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Drip Irrigation: Greenhouse Vegetable Production

Leon New and Roland E. Roberts
Extension Irrigation Specialist and Vegetable Specialist
 

Soil Moisture Control

Automatic Drip Irrigation is a valuable tool for accurate soil moisture control in highly specialized greenhouse vegetable production.  Total automation of drip irrigation offers a simple, precise method for sensing soil moisture and applying water.  Management time savings and the removal of human error in estimating and adjusting available soil moisture levels enable skilled growers to maximize net profits.

Available soil moisture is an important limiting factor in growth and productivity.  Greenhouse vegetable growers commonly estimate the availability of soil moisture by plant and soil appearance.  Slight wilting of succulent terminal leaves indicates water stress in plants.  Growers squeeze handfuls of soil taken from near the surface at several locations in the greenhouse.  Soil that does not stay compressed in a tight ball is considered too dry.

Water deficiency can be detrimental to plants before visible wilting occurs.  Slowed growth rate, lighter weight fruit and, in tomato, blossom end rot often follow slight water deficiencies.  Replacing traditional methods of estimating available soil moisture with a more accurate method is necessary to maintain optimum soil moisture levels.

Conventional irrigation methods usually wet plant's lower leaves and stems.  The entire soil surface is saturated and often stays wet long after irrigation is completed.  Such conditions promote infection by gray mold-rot (Botrytis) and leaf mold fungi.

Most greenhouse vegetable plants remove large amounts of water from soil at the 10" to 12" depth.  An accurate estimate of available soil moisture at this important depth cannot be made by testing the top few inches of soil.  In a greenhouse on a sunny day, transpiration and evaporation can occur so rapidly that excessive water loss can cause plant damage before sufficient water can be applied to correct moisture stress.  Water stress, no matter how slight, will cause a significant reduction in harvest weight.

Drip irrigation is a slow water delivery system in which water can be applied, drop by drop, to the soil surface near the base of the plant.  A properly designed automatic drip irrigation system can remove much guessing about when to irrigate and how much water to apply.  Water is applied whenever the sensor indicates a sub-optimum soil moisture level.  Using automatic drip irrigation systems, skilled greenhouse managers can:

*Apply correct water amounts precisely when required to maintain optimum available soil  moisture in the root zone.

*Reduce management time required for observing plant water needs and manually  controlling irrigation systems.

*Keep leaf surfaces and stems drier because water drips directly on the soil instead of  spraying in the air.

*Prevent water puddleing and splashing by applying water no faster than it will percolate  into the soil.

*Reduce incidence of leaf mold, gray mold-rot and other foliage diseases.

*Reduce evaporation losses and fruit deterioration by keeping more soil surface dry.

*Increase production if other factors are not limiting.
 

Planning a Drip Irrigation System

Uniform water application, operating convenience and minimum cost are important objectives in planning a greenhouse drip irrigation system.  Carefully study this section's ideas on achieving these objectives before selecting drip irrigation system components.

Divide the total greenhouse area into equal or similar sections or into individual houses.  Plan irrigation systems so that each house or section can be irrigated independently.  (See Figure 1.)  Plan total irrigation systems in conjunction with other greenhouse water needs to prevent exceeding water supplies.

The total amount of water available for all greenhouse uses, often described in gallons per minute, is a useful figure.  Using a portable water meter, the well or other supply source usually can be measured.  Water delivery rate from small wells often is determined by measuring the time required (in seconds or minutes) to fill a container of known volume (such as a 30-gallon garbage can or a 55-gallon barrel).  When greenhouse water requirements exceed the well delivery rate, a storage tank can increase the quantity available during the peak usage.
 

Water Requirements

Drip irrigation requires less water than lay-flat perforated hose, flooding or other frequently used water distribution procedures.  Plan irrigation piping for each separately irrigated greenhouse section or individual greenhouse to distribute 1.6 to 2.4 gallons per minute for each 1,000 square feet.  This is 8 to 12 gallons per minute for each 5,000 square feet of growing area.  Less water may not fully pressurize the irrigation system piping, causing uneven water application.  Uneven water distribution often creates dry or over wet areas.

