Cost of production is constantly increasing but price for vegetable crops remains steady. Consequently, vegetable growers are looking for new ways to achieve higher yields without impacting produce quality while seeking better prices of early spring markets. Plasticulture systems—which combine raised beds, plastic mulch, drip irrigation, and fumigation—have helped an increasing number of farmers reach these goals. Particularly, growers using plasticulture systems have doubled and tripled yields and harvested their crops two to three weeks earlier than is possible with traditional growing practices.

Throughout this publication, we will emphasize that plasticulture is a system that cannot function optimally without all its components. The benefits of these components are described below.

• Raised beds help provide more uniform soil moisture content, fertilizer distribution, and management of soil temperature in the crop root system.
• Plastic mulch, which gained wide acceptance in the early 1980s, also helps manage soil temperature and reduces water loss via evaporation. The mulch protects fruits and leaves from disease by preventing direct contact with the soil (Figure 1).
• Drip irrigation provides a uniform supply of water and fertilizer, which keeps the soil moisture at ideal levels under the plastic once properly managed. In the Southeast, in particular, most soil has limited water-holding capacity and do not re-wet easily once they have dried out. Therefore, drip irrigation helps eliminate this problem by frequently supplying small amounts of water to maintain a more constant moisture level.
• Fumigation controls weeds, insects, and diseases. This control is an important element of plasticulture because the high temperatures induced by plastic mulch can stimulate pest activity (Figure 2).

The success of plasticulture depends on the specific management within your operation. This publication provides information to help you decide if plasticulture is right for you, and if you decide it is, how to prepare your site and manage the system.

Figure 1. Tomato in white mulch

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Figure 1. Tomato in white mulch

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Figure 2. Soil fumigation process

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Figure 2. Soil fumigation process

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Plasticulture is a management-intensive system that requires careful planning and close attention. However, before you invest in plasticulture, you should carefully weigh the advantages and disadvantages of plasticulture versus traditional practices.

Advantages

Earlier Production

Probably the greatest benefit of growing crops on a plastic mulching system is the ability for earlier production. As aforementioned, plastic mulches raise the soil temperature, which helps plants grow more quickly and mature earlier compared to plants grown in bare ground. Spring vegetables grown on black plastic can be harvested in some cases 7 to 21 days earlier than they can be on bare-ground production systems. Harvesting one to two weeks earlier often significantly increases market advantage and the prices growers receive.

Reduced Leaching of Fertilizer

Because many fertilizers are not held tightly in some soils, rainfall and excessive irrigation may leach nutrients below the roots of plants grown on bare ground. Nitrogen, potassium, magnesium, and some formulations of micronutrients are subject to leaching, especially in coarse-textured soil, like sandy soils. A plastic mulch covering the bed (or portion of it) prevents rainfall from percolating through the soil and moving nutrients below the crop root zone. Preventing leaching improves the efficiency of production by eliminating the need and uncertainty of pinpointing the field to resupply leached nutrients, thereby saving time, fuel, and fertilizer. In addition, it helps prevent reduced quality and lower yields resulting from (1) hidden hunger—early stages of nutrient deficiency that may harm plants even before they begin to show symptoms and (2) lag time—the time from when plants first show symptoms of a deficiency until nutrients are replenished in the plant tissues. Finally, minimizing the amount of leaching presents an environmental benefit, helping to protect groundwater from fertilizer contamination via deep nutrient percolation (Figure 3).

Fewer Weeds

Plastic mulch can prevent the growth of most weeds, except nutsedge. Clear plastic does not prevent weed growth because light can penetrate it. So, if you are going to use clear plastic, you will also need to use an herbicide or soil fumigant. Herbicides also control weeds on bare ground between the plastic-covered beds.

Increased Plant Growth

Plants grow faster on plastic mulching systems due to their buffer effect on soil temperature. At a 2-inch depth, soil temperature can increase by up to 10°F under black mulch and up to 15°F under clear mulch under certain environmental conditions [high radiation, adequate soil moisture, etc.]. Alternately, silver-reflective and white mulch can maintain a lower temperature of the soil under high radiation and high air temperature conditions, providing a more adequate environment for the plant roots.

