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 or 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. Basically, 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 (see Table 12 below). 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 to 0.72
Fine sand 0.48 to 1.08
Loamy sand 0.72 to l.44
Sandy loam 1.32 to 1.80
Fine sandy loam 1.68 to 2.16
Loam and silt loam 2.04 to 2.76
Clay loam and silty clay loam 1.68 to 2.52
Silty clay and clay 1.56 to 2.16

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 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.

Soil tensiometers for measuring soil moisture are available at a cost of $75 to $100. They can be purchased through several field equipment suppliers.

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 to 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. During irrigation, the tensiometer indicates when field capacity has been attained at the depth of the porous tube.

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.  

Remember, if you wait for crop symptoms (wilting) to decide when to irrigate, the crop will already be damaged.

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. But 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.

Critical Periods for Moisture Needs

Vegetable crops should not be under stress at any time. However, 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. (See Table 13 below).  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