Soil texture controls the rate at which water can be absorbed by the soil.  To prevent puddling and runoff, plan lower water delivery rates for heavy clay soils with characteristic lower water intake rates.  Be careful to plan the water delivery rate to be no greater than the soil water intake rate.  See further discussion under water application in this section.
 

Flow Control Valve

The quantity of water that can enter any independently controlled or operated portion of the drip irrigation system should be regulated with a flow control valve.  Each greenhouse section or separate house should have one flow control valve which is sized and selected according to reliable plant water requirement values.  Flow control valves are usually available in 1 and 2 gallons-per-minute size increments.  Sizes of pipe connections usually are 3/4 and 1 inch.  The flow control valve should be located upstream from the solenoid valve where the irrigation system is automatically controlled.

A minimum water supply pressure of 15 pounds per square inch is required for proper operation of most flow control valves.  Valves will function properly, however, with pressures as high as 60 to 80 pounds per square inch.  For best performance, maintain pressure in the main water supply line between 20 and 40 pounds per square inch.

The flow control valve assures application of a constant quantity of water by the irrigation system as long as the greenhouse water system pressure is within the allowable range.  Flow control or other valves must limit water entering each greenhouse irrigation system when low pressure drip emitters are used to prevent puddling, over watering and plant splashing.  A properly sized flow control valve reduces water pressure range of 2 to 4 pounds is correct for low pressure drip irrigation emitters.
 

              

Piping

Black polyethylene (PE) and polyvinyl chloride (PVC) pipe are used most commonly in drip irrigation systems.  Low costs and handling ease are primary factors.  Low pressure, rigid polyvinyl chloride pipe often is used for supply and header lines because the connections and fittings can be solvent bonded.  Polyethylene connections, however, must be clamped or held by a tight fit.  Polyethylene pipe (usually 80 pounds per square inch, non-sanitation-foundation approved) normally is used for emitter laterals because it is flexible and easy to handle.  Flexible polyvinyl chloride pipe is less sensitive to high temperature and sunlight and is more durable, but it is also more expensive.  Water for processing and human consumption should not be supplied through non-sanitation-approved pipe.

A ½ inch polyethylene pipe is adequate for emitter lateral lines.  (See figure 3).  Where manufactured emitters are used, ½ inch pipe will provide equal water distribution and uniform water application throughout the usually 100 to 150 foot greenhouse irrigation runs.  Of course, other irrigation system components must be properly designed and selected.

Plan a ½ inch lateral emitter line for each plant row unless experience suggests a successful alternative.  Place lateral pipe containing emitters at the base of plants.  Keep the pipe in line on the inside of the plant row with respect to the working aisle.  Soil in the working aisle can become compressed by traffic.  With emitter lines on the aisle side of the plants, water tends to flood the aisles.

When drip irrigation is installed in an existing greenhouse with rows spaced 20 inches apart or closer, one emitter line may be sufficient for two plant rows.  However, production is normally significantly higher where plant rows are spaced further apart.  Consider wider row spacing for future crops and plan the system accordingly.

Do not place a single emitter line serving two rows in a furrow or ditch.  Water will not move up the soil incline and across the row to properly wet soil in the aisles.  In this case, no roots develop in the aisle soil and plants tend to stress for water sooner during high water requirement periods.

Polyethylene pipe, which is sensitive to high temperature, will contract and expand and can move out of position.  Light springs, such as screen door springs, can be attached to the lower end of emitter lines with the other end fastened to a stake or wall.  This helps hold the pipe next to the plants.  Adjust the fasteners tension while the system is irrigating and the pipe is cold and contracted.

Pipe 3/4 to 1 inch in size is usually large enough for header lines, not main supply lines, for 5,000 square feet of growing area.  Make the connection to the supply pipe at the center of the header pipe rather than at the end.  A tee connection into the center of the header line equally divides the water supply and reduces water pressure losses.  (See Figure 1).

Plug or tape the ends of all piping and fittings during installation except when making the final connection.  This keeps soil and other particles out of the system and reduces drip emitter plugging.
 