Reduced Evaporation

Because growth rate on mulch is increased, plants may require more water even though evaporation is reduced. Mulching cannot substitute for a sound irrigation program. However, less water evaporates from soil under plastic mulch, and soil moisture is maintained more uniformly. Therefore, you actually need less water per unit of production.

Improved Quality

Vegetables grown on plastic mulch are cleaner and less subject to rots because soil has not been splashed on the plants or fruit (Figure 4).

Reduced Soil Compaction

Soil under plastic mulch remains loose longer, friable, and well aerated. Roots have access to adequate oxygen, and microbial activity is enhanced.

Better Surface Water Drainage

Excess water runs off the plastic-covered rows and out of the field, reducing drowning and other stresses related to waterlogged soil.

Cultivation Eliminated

There is no need for cultivation, except in the area between the mulched strips. Weed growth in these areas can be controlled with a labeled herbicide.

Better Fumigation

Plastic mulches increase the effectiveness of soil fumigation by retaining fumigants [when applied through the drip] in the soil and providing better pest control.

Disadvantages

Removal and Disposal

A major problem with a plastic mulching system is having to remove the plastic mulch and drip tubing from the field after cropping. Conventional plastic mulch does not break down and should not be disked into the soil. It can, however, be recycled. In addition, there are photodegradable and biodegradable mulches, which break apart into the soil and do not have to be removed, although they tend to be more expensive. Under optimal conditions, biodegradable mulches can degrade within one to two growing seasons. However, in cooler climates or less favorable conditions, degradation may take longer. It's important to note that while these mulches are designed to break down more readily than traditional plastics, complete degradation in natural soil environments can still require several months to a few years.

Greater Initial Costs

Because plasticulture requires an investment in specialized equipment—including bed press, mulch layer, and mulch transplanter (or plug-mix seeder)—initial production costs are significantly higher than for traditional practices. Yearly costs of plastic and drip tubing further increase production expenses. These costs, however, should be offset by increased returns from earlier harvests, better quality, and higher yields.

Increased Management

Plastic mulch and drip irrigation must be monitored daily to be successful. Although plasticulture technology offers several important advantages over conventional production practices, it is not necessarily for everyone. Whether plasticulture is right for your operation depends on several factors:

1. Are the vegetables you intend to grow suited to production on plastic?
2. Will the vegetables be intensely managed?
3. Are sufficient financial resources available or obtainable?
4. Do you have a good market, or prospects for one?
5. Have you established a successful track record growing vegetables on bare ground?

If you answered “no” to one or more of these questions, then plasticulture might not be a good alternative for you now at this moment. However, if you answered “yes” to each of these questions, plasticulture may be profitable for you.

Figure 3. Blue Dye Test

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Figure 3. Blue Dye Test

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Figure 4. Tomatoes grown in plasticulture system in the Eastern Shore of Virginia in 2022

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Figure 4. Tomatoes grown in plasticulture system in the Eastern Shore of Virginia in 2022

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Site preparation is an important component of a successful plasticulture system. The field should be plowed early, with special attention given to incorporating organic matter. If cover crop residue is not plowed under well and early enough [1 to 3 weeks, depending on weather conditions and overall biomass] to allow it to decompose, it will disrupt fumigation and require more fumigant to treat the soil.

Soil Preparation

The first step in preparing soil is to take composite soil samples in the early fall and create two subsamples. Have one subsample test for mineral content and one for nematodes. If there is a nematode problem, fumigate the soil at the time the plastic mulch is laid. By using a multipurpose fumigant (such as Pic-clor 60 [check label for crops]) you can improve your management of weeds, nematodes, soil-borne insects, and diseases. Fertilizer is often applied according to the results of the soil test. Usually, 25–30% of the nitrogen and potassium and all of the phosphorus is applied at the time plastic is laid. The remaining nitrogen and potassium should be injected through the drip tube using soluble fertilizers. For specific crop fertilizer recommendations consult the Southeastern U.S. Vegetable Production Handbook, available online or from your local N.C. Coopeartive Extension Center.

