Placed on the Web September 2002, by the Center for Integrated Pest Management, NCSU

General Production Recommendations

IRRIGATION

Basic Principles. Vegetables are 80 to 90% water. Because they contain so much water, their yield and quality suffer very quickly from drought. Think of vegetables as a "sack of water" with a small amount of flavoring and some vitamins. Irrigation is likely to increase size and weight of individual fruit and to prevent defects like toughness, strong flavor, poor tipfill and podfill, cracking, blossom-end rot, and misshapen fruit. On the other hand, too much irrigation reduces soluble solids in muskmelons and capsaicin in hot peppers if applied during fruit development.

Growers often wait too long to begin irrigating, thinking "It will rain tomorrow." This often results in severe stress for the portion of the field that dries out first or receives irrigation last. Another common problem is trying to stretch the acreage that can reasonably be covered by available equipment. Both of these practices result in part or all of the field being in water stress. It is best that a good job be done on some of the acreage rather than a "half-way job" being done on all the acreage.

Drought stress can begin in as little as 3 days after a 1-inch rain or irrigation in such crops as tomatoes in soils throughout the Southeast. Thus frequent irrigation is necessary for maximum yield. Up to 1.5 inches of water is needed each week during hot periods to maintain vegetable crops that have a plant spread of 12 inches or more. This need decreases to 0.75 inches per week during cooler seasons.

Droplet size and irrigation rate are also very important in vegetable crops. Large droplets resulting from low pressure at the sprinkler head can cause damage to young vegetable plants and contribute to crusting when soil dries. Irrigation rate is also important in sandy soils, which absorb water more readily than clay soils. However, clay soils have a greater percent of the water available. Irrigation rate will depend on soil type, but application rates should not exceed 0.40 inch per hour for sandy soils, 0.30 inch per hour for loamy soils, or 0.20 inch per hour for clay soils. High application rates will result in irrigation water running off the field, contributing to erosion and fertilizer runoff.

Even relatively short periods of inadequate soil moisture can adversely affect many crops. Thus, irrigation is beneficial in most years, since rainfall is rarely uniformly distributed even in years with above-average precipitation. Moisture deficiencies occurring early in the crop cycle may delay maturity and reduce yields. Shortages later in the season often lower quality and yield. However, over-irrigating, especially late in the season, can reduce quality and postharvest life of the crop. Table 10 shows the periods of crop growth when an adequate supply of water is critical for high-quality vegetable production.

Applying the proper amount of water at the correct time 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. The ET rate is important in effectively scheduling irrigations. Numerous factors must be considered when estimating ET. The amount of solar radiation, which provides the energy to evaporate moisture from the soil and plant surfaces, is the major factor. Other factors include day length, air temperature, wind speed, and humidity level.

Table 10. CRITICAL PERIODS OF WATER NEED BY CROPS
CropCritical Period
Asparagus Brush
Beans: lima Pollination and pod development
           snap Pod enlargement
Broccoli Head development
Cabbage Head development
Carrots Root enlargement
Cauliflower Head development
Corn Silking and tasseling, ear development
Cucumbers Flowering and fruit development
Eggplants Flowering and fruit development
Lettuce Head development
Melons Flowering and fruit development
Onions: dry Bulb enlargement
Peas Seed enlargement and flowering
Peppers Flowering and fruit development
Potatoes: Irish Tuber set and tuber enlargement
Radishes Root enlargement
Squash: summer Bud development and flowering
Sweetpotato Root enlargement
Tomatoes Early flowering, fruit set, and enlargement
Turnips Root enlargement


Plant factors that affect ET are crop species; canopy size and shape; leaf size, shape. Soil factors must also be considered. Soils having high levels of silt, clay, and organic matter have greater water-holding capacities than do sandy soils or soils compacted (Table 11). Soils with high water-holding capacities require less frequent irrigation than soils with low water-holding capacities. However, when irrigated less frequently, a greater amount of water must be applied per application.

Another soil factor influencing irrigation practices is the soil infiltration rate. Water should not be applied to soils at a rate greater than the rate at which soils can absorb water. Table 12 lists the typical infiltration rates of several soils.


