It’s a given that sometime during the growing season supplemental irrigation is going to be required if one is to grow good quality vegetables. Uneven precipitation can cause plant stress, which will affect both crop productivity and quality issues in your produce. Proper timing of water applications can increase the yield and quality of most vegetable crops in most years.
Drip irrigation is a system I first worked with back in the early ‘80s, working with Henry Johnson of Johnson & Co. Irrigation in Advance, NC. Henry and I developed a trailer-mounted portable pumping and filtering unit to utilize growers’ farm ponds as water sources to demonstrate the components, installation, operation and maintenance of a drip irrigation system that has since been adopted across the country.
Over the years I conducted workshops on plasticulture and drip irrigation and have watched the quality of drip tape improve. Drip system emitters in the tape have very small openings – about the size of a pinhole. Various emitters of different companies have differing internal flow characteristics that determine how sensitive they are to pressure changes and the quality of the irrigation water. A 150- to 200-mesh screen, disk or sand filter is usually required for water filtration. Periodic chlorine injections are used to keep the system free of algae and bacterial slime. Acid injections, along with periodic system flushes, help remove mineral buildup in some systems. Backflow prevention devices should be used to keep the water supply from becoming contaminated by fertilizer being injected into the system.
Most emitters in drip irrigation tapes operate at pressures of 5 to 20 psi, with flow rates of 0.5 to 2.0 gallons/hour. Emitter spacing depends on the discharge rate and soil type, because most of the water will be distributed through the soil. Pressure moderating or flow control tape may be necessary to achieve uniform water application on rolling terrain.
Plastic mainlines and header pipes, ranging in size from one to four inches, are common. These pipes are often buried with risers coming up at intervals where an annual line is connected. Drip emitter lines run down rows to provide water to plant roots. Often fields are broken into zones in order to minimize the pump and mainline sizes. Typically, small electric, gas or diesel pumps of less than 100 gpm are used. The pump, filter, mainline, headers and drip lines are laid out in the field annually for most vegetable crops. These systems are quite easy to automate, and are often controlled by a timer.
There are many advantages of using a drip irrigation, including more efficient water usage than sprinkler systems; adaptability for fertigation; reduced fungal problems because leaves are not wetted; low energy requirement because of the low pump pressures required; and low labor requirements and a high degree of automation. There are some disadvantages, such as blocked emitters and broken lines and connectors caused by machinery operations or freezes. Drip systems are not suitable for frost protection.
Since each site, soil type and season is unique, direct soil moisture measurement is an essential safeguard to avoid over- or under-watering. Of the common soil moisture monitoring techniques available, the use of tensiometers is best for monitoring drip irrigation. Despite substantial cost and the need for maintenance, tensiometers can provide crucial information.
The first consideration is tensiometer placement. One set of instruments should be placed in the zone of most active root uptake. Depth will depend on the crop; shallow-rooted crops like celery or lettuce should be monitored at eight to 10 inches; tomato and vine crops are evaluated best at 12 to 15 inches. It’s also important to understand what is happening to soil moisture at lower depths; another set of instruments installed eight to 12 inches below the shallow set will provide this information. To be sure that representative readings are obtained, tensiometers at each depth should be installed at several locations in a field, because no irrigation system is totally uniform, nor are field soils homogeneous.
To understand exactly what the tensiometer readings mean, one needs to know the soil moisture release curve for each field being monitored. Few growers have access to that information, so generalizations must be used. Optimum soil moisture usually is assumed to be near field capacity (the maximum amount of water a soil can hold against the force of gravity). In most sandy soils, soil matric potential at field capacity is 7 – 12 centibars (cb); in loam soils, 12 – 20 cb. In coarse- to medium-textured soils, tensiometer readings at 20% available moisture depletion are 10 – 15 cb higher than at field capacity. This means that for stress-sensitive crops, irrigation should commence before tensiometers exceed 17 – 22 cb in sandy soil or 22 – 30 cb in loam or clay-loam soil.
Shallow tensiometers indicate the moisture status of the active root zone; the deeper instruments indicate whether the amount of water applied is correct. After an irrigation, deep tensiometer readings should go down near, or even below, field capacity. If they don’t, it means the application was too light. Between irrigations, the deep tensiometer readings should come back up near, or slightly above, field capacity, indicating that deep roots are not permanently saturated. Failure of deep tensiometers to rebound between irrigations means the application was too heavy, too frequent or there is restricted drainage, which prevents movement of gravitational water.
Another useful soil moisture monitoring tool is the portable soil capacitance probe. It emits a radio frequency wave and measures the attenuation of the wave by the soil around the probe tip. This instrument is best suited to compare the relative water content of different areas or soil depths, identifying under- or over-irrigated areas. It’s a tool to augment, not replace, tensiometers. The major advantages of the soil capacitance probe are its portability and quick response time. A major limitation is that it can be difficult to insert the instrument deeper than 12 inches in many field situations.
Jon Davidson, Berry Hill Irrigation, said last year he sold out of drip irrigation kits in the first several months that were supposed to last all year.
Trevor Hardy, Brookdale Farm Supplies, said he sees more farm ponds being put into supply water and also growth in the use of drip irrigation. He noted he sees growth in the use of tensiometers to help manage drip irrigation and also disk filters for water filtration.
From these comments and my own observations, I see more drip irrigation being used in the future as water resources need to be utilized more efficiently. And finally, a good resource from Penn State, “Drip Irrigation for Vegetable Crops,” can be found at extension.psu.edu/drip-irrigation-for-vegetable-production.
Contact Bill with feedback or ideas for future columns at firstname.lastname@example.org.