Tillage Methods

The tillage method you use should result in a soil of good tilth that can be easily formed into a bed. Normally, the soil is bottom plowed, disked one or two times, and then in-row subsoiled.

Bed Formation

Bed forming, or bed shaping, is an important practice in the plasticulture system. Most problems associated with plastic mulch are due to poor soil and bed preparation. The soil must be loose and friable and free of stones, clods, sticks, and undecomposed plant residue. Sharp stones and sticks will puncture the mulch film and allow soil fumigants to escape. Clods, stones, and undecomposed plant residue make laying the plastic difficult and prevent the desired contact between the soil surface and the plastic mulch. For the plastic mulch to lay properly, the bed must be tight, firm, and well-formed (Figure 5). If the bed is too soft, fruit such as muskmelons will cause a depression where water accumulates, and the melon may rot. Also, the bed should be crowned (1.5 - 2 inches higher in the center), so water will run off and not rest on the plastic.

Bedding machines are available in single- and multiple-row models. If the bed shaper is lined with Teflon, the bed surface will be much smoother. With other bed shapers, the soil is first lifted with hilling discs or double disc hillers and then compressed to a uniform height and density by a bed press pan (Figure 6). Be sure that enough soil is pulled up so that the bed has good, sharp corners. Bedded rows should be spaced on 5- or 6-foot centers, depending on your equipment. Bed dimensions can range from 24 to 16 inches at the base and 8 to 12 inches in height, depending on the crop and production system selected.

Figure 5. Tilled soil previous to mulching (A), and planting bed with white mulch (B)

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Figure 5. Tilled soil previous to mulching (A), and planting bed with white mulch (B)

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Figure 6. Mulching machine in North Carolina

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Figure 6. Mulching machine in North Carolina

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When to Apply Plastic and Fumigant

Laying the plastic mulch and fumigating in the fall has several advantages: (1) spring’s typically wet weather can result in delays in laying the plastic and transplanting; (2) better fumigation of the soil is possible in the fall because the soil is not apt to be too wet or too cold; (3) nematode populations are generally highest in the fall so fumigating then is most effective; and (4) fall-applied mulch often makes it possible to transplant 10 to 14 days earlier in the spring.

How much plastic is required per acre?

For 5-foot row centers, there are 8,712 linear feet of row per net acre, so you would need between 3.5 rolls (2,400 ft/roll) or 2.1 rolls (4000 ft/roll) of plastic mulch per acre.

Plastic-Laying Machines

A number of manufacturers make machines that raise beds and/or lay plastic. While machines may vary in design, they are all satisfactory if properly adjusted. Most machines have some fine points that set them apart, making it worth your time to investigate these differences. Some plastic-laying machines, for instance, have the ability to change rolls of plastic without stopping, and all have some method of holding an extra roll of film. Most also can control the tension of the film as it is being laid. Some machines form the bed, fumigate, and lay plastic and drip tube all in one operation. In general, the smaller, lighter machines are designed for small acreage (0-50 acres), and the larger, heavier machines for larger acreage (50+ acres). It is important that the plastic fit tightly on the bed and that the edge of the plastic, or the tuck, be held firmly in the soil. These measures reduce the chance of wind getting under the plastic and causing it to blow off or float up and down, which injures plants. Cutting a deep trench on the edge of the bed with a small furrow-opener attachment will aid in getting a tight tuck. Fumigation equipment may also be attached to the front of the mulch layer. The soil can be fumigated as the plastic mulch is being laid, or it can be done in a separate operation immediately before mulch laying. Drip irrigation tube applicators are normally attached to the front of the mulch layer and installed with the plastic.

Fumigation

The amount of fumigant to be applied per acre depends on row width and is a percentage of the broadcast rate. (Consult your state’s agricultural chemicals manual.) Air temperature should be at least 50°F, and the soil should be well-prepared. If both the air and soil are warm, most fumigants will escape through the plastic mulch after 21 days.

What is Drip Irrigation?