Emitters

Several kinds of drip irrigation water emitters, perforated hose and porous pipe are available for use in drip irrigation systems.  Drip emitter water application is described in gallons per hour, and emitters are made to apply a specific amount per hour.  Most are within a range of ½ to 3 gallons per hour.  Rate of water delivery from an emitter can be changed by increasing or decreasing the water pressure.

Drip irrigation emitters and perforated pipe and hose can also be further classified as low or high pressure.  When exposed to the same pressure increase, the water output from low pressure emitters increases at a rate of three to four times that of high pressure emitters.  Plan greenhouse drip irrigation systems so that each drip emitter applies 1 to ½ gallons per hour.

Low pressure emitters operate best when pressure in the emitter lateral pipe is 2 to 4 pounds per square inch.  Level the greenhouse soils surface so that elevation does not create a difference in pressure.  A properly sized flow control valve reduces the typically higher greenhouse water system pressure to about 2 to 4 pounds in emitter laterals.  The Melnor-Tirosh emitter, Submatic insert emitter, Chapin Twin-Wall, ANJAC Bi-Wall (or other perforated hose), Triklon Microtube, and 3 to 12 inch lengths of .036 or .045 inch diameter spaghetti tube are examples of low pressure emitters and perforated hose.

High pressure drip emitters can also be used in greenhouses, but their cost is usually greater.  High pressure emitters are designed to apply ½ to 3 gallons per hour at 15 to 25 pounds of pressure.  Do not install a constant flow control valve when high pressure emitters are used.  The valve reduces the lateral or emitter line pressure below that required by high pressure emitters to apply adequate water.  Instead, install Globe valves, solenoid valves with flow control, or other controls that allow higher pressure.  Use them in conjunction with a pressure gauge to manually set the system pressure near 15 pounds or to apply 1 to 1-1/2 gallons per hour from each emitter.

Emitter and hose durability and ease of installation are important considerations in emitter selection.  While spaghetti tubing is more economical than manufactured emitters, more labor is required for its installation.  Perforated hose is installed more easily, but is not as durable.  Emitters, pipes and fittings should be black to prevent algae growth inside the piping system.
 

Emitter Spacing

Space emitters about 3 feet apart in the ½ inch polyethylene lateral lines.  Allowing 24 to 30 inches between emitters provides more uniform soil moisture in extremely sandy soils where water lateral movement is limited.

Where only 20 inches exist between plant rows in a pair and single ½ inch emitter line is installed to irrigate the two rows, space emitters 24 to 30 inches apart.  Where plants are in rows more than 20 inches apart, allow one emitter line for each plant row.  A spacing of 28 to 32 inches between rows in a pair is preferred for maximum foliage exposure to sunlight.  Place emitter pipelines along rows on the side of plants away from walking aisles, as shown in Figure 3.
 

Water Application

The key to successful drip irrigation is applying small quantities of water very slowly and as frequently as required to maintain soil moisture content at a uniformly high level.  Application rates range from 0.15 to 0.23 inch per hour, or from 1.6 to 2.4 gallons per minute per 1,000 square feet of greenhouse area.  Short on-and-off operating cycles, such as 15 minutes on and 15 minutes off, allow additional time for water to move into the soil and equalize soil moisture.

Intermittent irrigation cycles help prevent puddling and surface runoff from heavy clay soils with slow water intake.  If intermittent irrigation cycles are utilized for soils with extremely low water intake rates, a lower water application rate such as 0.10 inch per hour may be required to prevent water puddling and runoff.  This low application rate may not provide enough water volume to fully pressurize the piping in a system using low pressure emitters, and water distribution will not be uniform.  High pressure emitters that apply ½ gallon per hour are more likely to provide even water distribution using smaller water quantities and should be used where an intermittent irrigation cycle is not employed.