Table 11. AVAILABLE WATER HOLDING CAPACITY BASED ON SOIL TEXTURE
Available Water Holding Capacity
Soil Texture water/inches of soil
Coarse sand0.02-0.06
Fine sand0.04-0.09
Loamy sand0.06-0.12
Sandy loam0.11-0.15
Fine sandy loam0.14-0.18
Loam and silt loam0.17-0.23
Clay loam and silty clay loam0.14-0.21
Silty clay and clay0.13-0.18


There is no simple method to accurately schedule irrigation because all the above factors interact to determine water loss. Research is under way to develop methods for growers to use in scheduling irrigations. The following factors should be kept in mind when deciding when and how much to irrigate.

  1. Soils vary greatly in water-holding capacity and infiltration rate. Silt and clay soils and those high in organic matter can hold much more water than sandy soils low in organic matter.

  2. Water loss from plants is much greater on clear, hot, windy days than on cool, overcast days. During periods of hot, dry weather, ET rates may reach 0.25 inch per day or higher. ET can be estimated by the use of a standard evaporation pan. (Check with your county Extension office for information on using pans.)

  3. Recent research indicates that maintaining soil moisture levels in a narrow range, just slightly below field capacity (75% to 90% available soil moisture), maximizes crop response. This may mean that more frequent irrigations of smaller amounts are better than delaying irrigations until the soil moisture reaches a lower level (40% to 50% available soil moisture) and then applying a heavy irrigation.

  4. Plastic mulches reduce evaporation from the soil but also reduce the amount of rainwater that can reach the root zone. Thus, much of the natural precipitation should be ignored when scheduling irrigations for crops grown under plastic mulch.

  5. In general, apply 0.25 inch of water or more in any one irrigation, except when used for establishing crops.


Table 12. SOIL INFILTRATION RATES BASED ON SOIL TEXTURE
Soil Texture Soil Infiltration Rate
(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

Drip Irrigation. Drip irrigation is a method of slowly applying small amounts of water directly to the plant root zone. Water is applied frequently, often daily, to maintain favorable soil moisture conditions. The primary advantage of drip irrigation systems is that less water is used than with sprinkler or surface irrigation systems. In many cases, one-half of the water applied with sprinkler or surface systems is required with drip systems. In addition, fertilizers applied through the drip irrigation system are conserved along with water. Drip irrigation is used on a wide range of fruit and vegetable crops. It is especially effective when used with mulches; on sandy soils; and on high value crops, such as muskmelons, watermelons, squash, peppers, eggplants, and tomatoes.

Drip irrigation systems also have several other advantages over sprinkler and surface irrigation systems. Low flow rates and operating pressures are typical of drip systems. These characteristics lead to lower energy and equipment costs. Once in place, drip systems require little labor to operate, can be automatically controlled, and can be managed to apply the precise amount of water and nutrients needed by the crop. These factors also reduce operating costs. With most drip systems, disease is reduced because leaves are not wetted. The areas between rows also remain dry reducing weed growth between rows and reducing the amount of water lost to weeds. In addition, field operations can continue during irrigation.

There are several potential problems unique to drip irrigation systems. Most require a higher level of management than other irrigation systems. Moisture distribution in the soil is limited with drip systems. In most cases, a smaller soil water reserve is available to plants. Under these conditions, the potential to stress plants is greater than with other types of irrigation systems. So the drip system must be carefully managed.

The equipment used in drip systems also presents potential problems and drawbacks. Drip equipment can be damaged by insects, rodents, and laborers, and often has a higher initial investment cost than other system types. Pressure regulation and filtration require equipment not commonly found on sprinkler or surface systems. The drip system, including pump, headers, filters, and connections, must be checked and ready to operate before planting. Failure to have the system operational could result in costly delays, poor plant survival or irregular stands, and reduced yield. In addition, it is not practical to use drip systems for frost control or the irrigation of solid-stand crops, such as forages and cereals.

Calculating the length of time required to apply a specific depth of water with a drip irrigation system is more difficult than with sprinklers (Table 13). Unlike sprinkler systems, drip systems apply water to only a small portion of the total crop acreage. Usually, a fair assumption to make is that the mulched width approximates the extent of the plant root zone and should be used to calculate system run-times. Table 13 calculates the length of time required to apply 1-inch of water with a drip irrigation system based on the drip tube flow rate and the mulched width. The use of this table requires that the drip system be operating at the pressure listed in the manufacturer's specifications.


