Irrigation

In most years there are at least some periods of inadequate rainfall. Even a short dry spell can adversely affect crop yield and quality. Irrigation requirements differ somewhat among the various kinds of vegetable crops, but they all benefit from supplemental irrigation when needed. It is critical to manage soil moisture to provide an ample and steady supply of water to crops without overwatering. Table 13 lists periods of critical water need by vegetable crops. Special crop requirements will also be discussed in each crop section of this Guide. General irrigation guidelines are presented here.

Contact your local Extension office to determine what information is available in your state. There are also several knowledgeable irrigation equipment suppliers who serve the New England area who can be very helpful. As you begin to plan your irrigation program, keep in mind that it is usually best to look first at the highest value crops you grow as well as the anticipated increases in yield and income with the use of irrigation. When buying equipment and developing water supplies, consider future needs.

General Irrigation Guidelines

Soil Moisture

A soil is saturated when all of its pore spaces are filled with water. This is likely to be the case after a heavy rain or irrigation. After one or two days, when excess water has drained due to gravitational pull, the soil is at field capacity. At this point, the remaining water is attracted strongly enough to soil particles to prevent further drainage, yet it is readily available for uptake by plant roots. Soil water is further depleted by evaporation from the soil surface and transpiration through the plant leaves. The combination of these processes is called evapotranspiration (ET). As soil water is depleted, what remains is held more tightly by soil particles. As this happens it is increasingly difficult for plant roots to extract moisture from the soil. Eventually a point is reached where the remaining water is so tightly held by soil particles that it is unavailable to plants. This is the permanent wilting point or wilting coefficient. Soil moisture in the range between field capacity and the wilting coefficient is usable by plants and is called available water, although moisture stress will occur as the lower end of this range is reached. It is advisable to begin irrigating vegetables before half the available water has been used. If soil moisture is depleted below this point, plants will be under increasing stress, even though they may not show visible wilt symptoms at first. Moisture stress can greatly reduce yield and cause numerous disorders such as tip burn of leafy crops and blossom-end-rot in tomatoes and other fruits. It may be advisable to begin irrigation when as little as 30% of the available moisture has been used.

To achieve the most benefit from an irrigation system, it is necessary to apply the correct amount of water at the right time. This means replacing the water lost through ET and doing so before plants are under stress. The rate of ET is affected by a number of environmental factors including solar radiation, temperature, wind speed and relative humidity. When it is hot, sunny and windy with low relative humidity, up to 1/3" of water per day can be lost through ET. That is about 2" per week. When it is cool, cloudy and damp with little wind, losses are quite low. As canopy area increases, evaporation from the soil decreases due to shading, but transpiration from the leaves increases and generally ET increases. If weeds are present, their leaf canopy increases ET losses to the detriment of the crop. Evaporation from the soil surface is reduced by the use of organic mulch and nearly eliminated under areas covered with plastic mulch.

Evaporation pans can be used to estimate ET loss. Pans should be filled with a measured amount of water, such as 1", and placed in or next to the field in a sunny spot. The loss of water from an evaporation pan will approximate the amount lost through ET. Although this is not exact, it provides a good indication of the rate of loss when sprinkler irrigation is used. When irrigation occurs, the evaporation pans should be filled with the same amount of water as was applied to the field.

It is important to know the amount of available water that a particular soil can hold. This varies considerably with soil type. For example, a slit or clay loam can hold several times as much available water as a sandy soil (Table 12). Soil organic matter can substantially increase a soil's ability to store available water. It has been estimated that for each percent of soil organic matter, water holding capacity is increased by about 1/2" per foot of soil depth. This depends on the state of decomposition, but it is clear that organic matter has a profound influence on moisture holding capacity. Crops growing on soils with a high available water holding capacity require as much water as those on a soil of low available water holding capacity, because ET is about the same on both types, but the required frequency of irrigation is different. Soils with a high available water holding capacity need less frequent irrigation than those with a low capacity. However, when irrigated less frequently, a greater amount of water should be applied per application. This results in less labor for moving and setting up pipes and sprinklers.