Drip irrigation emitters spaced 3 feet apart should each apply 1 to 1 ½ gallons of water per hour.  Corresponding water applications using factory perforated pipe or hose is 1/3 to ½ gallon per hour per linear foot.  Estimate the average emitter application for a greenhouse area by multiplying the flow control valve size in gallons per minute by 60 (minutes per hour) to establish the gallons per hour.  Dividing the gallons per hour by the number of emitters in the sub-area or house gives the average gallons per hour per emitter.  This calculation can be made using the following formula.

Average Flow control valve

GPH/emitter    =     GPM x 60 min./hr.
                                     Number of emitters/house

To establish an emitter application rate of 1 to 1 ½ gallons per hour, it may be necessary to tentatively select the flow control valve size which will apply 1.6 to 2.4 gallons of water per minute for each 1,000 square feet.  The objective is to establish an emitter application rate of 1 to 1 ½ gallons per hour.  Choosing the proper size flow control valve is most important when using low pressure emitters.
 

Water Filtration

Water must be filtered before flowing into a drip irrigation system.  Very small diameter emitter orifices and hose perforations, ranging from .020 to .050 inch, are required to accomplish the slow water delivery technique of drip irrigation.  Sand, soil, plant and other foreign material which can easily cause plugging must be filtered from the water.  Water containing large quantities of sand, silt, or debris (such as that from canals) requires large capacity, extra fine filters.  A single filtration system normally can be installed on the main water supply line to serve the total irrigation system.  Water filtration is the key to successful performance of drip irrigation systems.  Only clean water insures trouble-free operation month after month.

Y-type, in-line strainers, containing at least 100-mesh screens and equipped with clean-out faucets, normally provide adequate filtration for minimum sand conditions.  Trapped particles can be flushed from the filter by opening the faucet, and screens can be removed for more thorough cleaning or replacement.  Daily flushing is necessary when the filter collects considerable material.  Install the filter with the screen housing and flush valve down.  This allows trapped particles to be washed from the filter rather than moving beyond the screen and into the downstream piping when the screen is removed for cleaning or replacement.

Replaceable cartridge filters, multi-mesh screens (such as 100 and 180 mesh) or other fine mesh filters are required where water contains larger quantities of sand.  Where sand is an extreme problem, a sand separator, sand trap, or sand settling basin is required in conjunction with filters.  Install each separator, trap or basin upstream from the cartridge or mesh filter.

Where the water supply is an open reservoir, canal or stream, install a rayon or Dacron cloth box filter on the inlet or pump suction pipe.  A box frame 3'x3'x3' fabricated from angle or rod iron can be covered with shirt grade Dacron cloth.  Where possible, the pump suction pipe should enter the box through a hole in the top side.  Install pressure gauges behind and ahead of the filter to identify the need for cleaning clogged filters.
 

              

Fertilizer Injector

Fertilizer, especially nitrogen, can be applied through the drip irrigation system.  A properly planned injection system can accurately distribute fertilizer to every plant in the greenhouse.  Connect the injector to the main irrigation supply line so that the fertilizer can be selectively routed to each greenhouse section.  Plan the water supply line connection so that fertilizer material flows through the drip irrigation system filter.  The injector can be charged or otherwise set to inject fertilizer whenever the automatic control activates the irrigation system.

Positive displacement pump injectors, or forced flow batch tanks can be planned as an integral part of the drip irrigation system.  The injection device must be selected for proper operation of the greenhouse water system pressure.  It must have the ability to inject the proper amount of fertilizer material using the irrigation water flow rate.  Convenient, variable feed selection available on pump injectors accurately controls fertilizer quantities.  Venturi proportioners require about 12 pounds of water pressure to operate and must be installed upstream from flow control valves used with low pressure drip irrigation emitters.
 

Automation

Greenhouse drip irrigation systems can be easily automated because of the piping arrangement and small quantity of water delivered.  Automation simplifies the irrigation task, reduces labor, provides continuous monitoring of soil moisture and supplies additional water as needed.  Water can be applied to soil to satisfy plant needs at any time of day and even when other production or harvest operations demand full attention from the entire work force.  Typical components and a wiring diagram for automatic irrigation control are shown in Figure 2.