Drip irrigation was first developed in the 1940s in England, but it was not until the advent of polyethylene plastics in the 1960s that the principle of frequent low-volume irrigation was put to field use. This system of irrigation was first popular in Israel and soon spread to many parts of the world. Yields and quality of fruits and vegetables were increased severalfold. The main reason this system is so successful is that drip irrigation puts water near the roots of the crop. Because only the water needed by the crop is used, waste due to runoff and evaporation is reduced. In fact, roots of plants that are drip irrigated tend to be most plentiful near the dripper tubing and seem to grow around the tube and “lock on” to get their water.

Drip irrigation management is very different from other types of irrigation. Because most drip systems apply only 0.05 to 0.10 inches per hour, it takes a long time to supply enough water. Frequency of irrigation depends on soil type, bed dimensions, and stage of crop growth. Tensiometers or electrical resistance blocks at 6 and 12 inches below the surface of the soil in mulched beds are recommended to help determine irrigation needs. Normally, the area around the drip tube is very soft to the touch; the side of the row away from the tube should be only slightly soft. Generally, drip irrigation is applied daily or even more frequently. Once the ideal soil moisture level is established, only slight variations from this set point should occur.

Major Components of a Drip Irrigation System

A drip irrigation system has five major components:

1. Delivery system - drip tape or retrievable drip tubing.

2. Filters - sand, screen, or disc.

3. Pressure regulators - spring or valve.

4. Valves - hand-operated, hydraulic, or electrical.

5. Controllers - simple electric clock or computer.

How you put these components together, and which options you choose, will depend on the size of the system, the water source, the crop, and how sophisticated a system you need. It is essential to consult with a professional in irrigation system design to evaluate your specific field conditions before making any investment.

Delivery System

Most vegetable growers use a drip tape supply system. Water passes through holes or emitters in the tube’s inner chamber to a smaller outer chamber, where it is forced through tiny passageways that serve to reduce the water’s pressure. When the water reaches the exit holes to the soil, it has little or no pressure and simply drips out. You can specify the spacing between exit holes. In general, the sandier the soil, the closer together the holes must be.

Most drip tapes are usually used for only one year and then discarded because attempts to retrieve and store it have often resulted in clogging or extensive rodent damage and troublesome repair the second year. Application rates can range from 0.05 to 0.10 inches per hour per acre.

Some vegetable growers use retrievable drip tubing, which has a heavier construction than the drip tapes (usually 20- to 30-mil thick polyethylene). These systems use in-line emitters, which apply 1 to 4 quarts per emitter per hour; irrigation in inches per hour per acre will depend on spacing. Emitter systems are initially more expensive than drip tape but are reused for several years and may be less expensive over time than drip tape systems, if properly managed. Emitters used for vegetable crops are usually diaphragm or turbulent-flow type. The diaphragm types have internal pressure compensation and can be used in rolling areas where change in elevation affects water pressure.

For single-row-per-bed crops such as tomatoes, cucumbers, muskmelons, honeydews, watermelons, and pumpkins, the drip tube should be placed 2 to 4 inches from the center of the bed and 1 to 2 inches deep with the emitters facing upward. For crops like summer squash, okra, eggplant, peppers, beans, peas, lettuce, and sweet corn that are often planted with two rows per bed, the drip tube should be placed directly on the center.

Filters

The type and size of filter you choose will depend on the quality of your water source. Your equipment dealer will be able to assist you in making this choice. The most commonly used types of filters are sand, disc, centrifugal, screen, or a combination of two or more.

Sand filters are an absolute necessity for any surface water source. Most surface waters, such as ponds, streams, and rivers, have enough contaminants at some time in the cropping season that sand filters are the only practical way to remove them. These filters contain 12 to 18 inches of sand, which traps any contaminants as the water is pumped through it. The advantage of sand filters is that they can be flushed for cleaning. Flushing is a very quick process and can even be automated. Also, for their cost, sand filters have a large filter area. At least two sand filters are needed during the back flushing operation; clean water from one filter is used to remove contaminants from the other filter. Sand filters come in varying diameters, from 14 inches to 48 inches. If your water source is flowing water, such as from streams or rivers, you will probably need to use a sand separator to remove sand from the water before it enters the filter.