Table 12: Available Water Holding Capacity Based on Soil Texture

Soil Texture Available Water Holding Capacity
(inches of water/foot of soil)
Coarse sand 0.24-0.72
Fine sand 0.48-1.08
Loamy sand 0.72-l.44
Sandy loam 1.32-1.80
Fine sandy loam 1.68-2.16
Loam and silt loam 2.04-2.76
Clay loam and silty clay loam 1.68-2.52
Silty clay and clay 1.56-2.16

When to Apply Water

In general, if you wait for crop symptoms (wilting) to decide when to irrigate, the crop will already be damaged. While experienced growers learn their soils and how they interact with water over seasons of experience, utilizing a soil moisture measuring device or sensor of some sort enables a grower to attach a number to their observations and track trends over time. Readings provided by sensors can also be utilized to fine tune irrigation management strategies and better manage the growing environment for specific plants.  Proper installations of sensors is critical for accurate readings. Sensors can quickly and easily be moved from one location to another in order to better understand the dynamics of soil moisture in relation to soil types, irrigation cycles, topographical changes, etc., so long as the installation instructions are followed along with each move.

Soil tensiometers for measuring soil moisture are available at a cost of $75 to $100. They can be purchased through several field equipment suppliers. To use a tensiometer, place the porous tube of the tensiometer at the depth you desire moisture measurement. You can calibrate your tensiometer to a particular soil so irrigation is done when the tension on the gauge reads a certain value (a number specific to your soil and the crop you are growing). A maximum value (usually 30-35) would be used for a sandy loam soil and this value may vary with the particular tensiometer purchased. In utilizing tensiometers, be certain you are aware of soil variability within a given field. Three to four tensiometers per field may be needed to adequately account for this variability and, in addition, two depths (commonly 6" and 12") may be necessary to adequately reflect the most critically stressed areas. The 6" unit indicates when soil moisture near the surface is being depleted (begin irrigating) and the 12" one shows when the moisture has moved to the bottom of the root zone (stop irrigation). In a drip irrigation system, a rule of thumb is to place the tensiometer about 6" from the tape at a depth of one-third the entire root zone. During irrigation, the tensiometer indicates when field capacity has been attained at the depth of the porous tube.

Another type of soil moisture sensor gaining popularity in the northeast is the granular matrix sensor. This category of sensors provides a reading based on the electrical resistance between two electrodes embedded in the granular matrix within the sensor. The more soil moisture available in the soil, the lower the resistance and the number on the reader. This resistance reading is reported in kilopascals (kPa) or centibars. This measure of resistance can be used to better understand the force a plant root must overcome to extract water from a given soil. 

Established guidelines for maintaining soil moisture in specific crops and soil types are available. Using these recommendations, paired with visually observing the crop and soil, provides growers with additional information on which to base their irrigation decisions. In most soils, other than heavy clay, the decision to irrigate would generally happen in the range of 30 to 60 kPa. Differences in soil type should be considered when determining the appropriate range in which to irrigate. This is because different soil types have varying levels of plant available water at various soil moisture readings. To clarify, a soil moisture tension reading of 40 kPa in a sandy loam would mean that approximately 50 percent of the water in the soil is plant available. Comparatively, a loamy sand soil might have only 35 percent plant available water at the same 40 kPa reading. 

This reinforces the importance of knowing your soil type, along with monitoring soil moisture and visually observing crops and soil to make an informed irrigation decision. Additionally, the irrigation method makes a difference as to when a grower might decide to irrigate. For example, overhead irrigation is recommended to begin when the available soil moisture is no less than 50 percent, whereas drip irrigation, taking comparatively longer to distribute substantial volumes of water, should be started before the plant available water drops below 80 percent. 

When using drip irrigation on plastic-covered raised beds, during rain events where less than one inch of rainfall has fallen, run the drip irrigation system as normal.  When greater than one inch of rainfall has occurred, delay the application of water through the drip irrigation system.  

Critical Periods for Moisture Needs

Vegetable crops should not be under stress at any time. Each crop has its particular periods of critical moisture needs. In many crops (such as sweet corn, beans, and peas), the most critical period is during or just after flowering. These crops have flower development in a much more concentrated period of time. Other crops (tomato, peppers, eggplant, and potato) also have a critical moisture need during fruit or tuber development. Check individual crops for details (Table 13). Some crops, such as onions, potatoes, pumpkins and winter squash, benefit from dry conditions at the end of the growing season when the crop is curing.

