When I was a child, one of my responsibilities included watering our backyard vegetable garden. To save water and time, my folks would often wait until the plants began to wilt before giving me permission to irrigate. Consequently, I grew up with a simple but flawed concept of irrigation scheduling. When plants wilt, give them a drink. End of story.
Fortunately, my understanding of crop water management has matured over the years. Today, I appreciate the cumulative harm that drought stress inflicts on produce's quality and yield.
Home and market gardeners can schedule irrigation several ways. Two methods we use at the Noble Research Institute include the soil ball squeeze test and tensiometers.
The squeeze test consists in using a soil probe or a short piece of thin walled pipe to extract a 12-inch column of soil from the garden. Remove a handful of soil from the probe and squeeze firmly with your fingers. The ideal soil moisture content is indicated when, upon releasing your grip, a wet outline remains on your hand. If there is none, the soil is too dry and should be irrigated. If water drips from the ball when it's squeezed, the soil is saturated. Plants can tolerate short periods of soil saturation. The problem arises when roots are deprived of oxygen for extended periods, because plants stressed as a result of oxygen deprivation develop slowly and are predisposed to disease. All but the coarsest of soils undergo saturation briefly during or immediately following an irrigation or rainfall: wait a couple of hours after watering before performing a squeeze test. Although the soil ball squeeze test is imprecise, with experience and judgment you should be able to schedule irrigation reasonably accurately.
If your soil is prone to waterlogging, consider constructing raised beds and installing drip irrigation. Elevating the growing medium will improve drainage. When properly managed, drip irrigation prevents soil saturation by delivering a low-volume supply of water in frequent doses.
Tensiometers, devices that measure how tightly water is held in the soil, more accurately indicate soil moisture. 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 moves from the tube through the tip, creating in the tube a vacuum, or tension, that can be read on the gauge. When the soil water content increases through rainfall or irrigation, water enters the tube through the tip, lowering the gauge reading.
Soil tension levels used to schedule irrigation vary with soil texture. In a sandy loam, irrigation should begin when soil tension reaches 20 centibars (a unit of pressure) and cease when it falls to 10 centibars. A soil tension reading of zero indicates complete saturation. In a finer-textured soil, such as silt loam, there is no need to irrigate until soil tension reaches 30 centibars.
For information on the use and care of your tensiometer, refer to the owner's manual. With proper care, a tensiometer can provide many years of useful service.
To water, or not to water: that is the question. Instead of guessing, consider using a tensiometer or performing a squeeze test. Proper scheduling of irrigation will help ensure maximum production with minimum watering.
For more detailed information on drip irrigation and water management, ask for Noble Research Institute publications Permanent Raised Bed Gardening, volumes I and II.