Availability of soil water is a major factor that determines yield and quality of the potato crop. Too little water reduces yields, induces tuber malformations, or increases severity of common scab or Verticillium wilt after infection has occurred. Excess or poorly timed irrigation may reduce yields and quality, cause several disease problems in the field or in storage, or leach nutrients from the root zone. Fluctuations in water availability favor disorders such as second growth and brown center or hollow heart and sugar ends.
Efficient irrigation requires finding out how much available water the soil can hold. Available water is that portion of the soil water that can be withdrawn by plants. During the growing season, irrigation is needed when a certain proportion of the available water, the allowable depletion, has been used. The allowable depletion in a particular field varies according to soil type, stage of crop growth, amount of available water, weather conditions, and irrigation cost. Because potatoes are sensitive to water stress, the allowable depletion is no more than 30 to 40%. To minimize common scab infection, the allowable depletion is no more than 20% during tuber initiation.
Potatoes are a shallow‑rooted crop; 90% of the roots grow in the top 12 to 18 inches of the soil. You can determine from the clay content and soil texture in the top 18 inches how much available water the soil can hold. For a soil profile that includes layers of different soil types, calculate the available water separately for each layer; then add them together to obtain the total available water in the rooting zone. Take salinity into account in estimating available water; if the soil or irrigation water contains high levels of salts, plants withdraw less water, so the available water will be less.
Sprinkler irrigation
Most potatoes are irrigated with sprinklers. Center pivot, wheel line, and solid set systems are most commonly used. Sprinkler systems are more versatile than furrow irrigation systems and can apply fertilizers and some pesticides effectively. Uniform water application is most easily achieved with sprinkler systems. Sprinklers are readily adapted to uneven ground. When preparing fields, be sure not to leave any low spots where water will collect.
Sprinkler irrigation provides conditions in the potato canopy that are favorable for certain diseases, such as early blight, late blight, bacterial stem rot, and white mold. To reduce spread of these diseases, allow foliage to dry between irrigations. An advantage of center pivot irrigation systems is that the water applied is added relatively quickly to the plants. However, regardless of irrigation technique, watering during late afternoon or early evening may allow foliage to stay wet all night, providing a favorable environment for late blight. Allowing solid set sprinklers to run more than 6 hours, regardless of the time of day, has the same effect. Also, foliage near the middle of center pivot circles tends to remain wetter and more prone to foliar diseases and produces poorer quality potatoes, so this area should not be planted.
Drip irrigation
Drip irrigation systems are the most efficient, typically requiring 10 to 20% less water than sprinklers. The risk of foliar diseases is lower with drip systems, and they can apply fertilizers and some pesticides effectively. However, drip systems may create challenges with tillage and harvest operations and are particularly challenging when used on very coarse soils. Mite infestations may increase with the use of drip irrigation. The expense of drip systems generally makes them uneconomical for commercial potato production.
Subirrigation
Subirrigation can be used where the water table can be raised easily, soil is of uniform texture and structure, and fields are relatively level. The low, flat, peat soils of the Sacramento and San Joaquin Delta area of California are favorable for the use of subirrigation, but the lack of a high degree of soil uniformity makes other irrigation systems more advantageous.
Preirrigation
Soil moisture should be at 60 to 80% of field capacity at planting time. If rainfall is not adequate to fill the soil reservoir, use a fall irrigation or irrigate before planting. Avoid irrigations between planting and emergence. Irrigations at this time can increase blackleg, Rhizoctonia stem and stolon canker, and seed piece decay. It is best to have soil moisture high enough so that the first irrigation is not needed until plants have emerged. However, irrigations must be used if the soil becomes excessively dry, and they may also be needed to reduce wind erosion.
Postplant irrigations
With sprinklers, each postplant irrigation should bring the top 18 inches of soil back to field capacity; do not irrigate to a depth of more than 24 inches. The timing and amounts of postplant irrigations depend on the water‑holding capacity of the upper 18 inches of soil and the rate at which the water is used by evapotranspiration. Maintaining adequate soil moisture is critical during the tuber initiation and tuber growth phases; water stress during these periods may cause tuber malformations and translucent end, especially in Russet Burbank. Dry conditions before tuber initiation discourages Verticillium wilt but favor common scab infections; be sure to know which disease is of greater importance in your area. Overly wet soils favor powdery scab infections.
Final irrigations in most growing areas should be timed to allow soil moisture to drop to about 60% of field capacity at the time of vinekill. The optimum moisture level depends on soil type, the variety, and its bruise potential. On sandy soils, higher moisture content can usually be tolerated and some varieties perform better at 70 to 80 % of field capacity until harvest. This level of soil moisture encourages proper development of the tuber skin and decreases the chance that tubers will be infected during harvest by early blight, late blight, or soft rot pathogens. However, if soil moisture falls below 50% during vinekill, stem end browning may result. In hot growing areas, light irrigations may be continued until harvest to keep soil temperatures down. Excess irrigation at this time may reduce oxygen levels in the soil and cause tuber rot or black heart.