Table 13: Critical Periods of Water Need of Vegetable Crops

Crop Critical Period
Asparagus Brush growth
Snap beans Pod enlargement
Broccoli, cabbage, cauliflower Head development
Carrots, radishes, turnip, rutabaga Root development
Corn Silking/tasseling and ear development
Cucumbers, squash, melons Flowering and fruit development
Eggplants, peppers Flowering and fruit development
Lettuce Head development
Onions Bulb development
Potatoes Tuber set and enlargement
Tomatoes Early flowering, fruit set and enlargement

Note: These are stages of critical water demand, but vegetable crops should not be subjected to stress at any time during growth

Water Application Rate

The application rate of irrigation should not exceed the infiltration and percolation rates of the soil. Infiltration is the entry of water through the soil surface and percolation is the downward movement of water through the soil. If the application rate exceeds either the infiltration or the percolation rate, water will accumulate on the surface and is subject to run-off and erosion. Soil compaction inhibits both infiltration and percolation and should be minimized. Soil crusting also interferes with infiltration. Soil organic matter reduces soil compaction and crusting. Compaction and crusting can also be minimized by using appropriate tillage practices and restricting traffic over the soil.

Sprinklers

Sprinkler size should be chosen based on the crop, distance between laterals, pumping pressure and volume, and the infiltration and percolation capacities of the soil. Sprinkler placement should be staggered with those on adjacent laterals. This provides a triangular pattern in the field. Sprinklers are designed to operate with patterns that overlap according to the manufacturer's specifications. A triangular arrangement with overlapping patterns provides the most uniform coverage.

Wet conditions are favorable to most diseases. Time the use of sprinklers so that foliage can dry rapidly when irrigation is complete. This is usually in the morning. Irrigating thoroughly to achieve field capacity to a depth of the majority of the root system, and never permitting plant stress, can reduce diseases. Irrigating more often than necessary may encourage disease development by maintaining wet foliage for long periods of time. This can also leach nutrients.

Frost Control

Early- or late-planted vegetables can be subjected to freezing temperatures in spring or fall. Using an overhead sprinkler system that applies about 1/10" of water per hour, during periods when the air temperatures in the crop canopy are below freezing, can reduce or prevent crop losses. Greater or lower applications may be necessary depending on minimum temperature, length of freeze period, and wind speed. Be certain to start irrigating before the temperature reaches freezing and continue irrigating until ice melts. The degree of protection depends on wind speed and other factors. However, protection below 20°F has not been attained.

Trickle or Drip Irrigation

A well designed trickle or drip irrigation system benefits the environment by conserving water and fertilizer and requires little labor to use once it is set up. Water is applied either on the surface, next to the plant, or subsurface, near the root zone. In dry years, fewer weed seeds germinate between rows because there is less water available beyond the plant root zone, although weed roots may grow toward the wet zone, especially when plastic mulch is used. With drip irrigation there is less evaporation than with sprinklers. Evaporation losses from the soil surface are further reduced when drip irrigation is placed under plastic mulch.

It requires some expertise to install and operate a trickle system. Consultation with a knowledgeable professional is wise. A poorly designed system can result in yield variability in the field due to areas of over- or under-watering and clogged lines. Trying to save money by cutting costs on initial equipment purchases will likely be more expensive in the long run. Any or all of these problems can completely offset the potential cost savings from using drip irrigation, but they can be avoided by using good equipment and proper design and installation.

Water source. Organic materials such as plant materials, algae, small living organisms, and inorganic sand, silt, and clay are likely to be found in surface water such as a pond or stream. Well water is likely to have some sand, silt or clay particles, although not as much as most surface supplies. These particles can clog the small diameter emitters in the tape. Filters are used to remove particulate matter as discussed later. Surface water might have contaminants from run-off, which can include plant pathogens such as Phytophthora.

Slope. A slope of 2% or less is the ideal for drip irrigation. Many fields in New England have slopes greater than 2%. A difference of 2.3' in elevation will change water pressure by 1 psi, decreasing as elevation increases and vice versa. The length of lateral lines, the pump size, and pressure regulators are chosen based on the slope. It is best to run the rows horizontally across a slope, but if rows must run up and down, it is best to have the water flow downward through the drip tape. Water should also flow downward in headers. This probably requires that water be pumped to the top of the field so it can flow downward from there. With slopes greater than 5%, pressure-compensating drip tape is needed.

Soil. The soil type determines the soil wetting patterns. Soil wetting patterns in turn influence the depth of the drip tape and the distance between emitters. The duration and frequency of irrigation are also determined by the soil type. Over-watering can move fertilizer away from the root zone. On sandy soils, water goes primarily downward rather than horizontally so emitters should be at relatively close spacing. Spacing between emitters can be greater in heavier soils where there is considerable movement laterally. In sandy soils, irrigate more frequently, but run the water for a lesser amount of time. In heavier soils, irrigate less often, but run the water for a longer duration. In both cases, this should lessen the chance of leaching fertilizers away from the root zone.