Screen filters are used as secondary filters with surface water systems or as primary filters with well or municipal water sources. Screen filters consist of a cylindrical outer housing and an internal cylinder of a fine screen. Water enters the outer chamber and is forced through the inner chamber to an exit port. Screen filters range in diameter from 3⁄4 inch (used only for 1⁄2 acre) to 10 inches (used with several acres). Some screen filters have automatic backflush capabilities, but most must be flushed manually.

Disc filters are used as secondary filters with surface water systems or as primary filters with well or municipal water sources. These filters contain a series of grooved plastic discs that remove contaminants as small as 200 microns (about 1⁄1,000 inch). An advantage of using disc filters is that they have more surface area than screen filters and are easier to clean.

Centrifugal (Hydrocyclone) filters use centrifugal force to separate heavier particles (e.g., sand) from water. These are commonly best for water with high sand content, typically from wells or rivers. They require low maintenance and are effective for removing sand and grit but are not very effective for fine or organic debris.

If the water is very dirty, or if it contains a lot of iron or large amounts of algae, use small screen or disc filters even on submain lines.

Pressure Regulators

Usually, pressures in a main line range from 30 to 50 pounds per square inch (psi). However, most drip irrigation systems operate at line pressures between 8 and 16 psi, so water pressure must be reduced before it reaches the drip tape. This is accomplished by spring-type or diaphragm-type pressure regulators. Spring-type regulators are used on systems of up to 5 acres; systems larger than 5 acres require diaphragm-type regulators.

Valves

All systems need an on-off valve, and if your water source is a well or municipal water, you must use an antisiphon backflow preventer. The simplest valve for a drip system is a gate valve, and on very small systems where the pressure is constant, the gate valve can also serve as a pressure regulator. Solenoid valves, either electric or hydraulic, are essential if you plan to automate your operation or separate the field into zones. Solenoid valves allow you to switch from one zone to another zone and to turn the system on and off. Solenoids can be manually switched, but usually, some sort of electric controller is used.

Controllers

There is a wide variety of controllers available, ranging from a simple time clock to a complex computer-controlled unit that can run many zones. The computer controllers can also sense flow rate and pressure and will shut the system down in case of a malfunction. With some clocks you can run one zone for one period and another zone for shorter or longer time periods.

Irrigation Scheduling

Irrigation scheduling means determining when to irrigate and how much water to apply. Both decisions depend on the desired soil moisture level, the crop's water requirements, the water supply in the root zone, the water-holding characteristics of the soil, and the efficiency of the irrigation zone.

It is important to schedule irrigation carefully, because frequency of irrigation and quantity of water applied affect the cost of production as well as plant growth and development. Irrigation scheduling becomes even more critical when drip irrigation is used in combination with plastic mulch and fertigation.

Remember that drip irrigation is different from other types of irrigation because it maintains a constant optimal moisture level in the root zone by frequent applications of relatively small amounts of water. These small but frequent applications help to minimize loss due to water percolating deep into the soil. Although drip irrigation uses water efficiently, the soil water content must still be monitored frequently to make sure crops are receiving the ideal amount.

Soil Moisture Management

Soil moisture can be measured with soil moisture sensors. The following are some general guidelines for using tensiometers to schedule drip irrigation, although modern sensors are more efficient; for more detailed information, contact your local N.C. Cooperative Extension Center.

Tensiometers and moisture blocks are usually set up in pairs. Each pair is called a station, and there should normally be one station per irrigation zone.

At each station, one of the sensors should be placed near the middle and the other near the bottom of the root zone. The mid-zone sensor detects moisture loss in the upper portion of the soil and indicates when irrigation should begin. The deep sensor detects penetration of water to the lower roots and indicates when to stop irrigating.

Placement of sensors in relation to the row and the drip tube is critical. Drip tubes wet only a portion of the soil. The soil wetting pattern may extend only 6 to 8 inches from the emitter in sandy soil to several times that distance in heavier clay soils.

Where one drip tube is used per row (for example, tomatoes on 5-foot centers), the sensor should be placed on the opposite side of the row from the drip tube halfway between two plants and about four inches off row center.

For crops having one drip line located between two rows (for example, double rows of peppers spaced 12 inches apart), the sensor should be placed on the outside of one of the rows halfway between two plants and about four inches off plant row center.