The switching tensiometer is an accurate and reliable soil moisture sensor and an automatic controller for greenhouse irrigation.  The automatically controlled drip irrigation concept is attractive to greenhouse growers.  Automatic irrigation systems allow maintenance of uniformly high optimum soil moisture levels throughout the root zone.

In addition to the switching tensiometer, a typical automatic control system requires an electric solenoid valve, a transformer and a relay.  A time clock or other primary control device is recommended to irrigate in frequent on-and-off cycles.
 

Soil Moisture Sensor

The soil moisture sensor or switching tensiometer is the automatic control system's <ì>brain.  With exceptional management, a water-tight roof and uniform plant environment, one instrument can accurately control the irrigation system for 10,000 square feet or more.

The switching tensiometer consists of an enclosed water column, a porous ceramic sensing tip, a vacuum gauge and an electric contact switch.  The instrument senses and registers the suction required to pull water from the soil.  It is the same suction required to transfer water from the soil to plants root hairs.

When the soil is drying, water from the instrument's water column moves from the porous tip to the soil.  A vacuum created inside the column during the water removal pulls water from the soil through the porous tip back into the tensiometer as the soil is re-wetted.  The vacuum gauge dial registers the pressure changes associated with changes in available soil moisture.

The electrical contact switch must be pre-set manually by its orientation on the vacuum gauge dial to close and start the irrigation system.  The electrical switch is set at the driest soil moisture level allowed.  When the soil has been irrigated sufficiently for the dial to register a lower reading than the switch setting in centibars ( a higher moisture level), the switch opens and stops irrigation.  The tensiometer switch setting is critical in maintaining accurate soil moisture control.

The switching tensiometer operates on 24-volt electricity.  The switch is rated 12 watts and ½ ampere.  Therefore, the solenoid valve must be of very low wattage (2 to 3 watts) to be connected to the same circuit and controlled directly by the tensiometer switch.  Use a 24-volt control circuit utilizing a relay to prevent electrical overload in the tensiometer switch and extend its life.  The tensiometer switch is electrically connected with the transformer and relay, which each must be 24 volts, to form the automatic control circuit.  When soil dries to the switching tensiometer setting, the tensiometer switch closes the control circuit and the relay causes the solenoid valve to open, allowing water to enter the portion of the irrigation system controlled by this particular tensiometer.  When the soil has been re-wetted, the switching tensiometer senses the wetter soil; the tensiometer switch opens, causing the solenoid valve to close an turn off the irrigation system.  Figure 2 shows a typical wiring diagram using a 24-volt relay and control circuit.
 

Time Clock

There is a time delay for water applied by the irrigation system to move down to the 8 to 12 inch moisture-sensing depth.  A time clock or operation sequencing device can be used as a primary control to cause the irrigation system to apply water intermittently.  Intermittent water application helps compensate for the water penetration delay and can prevent over-irrigation.

One time clock with two circuits, one normally on or closed and the other normally off or open (single pole double throw) can serve two greenhouse areas controlled by two tensiometers.  One area can be controlled by the normally on circuit while another is controlled by the normally off circuit.  This causes two sections or houses to be irrigated on intermittent cycles.  Each irrigation system is shut off by the switching tensiometer after adequate irrigation.

A multi-station sequencing control can provide similar primary irrigation system control.  A sequencing controller with a long number of stations, however, could delay irrigation too long on hot days when the full number of stations is being checked or allowed time to irrigate.  With drip irrigation, the idea is to establish and maintain a high soil moisture level within the optimum range of 10 to 20 centibars.
 

Automatic System Operation

Irrigation Frequency

Water application frequency and watering time of individual irrigations are highly influenced by air temperature, relative humidity, sunlight and plant size.  Record the system operating frequency and total operating time for at least one or two sections or houses.  Operating time recorders can be connected to the relay in the tensiometer control circuit using a double pole relay.  Regular time clocks can be wired similarly to run only when the irrigation system is on.

Recorders on automatic drip irrigation systems show that average irrigation frequency is almost 4 days.  The frequency ranges from 1 to 7 days but most commonly is 3 to 5 days.  The interval between irrigations tends to be slightly longer during winter months.
 