Drip tape should apply water uniformly throughout the crop root zone. The emitters should be close enough so that there is uniform wetting of the soil. As discussed above, the spacing of the emitters is affected by soil type. Drip tape should have a coefficient of manufacturing variation (CV) number that reveals how much variation in uniformity there is from one emitter to the next. A CV of 0.05 is considered excellent and a CV between 0.05 and 0.1 is acceptable. The rate of water delivery is a function of the size and spacing of the emitters and typically ranges from about 2.5-5.0 gallons per minute per 1,000' of tape (0.25-0.5 gpm per 100').

The length of the drip lines is another important consideration. The length is determined by the pump size, the field size(s), and the slope of the land. Any one of these factors will influence wetting uniformity because the emitters will discharge water at different rates if there are changes in pressure along the line. Because of variation in water pressure, tape is rarely laid out longer than a length of 400' on fairly level land and less on slopes. Tape should run across the slope, but if this is not practical, it should run downhill so the pressure loss due to friction is counteracted by the gain as the elevation decreases. If the tape runs up hill, there is a double loss due to friction and increasing elevation.

The choice of tape thickness, measured in mils, is based on how long you want the tape to last and the expected highest water pressure in the lines. The longer the tape is expected to be in the ground, or the higher the pressure in the lines, the thicker the tape should be. Tape thickness is usually between 4 and 10 mil, though thickness of up to 25 mil can be purchased. Tape can be reused for two to three years, but the labor costs of retrieving and cleaning the tape usually make this uneconomical for annual vegetable crops.

Placement of drip tape. The tape should be placed as close to the plant as is practical for the specific crop. This is critical on porous soils, but if necessary, tape can be placed between 6" and 12" from the plants on soils with good lateral water movement. Tape is often laid between double rows of crops such as peppers. The soil will be wetter on the side of the crop where the tape is and most of the roots will be concentrated there. The tape should be placed so that the emitters are pointed upward so that any particulate matter will settle away from the emitters after the water stops flowing.

Tape can be placed on top of the soil or a few inches below the surface. If tape is on the surface, it is easy to observe wetting patterns and to make repairs if needed. The disadvantages are that there is greater evaporation in the initial stages of the crop's growth and the tape is more likely to be damaged by production practices, wind, and animals. As tape is heated by the sun during the day, it expands and takes on a snake-like pattern in the row. This is particularly a problem with higher temperatures under plastic. This can be avoided by burying the tape.

Pumps. For a given area, trickle irrigation uses less water and at a slower rate than a sprinkler system. Therefore smaller pumps can usually be used. The required pump capacity is determined by the flow rate of the tape, total length of tape to be used at a given time and pressure loss due to friction and increases in elevation. Pump size should be determined when designing a system.

Filters. The choice of filter is based on the quality of the water passing through the system and drip application requirements.  Typical screen mesh size is 150 mesh (100 micron). The filter should be sized for the longest application flow rate needed.  Water particulates change through the growing season and filter flow rates are affected with dirtier water and more suspended particulates.  It is better to design a system larger than is required, compared to smaller.

Screen filter. Screen or mesh filters are inexpensive and easy to install. Mesh filters work well if there are moderate to low contaminants in the water, such as those coming from a well or public water supply. Screen filters have a limited ability to store contaminants. If the water comes from a river or pond, the screens will probably have to be flushed often. This could result in considerable down time and labor.

Mesh screen sizes are between 20 and 200 mesh. The larger the number, the smaller the particle the screen will filter out. The screens are made from stainless steel, nylon, or polyester. Follow the recommendation of the tape manufacturer as to mesh.

Disc filter. A disc filter consists of a series of discs that are stacked on top of each other. The discs have microscopic grooves that radiate out from the center and filter out particles. Equivalent mesh sizes are between 40 and 600 mesh. They require less water for cleaning than do sand filters.

Sand filter. Sand filters are preferred over screen filters if the contaminant load is moderate to heavy. A sand filter can run longer than a screen filter before it needs to be cleaned by back flushing. This results in less down time and labor. The filters can be set up in pairs so that clean water from one filter is used to back flush the other filter. A screen filter can also be used to provide clean water for back flushing a sand filter.

The correct filter size is important. Under-sizing will increase pressure loss and there is considerable down time for cleaning. It is better to be too big than too small. The sand used for trickle irrigation should be of the correct type, made up of crushed, sharp edged silica or granite. 