For sands and sandy loam soils, irrigation should begin when the shallow sensor indicates 20 to 30 centibars (35 to 50 for clays) and should be stopped when the deep sensor indicates 10 centibars (15 for clays).

It is good management to both irrigate and fertilize with the drip irrigation system as long as supplying the crop water requirements is closely coordinated with providing the crop nutrient requirements. Drip irrigation is a management-intensive system; to be successful, crops must be watered daily or even hourly.

What is Fertigation?

Fertigation is the process of feeding crops by adding soluble fertilizers into an irrigation system. A drip system produces uniform water distribution and, therefore, can also regulate crop feeding. Once a drip system and all its hardware are installed in the field, it makes sense to use the drip system to distribute fertilizer to the crop. Fertilizer can be applied with several types of devices, all of which are called injectors.

Types of Injectors

Vacuum injectors (Venturi-type). Vacuum injectors use the venturi principle for injecting solutions into the irrigation line. Venturi units are available in various sizes, up to 2 inches in diameter. A venturi applicator draws fertilizer solution out of a tank by differential pressure created within the venturi. Because pressure in an irrigation system is variable, there will be surges at times; therefore, application rates are not as uniform as with other methods.

Pumps. Various types of pumps are available for injecting fertilizer into the irrigation stream, including small electrical pumps which must be calibrated and require electric power running to the location, and pumps that are powered by irrigation water.

The TMB pump is constructed of stainless steel and has rubber diaphragms. This pump is driven by the irrigation water and does not require electric power. It is durable, reliable, and accurate. Water drives the piston, which draws a known volume from a tank and forces it into the irrigation line. If chlorine is injected and left overnight in the pump, the stainless steel can be stressed, shortening the life of the pump. TMB pumps come in multiple sizes, pumping from 13 to 300 gallons per hour (gph).

The Amiad pump is made entirely of plastic and also uses water to drive the piston. It, too, is accurate and durable. Amiad injection pumps come in multiple flow rates and often operate between 0.5 and 8 bar. Because of the plastic construction, chlorine does not harm this pump.

The Dosatron is a fertilizer injector. It uses hydraulics to suck fertilizer from a tank, and then it can pump it into the system while attached in-line without requiring a bypass.

Scheduling Fertigation Events

Drip fertigation schedules vary from crop to crop. Most schedules are designed to supply the plant nutrients as needed. It is helpful to analyze plant samples in order to adjust fertigation techniques for a particular crop. Adding nutrients with a drip system can make up for any shortages you discover after the preplant fertilizer has been applied. For more information about recommended fertigation rates, please visit access the Southeastern U.S. Vegetable Crop Handbook. Fertigation should be performed weekly or more frequently. Dividing the recommended rate into smaller, more frequent applications often enhances fertilizer efficiency and improves plant performance.

Fertigation can seem complicated to new users and should be managed with care by even the most experienced fertigator. Listed here are a few important fertigation principles:

• The fertilizer type and amount must be water-soluble to dissolve effectively in the nurse tank. Keep in mind that colder water temperatures reduce fertilizer solubility.

• The irrigation system should be at full pressure (charged) at the farthest point from the pump before injecting fertilizer.

• The amount of time it takes to inject fertilizer should be at least as long as the time it takes to charge the system.

• Undissolved materials should be filtered.

• The system must be equipped with proper backflow prevention.

Solubility

Solubility of materials varies in both the rate with which they dissolve and in the amount that can be dissolved in water. Most materials dissolve best in water with a pH of 5.5 to 7. Some common fertigation materials and their solubility are listed in Table 1.

If you attempt to dissolve more fertilizer than is soluble in a given volume of water, “salting out” can occur, meaning that a mass of fertilizer settles to the bottom of the tank. Agitation by a mechanical screw or recirculation with a pump can help dissolve fertilizer. Heating the water increases solubility of fertilizers and allows them to dissolve more rapidly. It is important that the material be dissolved because uniformity of application depends on each gallon of irrigation water containing the same amount of fertilizer. Undissolved materials lead to uneven application rates and nonuniform crop responses. When only a few applications are made, proper mixing becomes even more important.