Time of Individual Irrigations

Actual water application time of individual irrigations commonly ranges from 1 to 3 ½ hours.  In a system using 15 minute on-and-off cycles, the 1 to 3 ½ hours of irrigation occurs in a total time period of 2 to 7 hours.  Irrigation time varies less than the days between irrigations, indicating that soil dries to nearly the same level before each irrigation and that similar quantities of water are required to re-wet the soil each time.  Water application during 1 ½ hours of irrigation averages 0.45 inch.
 

System Management

An automatic drip irrigation system does not replace good grower management, but can be an elegant production tool for a skilled grower.  Indispensable to the success of the novice grower, an automatic drip irrigation system can simplify irrigation procedures, reduce irrigation labor and provide precise soil moisture control.  The system requires periodic operation checks and frequent observation, especially following initial installation.

Management tips will be helpful to growers who want to become fully acquainted with the versatility of automatic drip irrigation in increasing their plants productivity.
 

Plant, Soil, and Tensiometer Relationships

Water exists in plants as a continuous column from the leaf interior downward through they xylem vessels of the stem through the roots to the tiny root hairs.  Water molecules in the narrow xylem vessels are held to one another by strong, cohesive forces.  Water exists in the soil as a film of molecules around the grains of sand, microscopic clay micelles and particles of organic matter.

Transpiration is the process by which water evaporates from leaf surfaces and creates an upward movement of water through the plant, replacing water vaporized and released into the air from leaves.  A suction force is transferred from leaves downward, inside the xylem of stems to plant root hairs and finally to the water film around soil particles.  As water is removed from soil by roots, the film of water around soil particles become thinner.  An increasingly greater suction force is required to pull water molecules from soil particles to the root hairs.  If more water is released from the leaves than is taken in through the roots, the leaves wilt.  A natural effort to correct this moisture imbalance in tomato occurs by withdrawing water from around fruit.  This is the primary cause of blossom end rot.

A tensiometer contains a closed water column with a porous ceramic tip.  Water molecules can move through the porous tip to its exterior surface and then into the soil.  Water molecules on the surface of a tensiometerís porous tip are in contact with water molecules on root hair surfaces and soil particles.

The suction force created by plant transpiration on the water column is registered by a vacuum gauge on tensiometers.  The force required to remove water from the soil is similar to that to which the plant is exposed.  A tensiometer gauge registers the vacuum or suction force in centibars.  One centibar of vacuum is equivalent to 1/1000 of an atmosphere (14.7 pounds per square inch) or 0.147 pounds per square inch.
 

Tensiometer Location

Locate a switching tensiometer in the driest portion of the soil area it controls.  This is often near the exhaust fan end of the greenhouse, but the location should be 10 to 20 feet within the growing area.  Locate the tensiometer in a representatively dry area and not in a high place or an area receiving additional water.

When rows are equally spaced, locate the tensiometer midway between two drip emitters and toward the midpoint of two plant rows.  Some growers prefer the tensiometer to be positioned in the working aisle to insure optimum soil moisture levels.  Do not install the tensiometers in a plant row, for irrigation will be stopped before water moves to the entire soil root zone.  This limits the soil area wetted, and the adequate soil moisture is not likely to be available.

Where one emitter lateral is serving two closely spaced rows and is lying midway between the rows, place the tensiometer on the opposite side of the row toward the center of the aisle.  Protect and set the tensiometer at an angle to minimize interference with greenhouse traffic.
 

Tensiometer Sensing Depth

The porous tip of the tensiometer should be installed to sense the soil moisture level at 6 to 8 inches just after setting and while establishing plants.  As the plants grow, lower the sensing tip so that the tip is 10 to 12 inches deep to monitor the maximum moisture extraction rate.  A 12 to 18 inch sensing depth may prove best in extremely sandy soils.

The tensiometer usually can be pushed to desired soil depth.  Never push on the gauge or cap.  Grasp the shank of the tensiometer with both hands and push downward.  Be sure that the soil is uniformly and firmly in contact with the porous tip.  An oversized hole or loose, dry soil causes poor contact with the porous tip and inaccurate tensiometer readings.  Proper tensiometer sensing is essential for accurate irrigation system control.
 