Pipe/Mainlines. Mainlines can be metal pipe, PVC pipe or lay-flat hose. They deliver water from the pump to the submains and laterals. Proper pipe diameter is a function of the pumping rate and distance the water must travel. Larger diameters can move greater volumes and have lower friction losses, but are more expensive than smaller pipes. The longer the pipe and the more elbows or junctions, the more pressure loss due to friction. A qualified designer can help determine the appropriate pipe size for the system.

Care should be taken when laying the pipe to prevent soil or debris from getting into the system and clogging the lines. The lines should be flushed before the tape is connected to remove any dirt that got into the lines during installation.

Back flow preventers or check valves allow water to flow in one direction only. They are used to prevent water from flowing backward into a water source after the system is shut off. This is especially important to prevent injected fertilizer or pesticides from contaminating water supplies. They are required by law for systems with injectors. In some areas they may be required even if no injector is used. Vacuum-relief valves are installed to prevent soil from being sucked into the emitters when a vacuum is created after the system is shut-off.

Pressure regulators maintain the desired pressure as the water flows through the system. They are required to supply the appropriate pressure to tape and may be needed to protect filters and other components. They should be sized according to the rate of water flow. Drip tape typically operates at 12 PSI. 

Running the System

 

Fertigation

Fertigation is the injection of soluble fertilizer into irrigation water. Nitrogen and potassium are available in liquid or soluble solid form and can be applied through a drip system. Phosphorus, if needed, is usually broadcast at the beginning of the season.

By using a fertilizer injector, trickle irrigation can be used effectively to apply N and sometimes K during the growing season. The need for supplemental N can be determined using the PSNT as it is with other application methods. Samples for the PSNT should be taken from under the plastic, if used. Use a soil sampler to punch a small hole in the plastic and remove a core of soil. Be sure to avoid cutting the irrigation tape when sampling under plastic.

With conventional topdressing or sidedressing, it is common to apply all the N in one or two applications. With trickle irrigation, it is convenient to apply small amounts of N weekly or even daily, which is desirable from a N management standpoint. For example, if you want to apply about 50 lb N per acre, you can inject a little over seven lb N per acre per week for seven weeks, or about one lb per day if you prefer. Small weekly applications provide for more efficient crop use of N than one or two larger applications. Daily application offers little advantage over weekly application, but may be necessary if the injector cannot inject a week's worth of N during the appropriate irrigation run time. To prevent leaching, the irrigation system should not run longer than necessary to effectively wet the root zone of the crop. If there is not enough time to inject all the fertilizer needed for the week in one injection, then smaller, daily injections are preferable. Before injecting fertilizer, the entire system should be filled with water at full operating pressure. When all the fertilizer has been injected, the system should run long enough to flush all fertilizer from the lines. If fertilizer is left in the lines, clogging may occur due to chemical precipitates or growth of bacterial slimes.

Water Problems

Certain fertilizer materials may react with chemicals in irrigation water. If the water pH is below 7.0, there is little potential for problems, but at pH 8.0 and above, the risk is high. At levels above 40-50 ppm, calcium and magnesium are likely to react with phosphorus, if present in the fertilizer, causing precipitation of phosphates. If fertilizer containing calcium is added to water with concentrations of bicarbonates above 2 meq/liter, calcium carbonate may precipitate. Sulfates in fertilizers can react with calcium in the water resulting in the precipitation of gypsum. These precipitates can clog emitters.

Phosphorus- and sulfate-containing fertilizers, if needed, should be applied before planting because we are not concerned about these leaching. Nitrogen is the element that is most appropriate for injection into trickle irrigation water. Calcium nitrate has the potential to cause clogging if the water pH and bicarbonate levels are high, as noted above. If calcium nitrate causes clogging, potassium nitrate or urea can be used as an alternative N source.

Water testing labs can analyze water for pH, calcium, magnesium and bicarbonates. You can also perform a simple test: Mix fertilizer into a container of irrigation water at the same concentration it will be after injection into the trickle system. Cover the mixture to exclude dust and let it sit for at least the length of time it will be in the system before it reaches the soil. If the water becomes cloudy or a precipitate collects on the bottom of the container, you can expect this to happen in the irrigation system with the likelihood of clogging. If it is necessary to lower the water pH, acid can be injected into the irrigation water. This requires special handling precautions and special injection equipment. Be sure to carefully follow directions to avoid personal injury or damage to crops or equipment.