Irrigation scheduling
Timing of irrigations during the growing season can be based on various measures of soil moisture together with a water budget, which can be based on evapotranspiration or pan evaporation data. A combination of methods is usually best, and disease potential must be considered.
Always check soil moisture before applying water and estimate how much available water remains in the crop rooting depth. Use a soil tube or shovel to take soil from the rooting zone at several points in each field. The feel and appearance of the soil, as outlined in the table below can be used as a guide for judging the depletion level in soil taken from the root zone.
| Coarse-textured soils | Inches of water needed1 | Medium-textured soils | Inches of water needed1 | Fine-textured soils | Inches of water needed1 |
|---|---|---|---|---|---|
| Soil looks and feels moist, forms a cast or ball, and stains hand. | 0.0 | Soil dark, feels smooth, and ribbons out between fingers; leaves wet outline on hand. | 0.0 | Soil dark, may feel sticky, stains hand; ribbons easily when squeezed and forms a good ball. | 0.0 |
| Soil dark, stains hand slightly; forms a weak ball when squeezed. | 0.3 | Soil dark, feels slick, stains hand; works easily and forms ball or cast. | 0.5 | Soil dark, feels slick, stains hand; ribbons easily and forms a good ball. | 0.7 |
| Soil forms a fragile cast when squeezed. | 0.6 | Soil crumbly but may form a weak cast when squeezed. | 1.0 | Soil crumbly but pliable; forms cast or ball, will ribbon; stains hand slightly. | 1.4 |
| Soil dry, loose, crumbly. | 1.0 | Soil crumbly, powdery; barely keeps shape when squeezed. | 1.5 | Soil hard, firm, cracked; too stiff to work or ribbon. | 2.0 |
| 1 | Amount needed to restore 1 foot of soil depth to field capacity when soil is in the condition indicated. |
Water needs usually vary from one part of a field to another, especially if the field includes different soil types or slopes. Plants in a sandy streak or where root growth has been restricted show stress sooner than the rest of the crop. Watch these weak areas to gain advance notice of when irrigation is needed for the rest of the field. However, schedule irrigations according to the need shown by most of the crop.
Instruments are available that measure the moisture content of the soil. Tensiometers and neutron probes are frequently used for monitoring soil moisture. To obtain reliable readings, you must install these instruments in areas representative of the field, including spots where water stress occurs more readily. Aerial infrared photography can help identify areas of different moisture stress within fields. At each site install one soil moisture probe at the rooting depth of the current growth stage and a second probe 18 to 24 inches deep. Follow the recommendations of irrigation experts in using soil probes for irrigation scheduling.
A convenient way to monitor soil moisture indirectly is to use a water budget to estimate how much water the crop uses from day to day under prevailing weather conditions. After soil has drained to field capacity, further loss of soil water occurs mainly through evaporation from the soil surface or transpiration from leaves. The combination of evaporation and transpiration is called evapotranspiration (ET). If you now how much available water is in the crop rooting depth at field capacity and how much water is lost through ET each day, you can estimate the amount of available water remaining at any time by adding up the daily ET values. Newspapers and radio and television stations in some areas report ET figures for major crops, including potatoes. Water use information for most potato-growing areas is available on the internet from CIMIS for California locations (cimis.water.ca.gov). Information is also available from private consulting firms; check with local authorities regarding sources of information for your area.
You can also use daily pan evaporation and a water use curve for the crop to develop your own ET figures. To use this curve, determine the water use coefficient for the current stage of potato growth, for example, a value of 0.45 during tuber initiation. Multiply the daily pan evaporation by this figure to get actual crop water use. Pan evaporation can be obtained from local weather stations as well as the CIMIS Web site.
Water budgets provide estimates of crop water use. However, actual water use is affected by cultivar, disease, weeds, insects, physical characteristics of individual fields, and management factors. Use a direct measurement of soil moisture to make the final decision about when to irrigate.
A good irrigation program follows these basic guidelines.
- Start with soil at field capacity and avoid irrigation between planting and emergence.
- Monitor the soil moisture.
- Record daily ET.
- Keep records of irrigation and rainfall amounts.
- Do not irrigate deeper than 18 to 24 inches or more frequently than necessary.
- If late blight is a risk, irrigate only when foliage will dry before nightfall; begin irrigation during early morning only when foliage will dry within 6 to 8 hours; do not run solid set sprinklers longer than 6 hours, regardless of time of day. Turn off the sprinklers near the center of a pivot as much as possible.
- Avoid wet soil (soil with moisture in excess of levels recommended for the current stage of growth).
- Use irrigation systems designed to give uniform water distribution.
- Be sure to reduce irrigations, matching the decreased plant demand in late season, to avoid excess water that encourages development and spread of late blight as well as other tuber rots.