When dissolving fertilizer, it is best to half fill the tank with water and slowly begin adding dry fertilizer materials with constant agitation. Be careful not to add fertilizer so fast that material begins to settle to the bottom. Continue adding fertilizer and filling the tank with water until the proper fertilizer amount is in the tank. Some materials like potassium nitrate dissolve in an endothermic reaction (the solution cools as the fertilizer dissolves), which reduces the amount of material that is soluble. This type of reaction requires heat to help dissolve the materials. Some growers avoid these problems by purchasing mixes designed for fertigation purposes.

Micronutrient Needs

Whether micronutrients should be added to the fertilizer solution on a regular basis depends on soil type and conditions. If soils have a low nutrient-holding capacity, micronutrients should be injected. The area around the drip tube becomes the major source of all nutrients, and even though the general soil test may show good levels of micronutrients, there can be deficiencies further away from the drip tube. It is often necessary to use a mixture of micronutrients by supplying all those that are deficient. Check compatibility and solubility charts to avoid problems with micronutrients. Boron is one of the most commonly needed micronutrients and has been shown to enhance potassium uptake.

Charging the System

The irrigation system should be fully charged before any fertilizer is introduced. When the system is not fully charged, air surges occur and cause erratic application rates of fertilizers. If the system is at full pressure with no entrapped air, the flow of fertilizer solution is uniform and equal amounts will pass through each dispensing hole (dripper). Application time should be at least as long as the time it takes to bring the system to full pressure. This will allow each dripper to have similar contact time with fertilizer solution as it passes through the system.

Filtering Fertilizer

Some growers believe the misconception that undissolved fertilizer and impurities will dissolve as they pass through the filter or pipelines. Such an approach results in uneven fertilizer distribution and causes the dripper lines to clog because most fertilizer materials do not totally dissolve in the time it takes them to get through the system.

Because undissolved fertilizer is a particular problem in drip irrigation systems, the point at which fertilizer should be injected into the system is before the filters. Also, it is very helpful to allow the fertilizer solution to settle before pumping it into the system. Usually settling overnight allows adequate clearing time for most fertilizer solutions. With fertilizers such as calcium and sodium nitrate, which have a coating that floats, the water should be skimmed or drawn off through a hole near the top of the tank to remove the impurities. Other impurities sink, so there should also be a hole near the bottom of the tank through which undissolved material can be washed out. Finally, there should be an opening through which to remove the clear, usable fertilizer solution; this hole should be located about 12 to 18 inches above the bottom of the tank, which allows enough room for impurities to settle.

If a fertilizer has non-dissolving material (as with some potassium nitrate sources) or non-dissolving coatings (like calcium nitrate), your fertilizer supplier may suggest flocculating agents which help these impurities drop out of the fertilizer solution.

Backflow

Fertilizer that remains in the system when it is shut down will siphon back through the pump into the water source. This is called backflow, a process that may contaminate the water source, stimulate the growth of algae in surface water, and pollute groundwater.

Optional Components of Fertigation Systems

Drip tube can be attached to the submain line by using feeder tube or special connectors. The feeder tube system works well for most small systems, but these tubes do tend to loosen and blow out of the drip tube and should be checked frequently. Larger systems usually use connectors that lock onto the drip line. Some systems also use flush valves on the end of each drip line. These valves drain the line when the system is shut down and the pressure drops below 2 psi, and they do not close until the pressure is greater than 2 psi. Flush valves are not necessary if you open the lines once every couple of weeks to flush out sediment and other contaminants.

There are many kinds of plastic mulch, some of which are designed for very specific weather and soil conditions. The following are descriptions of some of the most widely used types of plastic.