Tensiometer Switch Setting

The ideal soil moisture level for greenhouse tomato production is slightly less than field capacity.  Field capacity describes a very high soil moisture content--all the water soil can hold against the downward pull of gravity.  The amount of water that can be stored in an acre-foot of greenhouse soil varies directly with the amount of clay.  Sandy soils hold less water available to plants than do loam or clay loam soils.

The moisture level at which the switching tensiometer starts irrigation must be set manually.  This setting is critical in successfully maintaining optimum soil moisture.  A high setting may allow plants to undergo stress before soil moisture is replenished.  A low setting allows too much irrigation time and establishes a detrimental over wet condition, causing water to leach fertilizer elements from the root zone.  Plant roots in soil which is too wet can experience oxygen starvation and consequent injury.

Identify the correct tensiometer switch setting through and experience, closely observing plants and fruits.  The following range of settings are recommended as guidelines.
 
 

Soil texture                                            Switch setting

sandy soil.............................................10 to 15 centibars
sandy loam soil....................................15 to 29 centibars
loams and clay loam............................20 to 25 centibars

Make the switch setting by orienting the electric switch unit over the recommended number on the gauge dial of the tensiometer.  Soil drying is registered as a rise in the gauge reading.  Conversely, a lower reading indicates wetter soil.  It is possible to keep soil moisture uniform and always within the optimum range using an automatically controlled irrigation system.
 

Tensiometer Sensing Delay

The tensiometer senses soil drying more quickly than re-wetting.  Because there is a short delay in tensiometer response to soil re-wetting following a drying period, operate the irrigation system on frequent on-and-off cycles.  The off period allows time for the tensiometer to sense soil wetting and for water to move horizontally in the soil.  On-and-off irrigation cycles of 15 minutes are satisfactory for West Texas sandy loam soils.  Heavy clay soils and higher water application rates may require a longer off time.

The switching tensiometer turns on the irrigation system with an accuracy of 3 to 4 centibars.  A time clock, sequencing controller or other electric or time control can be used as a primary override control to cause the system to irrigate on intermittent cycles.
 

Tensiometer Servicing

The tensiometer must be charged with water and the level kept near the top.  Keep alert to tensiometer water levels during routine greenhouse work.  Water will be removed faster during hot weather when plant water requirements are high.  Proper servicing may be best accomplished on a definite schedule, such as once a week.

When placing the screw top on the tensiometer water reservoir, tighten until the rubber stopper just barely contacts the inside base of the reservoir.  Then, tighten only 1/4 to ½ turn more.  Forcing the cap on too tightly distorts the threads and prevents a closed water column, which makes the tensiometer inoperative.
 

Support Tensiometers

Two or more regulr tensiometers can provide a check on switching tensiometers performance accuracy and can help identify soil moisture differences within individual greenhouse sections.  One procedure for using support tensiometers is to locate three tensiometers together to sense soil moisture at three depths, such as 6, 12, and 18 inches.  Another procedure is to install support tensiometers at three locations over the house to sense moisture at the same depth as the switching tensiometer.  The same tensiometers can be used together at one location for 7 to 14 days, then moved to three locations to limit the number required.

Emitter clogging remains a partially unsolved problem with drip irrigation.  Emitter clogging becomes considerably worse when the system filter is not properly attended and open pipe connections are not protected.  The system is likely to become inoperative.

Water application from emitters must be continuously observed during routine greenhouse work.  If excessive emitter plugging occurs, check [and perhaps improve] the filter system.  Measure emitter water delivery when the delivery rate appears inadequate or lacks uniformity.  A 100 milliliter graduated cylinder is handy to make measurements.  Water must be collected while the emitter pipeline is in its normal operation position.  Lifting the emitter pipe changes the pressure and the water delivery.  For some emitters, removing a small quantity of soil below the emitter and placing a separate container in the excavated area to catch the water may be necessary.

Milli-liters of water can be converted to gallons per hour using Table 1.

