is held in the soil which is an indirect indication of soil moisture content. A tensiometer consists of a sealed water-filled tube equipped with a vacuum gauge on the upper end and a porous ceramic tip on the lower end. As the soil around the tensiometer dries, water is drawn from the tube through the ceramic tip, creating in the tube a vacuum or tension that can be read on the gauge. When the soil water content is increased through rainfall or irrigation, water enters the tube through the porous tip, lowering the gauge reading.