• Black Embossed Polyethylene: 1.25-mil mulch film, 3 to 6 feet wide in rolls of 2,000 to 8,000 feet.
• Clear Polyethylene: 1-to 2-mil mulch film, or row covers, 3 to 6 feet wide in rolls of 2,000 to 8,000 feet.
• White Polyethylene: 1.25-mil mulch film used in summer to help cool the soil, 3 to 6 feet wide in rolls of 2,000 to 8,000 feet.
• Metallic, Silver, Reflective: Specialized type of reflective plastic mulch. Reflects light, disorienting flying insects such as aphids, whiteflies, and thrips. Keeps soil cooler by reflecting sunlight and reflects light back onto the lower leaves of plants, enhancing light interception and potentially increasing photosynthetic efficiency (Figure 7).
• White on Black Polyethylene: 1.50-mil mulch film used in summer to help cool the soil, 4 to 6 feet wide in rolls of 2,000 to 8,000 feet.
• Low-Density Polyethylene: 0.75- to 1.25-mil mulch film used to reduce weight and cost, may or may not be embossed, 4 to 6 feet wide in rolls of 2,000 to 8,000 feet.
• High-Density Polyethylene: 0.50- to 1.0-mil mulch film used to reduce weight and cost, stronger than same thickness of low-density polyethylene, may or may not be embossed, 3 to 6 feet wide in rolls of 2,000 to 8000 feet.
• TIF (Totally Impermeable Film): plastic mulch is a specialized type of plastic mulch designed to significantly reduce the permeability of gases, including fumigants, through the mulch layer.
• Infrared-Transmitting (IRT) Plastic Mulch: Semi-transparent film that allows infrared radiation to pass while suppressing weeds.
• High-Barrier TIF: Enhanced with additional barrier layers, often made with ethylene vinyl alcohol (EVOH) or similar polymers.
• Linear Low-Density Photodegradable Polyethylene: Degradable mulches are designed so that they start to break down after exposure to the sun. The breakdown times given for degradable mulches should be used only as indicators. Under high light intensity and warm weather, they will start to break down faster than they will under cool, cloudy conditions. The presence or absence of plant cover will also influence breakdown rate. Degradable films are not recommended for double cropping.
• Paper Mulch: A biodegradable material made from natural fibers such as kraft paper or recycled materials, used as an eco-friendly alternative to plastic mulches in agriculture and gardening. It provides similar benefits to plastic mulch, including weed suppression, soil moisture retention, and soil temperature regulation, while offering the added advantage of decomposing naturally in the soil. This eliminates the need for post-season removal and reduces plastic waste. As it decomposes, paper mulch contributes organic matter to the soil, enhancing its quality. However, it is less durable than plastic mulch and may degrade prematurely in heavy rain, wind, or high-traffic areas, requiring proper anchoring to remain effective. Additionally, rolls are heavier, and the max commercially available length is 1200 ft.

Figure 7. Tomato planted in reflective mulch

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Figure 7. Tomato planted in reflective mulch

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There are other challenges particular to plasticulture that may affect your decision about whether it is right for your operation.

Insects and disease control. Good insect and disease control is essential. Consult your state’s agricultural chemicals manual or your local N.C Cooperative Extension Center for recommendations.

Weed Control

For information on weed control under clear plastic mulch and in the row middles between black plastic mulch, consult your state’s agricultural chemicals manual, or your local Cooperative Extension Center. Only herbicides approved for a particular crop can be used between rows of plastic because this is not a fallow area.

Double Cropping

After harvesting the first crop, it is recommended to grow a second crop on the same mulch. However, avoid planting the same crop in the same field twice in one year. Rotating different crops allows for two production cycles per acre, maximizing the use of land, plastic mulch, drip tubing, and starter fertilizer.

The second crop can be fertilized using one of the following methods:

• Drip Fertigation: Apply soluble fertilizers through the drip line with a fertilizer injector.
• Overhead Fertigation: Distribute fertilizers using an overhead irrigation system.
• Localized Application: Place fertilizer in the holes of the plastic mulch between plants.

To terminate the first crop, herbicides such as glyphosate or paraquat can be used. Take care during the planting of the second crop to avoid damaging the drip irrigation tubing.

Plastic mulching system relies on careful control of its major components: raised beds, plastic mulch, drip irrigation, and fumigation. It can improve crop yields and anticipate fruit harvest, thereby increasing profits. While we encourage you to further investigate plasticulture’s benefits, it is important to growers to understand the work involved in maintaining the system before deciding whether plasticulture is right for you.