TABLE 1 HERE..., (Sorry I lost TABLE 1)
 

Filter

Filter screens usually can be cleaned with very dilute acid solutions and, in some cases, with high pressure air or water.

When the filter element or screen is removed to be cleaned or replaced, be careful not to move sand and other foreign particles from the filter into the downstream piping.  Install the filter so that the element or screen is moved downward and away when removed.  This allows loose water to flush trapped particles to the outside of the element housing.

Where water contains considerable sand, irrigation water supply connections into storage tanks should be at least 12 inches above the bottom.  A flush valve located near the tank bottom can be used to remove trapped sand from the tank.  When irrigation supply connections are near the tank bottom, more sand enters the irrigation system, causing filter clogging that can prevent continuous irrigation .
 

Fertilization

Highly soluble fertilizers such as potassium nitrate, calcium nitrate, ammonium nitrate and ammonium polyphosphate can be applied singly by the drip irrigation system.  First dissolve the fertilizer material in water and make the proper concentrate.

Determine the solubility of other fertilizers by mixing a small quantity of fertilizer material and irrigation water in a clear container.  Do this before attempting to inject other fertilizer material into the irrigation system.  Some fertilizer-water mixtures form precipitates which clog filters and drip emitters.

Mixing fertilizer materials is not wise.  For example, calcium ions in calcium nitrate fertilizer solution combine with phosphate ions in ammonium polyphosphate solution to form insoluble calcium phosphate which can plug drip irrigation emitters.  The fertilizer solution should be moved through the irrigation system quickly, but uniformly, so that sufficient time is allowed for flushing with clear water before the irrigation cycle ends.
 

Between Crops

The irrigation system header and lateral emitter lines can be tied overhead to the greenhouse structure to be out of the way for tillage and fumigation between crops.  Remove tensiometers from the soil and place immediately in a pail of water.  If algae has accumulated in the tensiometer water column reservoir, clean with a long, narrow brush.  Flush and refill with distilled water.
 

Soil Farming Following Tillage

Toto-tilling soil to liberate fumigant (MC-33) leaves soil in a loose condition.  Lateral water movement is limited until soil is refirmed so that soil particles are again close enough to conduct water by capillary action.  When plants are watered, soil immediately around the plants settles, but soil between rows remains loose.  Soil moisture is uneven until the whole soil profile settles.  Moderate farming such as with a roller or board pulled by a tiller helps refirm soil.  One heavy wetting with sprinklers also settles soil uniformly.  A slight rise in the firmed soil surface profile at the midpoint between rows in a pair with a slight fall toward the aisle encourages lateral movement of water away from emitters and results in more even wetting of the soil profile.
 

Mulching

Mulching with sterilized rice hulls, clean straw, peanut hulls, etc., minimizes water evaporation from the soil surface and provides a clean, dry cushion for heavy fruit clusters.  Lateral water movement in the soil is encouraged and soil compaction in the aisles is reduced.  Annual incorporation of organic mulch materials into the soil slowly increases soil water and nutrient-holding capacities.

Soil temperatures are usually 2 to 4 degrees warmer in the winter when mulches are used.  Earlier harvest often follows.  Mulching can prevent fruit contact with soil, especially in high yielding crops.  Fruit contact with the soil is a major cause of soil rot and low grade blemished fruit.
 

Air Humidity Control

Do not allow relative humidity to drop below 50 percent.  Low humidity causes dry pollen, excessive plant transpiration and water stress.  Where resistant varieties of plants are used, overhead fine mist nozzles coupled to a hygrometer can help keep relative humidity high.  The nozzles should apply a very fine mist that does not wet the plants.
 

Grower Management Required

Automation does not replace the necessity for overall management skill; it just reduces management time.  Growers must not ignore plants for more than a day at a time.  Be keenly alert for any wilting or slowed growth rate.  Powdery, bluish-green foliage can be caused by slight water stress.  More attention is required on hot, dry days and when plants are setting the first four clusters.  Growers, whether novice or experienced, can do a better management job with automatic sensing and soil moisture control.

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