Table 13. HOURS REQUIRED TO APPLY 1" WATER TO MULCHED AREA
Drip Tube Flow Rate Mulched Width (ft)
(gph/100 ft)(gpm/100 ft) 2.02.5 3.0 3.54.0
8 0.1315.5 19.523.527.031.0
10 0.1712.516.5 18.5 22.025.0
12 0.20 10.5 13.0 15.5 18.0 21.0
16 0.27 8.0 10.0 11.5 13.5 15.5
18 0.30 7.0 8.5 10.512.014.0
20 0.33 6.0 8.0 9.5 11.0 12.5
24 0.40 5.0 6.5 8.0 9.0 10.5
30 0.50 4.0 5.0 6.0 7.0 8.5
36 0.60 3.5 4.5 5.0 6.0 7.0
40 0.67 3.0 4.0 4.5 5.5 6.0
42 0.70 3.0 4.0 4.5 5.0 6.0
48 0.80 2.5 3.0 4.0 4.5 5.0
50 0.83 2.5 3.0 4.0 4.5 5.0
54 0.90 2.5 3.0 3.5 4.0 4.5
60 1.00 2.0 2.5 3.0 3.5 4.0



In many cases, it is inappropriate to apply more than 0.25 inch of water at a time with drip irrigation systems. Doing so can move water below the plant root zone, carrying nutrients and pesticides beyond the reach of the plant roots. Table 14 calculates the maximum recommended irrigation period for drip irrigation systems. The periods listed in Table 14 are based on the drip tube flow rate and soil texture. Soil texture directly influences the water-holding capacity of soils and, therefore, the depth reached by irrigation water.

In drip systems, water is carried through plastic tubing and distributed along the tubing through devices called emitters. The emitters dissipate the pressure from the system by forcing the water exiting from an emitter through orifices, tortuous flow paths, or long flow paths, thus allowing a limited flow of water to be discharged. The pressure-reducing flow path also allows the emitter to remain relatively large, allowing particles that could clog an emitter to be discharged.

Although modern emitter design reduces the potential for trapping small particles, emitter clogging remains the most serious problem with drip irrigation systems. Clogging can be attributed to physical, chemical, or biological contaminants. Filtration and occasional water treatment may be necessary to keep drip systems from clogging. Further information on drip irrigation systems can be obtained from the manufacturers, dealers, and the county Extension office.


Table 14. MAXIMUM IRRIGATION PERIODS (HOURS) FOR DRIP IRRIGATION SYSTEMS
Drip Tube Flow Rate
Soil Texture
(gph/100 ft) (gpm/100 ft)
Sand Loamy Sandy LoamClay Silt Loam
12 0.2
5.0 8.011.515.517.5
180.3
3.5 5.0 7.5 10.5 11.5
240.4
2.5 4.05.58.0 8.5
300.5
2.0 3.04.56.5 7.0
360.6
1.5 2.54.05.0 6.0
420.7
1.5 2.03.04.5 5.0
480.8
1.5 2.03.04.0 4.5



Chlorination. Bacteria can grow inside drip irrigation tubes, forming a slime that can clog emitters. Algae present in surface waters can also clog emitters. Bacteria and algae can be effectively controlled by chlorination of the drip system. Periodic treatment before clogging develops can keep the system functioning efficiently. The frequency of treatment depends on the quality of the water source. Generally two or three treatments per season is adequate.

Irrigation water containing high concentrations of iron (greater than 1 ppm) can also result in clogging problems due to a type of bacteria that "feeds" on iron. In consuming the dissolved (ferrous) form of iron, the bacteria secrete a slime called ochre, which may combine with other solid particles in the drip tubing and plug emitters. The precipitated (ferric) form of iron, known commonly as rust, can also physically clog emitters. In treating water containing iron, chlorine will oxidize the iron dissolved in water, causing the iron to precipitate so that it can be filtered and removed from the system.

Chlorine treatment should take place upstream of filters in order to remove the precipitated iron and microorganisms from the system.

