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Irrigation
Moisture management in the soil is critical to vegetable production and growers should consider irrigation needs along with soil, markets and terrain when determining site suitability for production. Water deficit related problems can occur during years of ideal rainfall and 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. This section provides information on crop water requirements, basic guidelines for irrigating vegetable crops, scheduling irrigation based on soil moisture levels, water supply considerations, irrigation equipment, and fertigation.
Crop Water Requirements
Applying the proper amount of water at the correct time and location is critical for achieving the optimum benefits from irrigation. The crop water requirement, termed evapotranspiration or ET, is equal to the quantity of water lost from the plant (transpiration) plus that evaporated from the soil surface. Many factors must be considered when estimating the rate of evapotranspiration and subsequently a crop’s irrigation needs and these factors are discussed here.
Environmental conditions affect the rate of evapotranspiration of a plant. The most important factor is the amount of solar radiation (sunlight), which provides the energy to evaporate moisture from the soil and the plant. Other important factors are air temperature, wind speed, and humidity level.
Plant factors affecting the crop water requirement are crop species and variety, canopy size, leaf characteristics (size, shape, wax coating, and orientation), plant population density, rooting depth, and stage of growth and development of the crop. The plant canopy size and shape influences transpiration, light absorption, reflection, and the rate at which water evaporates from the soil. Crops that feature a canopy with more surface area for transpiration and sunlight interception (mature sweet corn, potatoes, snap beans) use more water than crops that do not have an extensive canopy (onions, immature plants, recently transplanted crops). Rooting depths vary with crop species and may be affected by soil compaction, pH, and hard pans. Rooting depth determines the volume of soil from which the crop can draw water and is important when determining to what depth the soil must be wet during irrigation. Table 12 includes the extraction depth of various crops in unrestricted soils. With uniformly deep unrestricted soil, about 70%–80% of soil moisture withdrawal by plant roots is in the upper half of the rooting depth.
Table 12: Effective Root Zone Moisture Extraction Depth in Unrestricted Soils
| Crop | Depth | Effective Root Zone |
|---|---|---|
| Chives, endive, escarole, fennel, lettuce, radish, scallion, spinach | Shallow | 0”–6” |
| Celery, onions (dry), shallots, Swiss chard | Shallow–Moderate | 0”–12” |
| Beets, broccoli, cabbage, carrots, cauliflower, collards, cucumbers, eggplant, horseradish, kale, kohlrabi, okra, peas, peppers, potatoes, rutabagas, snap beans, sweet potatoes, turnips | Moderate–Deep | 0”–18” |
| Asparagus, corn (sweet), lima beans, melons, parsnips, pumpkins, squash, tomatoes, watermelons | Deep | 0”–24+” |
Plant growth stage influences vegetable susceptibility to moisture stress. During the first 1–2 weeks of seedling or transplant growth, the root system is not yet established in surrounding soil and irrigation can significantly increase plant survival, especially when soil moisture is marginal. Irrigation can also increase the uniformity of emergence and final stand of direct-seeded crops. For direct-seeded crops, reduce the rate of application and the total volume of water per application to avoid crusting (cohesion of soil particles at the surface). If crusting occurs, continue to apply low rates of water at high frequency while seedlings are emerging. Keeping the soil surface moist will reduce the force necessary for seedling emergence. Water use by vegetable crops increases up to full canopy and then will decrease thereafter.
Table 13 shows the periods of crop growth when an adequate supply of water is critical for high-quality vegetable production. 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, pepper, eggplant, and potato) also have a critical moisture need during fruit or tuber development. 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 Needs by Crops
| Crop | Most Critical Period |
|---|---|
| Asparagus | Brush (period following fern mowing) |
| Beans (lima) | Pollination and pod development |
| Beans (snap) | Pod enlargement |
| Broccoli, cabbage, cauliflower, lettuce (head) | Head development |
| Carrots, turnips, radishes, rutabaga, sweet potato | Root enlargement |
| Corn (sweet) | Silking and tasseling, ear development |
| Cucumbers, squash (winter), melons | Flowering and fruit development |
| Eggplant, pepper | Flowering and fruit development |
| Onion (dry) | Bulb enlargements |
| Peas | Seed enlargement and flowering |
| Potato (white) | Tuber set and enlargement |
| Squash (summer) | Bud development and flowering |
| 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. | |
Cultural practices also influence the rate of evapotranspiration. Cultivation, mulching, weed growth, and method of irrigation are factors to consider. Cultivation generally increases soil evaporation, but if crop roots are pruned or damaged by the cultivator, water uptake and transpiration may be reduced. Shallow cultivation may help eliminate soil crusts and improve water infiltration from rainfall or irrigation. Weeds compete with the crop for water and increase the volume lost through transpiration. Sprinkler irrigation wets the entire crop area and results in greater evaporation loss than trickle/drip irrigation, which wets only the area in the region of the plant root system. Trickle/drip irrigation systems require more frequent operation to prevent plant stress due to the relatively small wetted area.
Various soil factors must also be considered. When thinking about the relevance of soil to crop water requirements it is first helpful to understand how soil moisture levels interact with a plant’s capacity to uptake water from the soil.
A soil is considered saturated when all of its pore spaces are filled with water. This is likely to be the case after a heavy rain or irrigation event. 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 and is readily available for uptake by plant roots.
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. Soil moisture in the range between field capacity and the wilting point is the amount of water that a plant can withdraw from the soil and is called available water. Moisture stress can occur at the lower end of this range, so 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.
Soils with high levels of silt, clay, and organic matter have greater available water-holding capacities than sandy soils or soils that are compacted (see Table 14). Soil organic matter can also substantially increase a soil's ability to store available water. It is estimated that for each percent of soil organic matter, water holding capacity is increased by about 1/2" per foot of soil depth. Soils with high available water-holding capacities require less frequent irrigation than soils with low available water-holding capacities. Low water-holding capacity soils such as loamy sands and sandy loams require frequent irrigation in smaller amounts due to the low holding capacity.
Table 14: Available Water Holding Capacity Based on Soil Texture
| Soil Texture | Available Water-Holding Capacity (inch of water / inch depth of soil) |
|---|---|
| Coarse sand/compacted sands | 0.02–0.06 |
| Fine sand | 0.04–0.09 |
| Loamy sand | 0.06–0.12 |
| Sandy loam | 0.11–0.15 |
| Fine sandy loam/compacted loams | 0.14–0.18 |
| Loam and silt loam | 0.17–0.23 |
| Clay loam and silty clay loam | 0.14–0.21 |
| Silty clay and clay | 0.13–0.18 |
Other soil factors that influence irrigation practices are soil infiltration and percolation. 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 may lead to erosion from runoff and promote disease development. Soil compaction and crusting inhibit infiltration and percolation and should be minimized. 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. Poor infiltration and percolation can be problematic in silt and clay loam soils, particularly with sprinkler irrigation. Table 15 lists the typical infiltration rates of different soil types.
Table 15: Soil Infiltration Rates Based on Soil Texture
| Soil Texture | Soil Infiltration Rates based on Soil Texture (inch/hour) |
|---|---|
| Coarse sand | 0.75–1.00 |
| Fine sand | 0.50–0.75 |
| Fine sandy loam | 0.35–0.50 |
| Silt loam | 0.25–0.40 |
| Clay loam | 0.10–0.30 |