Chlorine is available as a gas, liquid, or solid. Chlorine gas is extremely dangerous and not recommended for agricultural purposes. Solid chlorine is available as granules or tablets containing 65% to 70% calcium hypochlorite. Liquid chlorine is available in many forms, including laundry bleach and postharvest wash materials. Liquid forms typically contain between 5% to 15% sodium hypochlorite. Use chlorine only if the product is labeled for use in irrigation systems.

Since chlorination is most effective at pH 6.5 to 7.5, some commercial chlorination equipment also injects buffers to maintain optimum pH for effective kill of microorganisms. This type of equipment is more expensive, but more effective than simply injecting sodium hypochlorite solution.

The required rate of chlorine injection is dependent on the amount of microorganisms present in the water source, the amount of iron in the irrigation water, and the method of treatment being used. To remove iron from irrigation water, start by injecting 1 ppm of chlorine for each 1 ppm of iron present in the water. For iron removal, chlorine should be injected continuously. Adequate mixing of the water with chlorine is essential. For this reason, be certain to mount the chlorine injector a distance upstream from filters. An elbow between the injector and the filter will also ensure adequate mixing.

For treatment of algae and bacterial, a chlorine injection rate that results in the presence of 1 to 2 ppm of "free" chlorine at the end of the furthest lateral will assure that the proper amount of chlorine is being injected. Free, or residual, chlorine can be tested using an inexpensive DPD (diethyl-phenylene-diamine) test kit. A swimming pool test kit can be used, but only if it measures free chlorine. Many pool test kits only measure total chlorine.

If a chlorine test kit is unavailable, one of the following schemes is suggested as a starting point:

For iron treatment:

For bacteria and algae treatment:

The injection rate can be calculated from the following equation:

(0.006) x (desired chlorine concentration in ppm) x (gpm of irrigation)

Injection rate of chlorine solution in gallons/hour = (0.006) x solution in gallons/hour (desired chlorine concentration in ppm) x gpm of irrigation % chlorine in bleach or concentrate


It is important to note that both liquid and solid forms of chlorine will cause water pH to rise. This is critical because chlorine is most effective in acidic water. If water pH is above 7.5, it must be acidified for chlorine injection to be effective.

Important Notes.

Fertilization. Before considering a fertilization program for mulched-drip irrigated crops, the grower should have the soil pH checked. If a liming material is needed to increase the soil pH, the material should be applied and incorporated into the soil as far ahead of mulching as practical. For most vegetables, adjust the soil pH to around 6.5. When using drip irrigation in combination with mulch, apply the recommended amount of preplant fertilizer and incorporate 5 to 6 inches into the soil before laying the mulch. If equipment is available, apply the preplant fertilizer to the soil area that will be covered by the mulch. This is more efficient than a broadcast application to the entire field.

The most efficient method of fertilizing an established mulched row crop is through a drip irrigation system, which is usually installed during the mulching operation. Due to the very small holes or orifices in the drip tubing, a high quality liquid fertilizer must be used. Since phosphorous is a stable non-mobile soil nutrient and can cause clogging of the drip tube emitters, it is best to apply the crops phosphorous needs pre-plant. Also applied pre-plant should be 20 to 40% of the nitrogen and potassium nutrient needs. The remainder of the crops nutrient needs can be applied through the drip system with a high quality liquid fertilizer such as 8-0-8, 7-0-7, or 10-0-10. It is usually not necessary to add micronutrients through the drip system. Micronutrients can be best and most economically applied pre-plant or as foliar application if needed.

The amount of nutrients to apply through the drip system depends upon the plant's growth stage. In general, smaller amounts of nutrients are needed early in the plant's growth with peak demand occurring during fruit maturation. The frequency of nutrient application is most influenced by the soil nutrient holding capabilities. The clay type soils with a high nutrient holding capacity could receive weekly nutrient application through the drip system. A sandy soil with low nutrient holding capacity will probably respond best with a daily fertigation program. An example of a fertigation program for transplanted watermelons is given in the following table.

Stage Amount (Lb/A/Day)
Pre-plant 30%N-100%P-30%K
Planting-Flowering 1.0 lb N & K
Flowering-Fruit Set 1.5 lb N & K
Fruit Set-Ripe Start 2.0 lb N & K
Ripe Start-Harvest 1.5 lb N & K
Maintenance 1.0 lb N & K