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ANR-1433

High Tunnel Irrigation and Fertigation

irrigation and fertigation have become standard

practice in the commercial production of quality

fruits and vegetables. irrigation is the process of

supplying water for the crop and fertigation is the

process of supplying nutrients through the irrigation

system. With the use and benefits of plastic mulches

came the necessity to supply water under the plastic

for most of the plant’s daily water needs. Along

with water, irrigation systems can be designed to

supply all or part of the nutrient needs of the

plants being grown.

High tunnel or sheltered production systems, much like plastic mulch, prohibit the natural delivery of water as rainfall. the artificial environment requires the producer to supply all the crop’s water needs. Water availability, water quality, irrigation design, irrigation maintenance, and all aspects of water management are critical in a high tunnel system.

Water Availability and Qualityirrigation water is a necessity for high tunnel production systems. Water sources may include wells, public water systems, harvested water, and surface water. because most of the water will be supplied through low pressure and low flow systems, the amount of water needed in volume per minute will be much less than what is used by sprinkler systems. the actual amount of water needed per minute will depend on the size of the high tunnel being used, type of tubing (example: drip tube, emitter spacing, and flow rate) used and number of irrigation zones operating at the same time.

Water quality is almost as important as availability. Poor quality irrigation water can cause physical plug-ging of emitters by algae and soil particles as well as chemical reactions with the fertilizers being injected for fertigation purposes. Physical water quality issues are more often encountered when using surface water and, in some cases, wells that are pumping sand or silt particles. However, all water sources can cause

problematic chemical properties in water, including high or excessive levels of carbonates, iron, phos-phates, chlorine, sodium, or other mineral elements.

A water analysis can be performed by the Auburn university soil testing laboratory for $16 per sample, which includes 15 different elements, pH, and soluble salts (electrical conductivity). total alkalinity can be run for $10 per sample. Find testing forms at www.aces.edu/go/295.

Irrigation Design Basicsthe design of a high tunnel irrigation system is just a small version of what would be done for a larger area. the major components are as follows:

1. Water source. A backflow preventer should always be installed between the water source and fertilizer injector to prevent possible contamination.

2. supply line.

3. Header.

4. laterals.

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design details can be found in system design, Anr-0645 in the micro-irrigation Handbook at www.aces.edu/anr/irrigation/microirrighandbook.php on the Alabama Ag irrigation info network. (see your county extension office for assistance.)

Irrigation Schedulingscheduling how much water to apply and how often to apply it depends on several factors: the crop being grown, growth stage of the crop, soil type, tempera-ture, and other climatic considerations. the Alabama Agricultural irrigation information network (www.aces.edu/anr/irrigation) provides a number of publi-cations to help you make these decisions, including information on water management and scheduling and commercial strawberry irrigation. Also see exten-sion publication Anr-1169, “basics of vegetable Crop irrigation” at www.aces.edu/go/306.

Fertilization and FertigationProper soil fertility is important to proper plant growth and production. Well in advance of planting and devel-oping a fertilization plan, you should perform a soil test to determine the soil fertility of your high tunnel site. soil test supplies for the Auburn soil testing labo-ratory are available from your county extension office, and some farm supply companies provide supplies for other soil testing labs.

Soil pHmost crops grow best when the soil pH is between 5.8 and 6.5. Your soil test results will indicate how much limestone (lime) should be applied to raise the soil pH to an optimum level. ideally, lime should be applied 3 to 6 months prior to planting your crop and mixed into the top 6 to 8 inches of soil. liming before construction of the high tunnel would make this task easier and should provide the proper pH range for 3 to 4 years. soil moisture is needed for the lime to react with the soil. therefore, if the high tunnel is already in place and covered, you should supply water to the area limed. Overhead sprinklers could be used for this purpose. supply approximately one inch of water every 2 or 3 weeks.

Plant NutrientsPlants need 17 essential nutrients for growth and devel-opment: carbon, hydrogen, oxygen (supplied through air and water), nitrogen, phosphorus, potassium, calcium, magnesium, sulfur (supplied through soil and maintained through fertilization and liming), iron,

manganese, zinc, copper, nickel, boron, molybdenum, and chlorine (supplied through the soil and occasion-ally added by fertilization to the soil or plant). if the soil pH is corrected, these elements already in the soil are more readily available to the plants.

the nutrients to be supplied are based on the soil test results and specific recommendations for the plants that will be grown in the high tunnel. before planting the crop, nutrients such phosphorus and potassium are normally applied to the soil and incorporated. Also, 20 to 40 percent of the required nitrogen may be applied and incorporated prior to planting. As soon as the crop is planted, the irrigation system should be available to water plants and supply additional nitrogen or other necessary nutrients. An injector will be used to supply most of the post-planting nutrients.

Key Fertigation Points• A functioning check valve (backflow preventer) is

needed between water source and injection.• Fertilizers should not be combined with pesticides

or chlorine.• injection site must be upstream of the filters.• irrigation system must be at full operating pres-

sure before injection begins.

Injectorsthe purpose of the injector is to take a concentrated solution of fertilizer and move it into the irrigation system while it is operating. A diluted nutrient solu-tion will be supplied as the plants are being watered. irrigation suppliers can help with the type and size of injector that you will need for your system. there are three types of injectors that may be used: pressure differential or venturi types, water-powered piston or diaphragm pumps, and externally powered positive displacement pumps. ARCHIVE

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Calculating Fertigation Injection Amounts (source: Anr-653, “Applying Fertilizers through the system” at www.aces.edu/anr/irrigation/Anr-653.php)

simple arithmetic can be used to calculate amounts of fertilizer materials to inject. the steps are slightly different for dry and liquid materials, but in both cases the purpose is to figure out amounts of as-purchased fertilizer materials to apply, based on the recommended fertilizer rate per acre, percentage to be applied per application, number of acres irrigated (the zone size), and percentage analysis of the materials.

For dry fertilizer materials, the formula to use is as follows:

pounds actual nutrient needed per acre = pounds actualnumber of weekly applications week

example:

120 pounds N (nitrogen) per acre = 10 pounds N/acre12 weekly applications week

example: Ammonium nitrate fertilizer is 34 percent nitrogen, so a 40-pound bag contains 13.6 pounds actual nitrogen. (remember: 34 percent = 0.34.)

40 pounds ammonium nitrate (34 percent N) × 0.34 = 13.6 pounds N

to determine how much ammonium nitrate (34 percent n) is needed for each weekly fertigation, divide the actual amount of n needed (10 pounds nitrogen per acre per week) by the percentage of actual nitrogen contained in the product (34 percent). (remember: 34 percent = 0.34)

10 pounds N per acre per week = 29.4 lb ammonium nitrate per acre per week0.34

this calculation provides the amount of fertilizer product to inject per acre per week but further math is necessary if the area inside a high tunnel is less than an acre. the process is the same no matter the size of the high tunnel.

1 acre = 43,560 square feet

to determine the percentage of an acre that should be fertigated, determine the number of square feet per row. inside the high tunnel are 8 rows that are each 90 feet long and 2.5 feet wide (90 feet × 2.5 feet = 225 square feet per row).

225 square feet per row × 8 rows = 1,800 square feet total

1,800 square feet = 0.04 acre43,560 sq. ft.

With a crop space that is only 0.04 of an acre, adjust the 29.4 pounds ammonium nitrate per acre per week to pounds ammonium nitrate per 0.04 acres per week.

29.4 pounds ammonium nitrate per acre per week × 0.04 acre = approximately 1.2 lb ammonium nitrate per week

the process might seem complicated at first, but it is important to become comfortable calculating fertil-izer rates on a number of different sizes of areas, for different nutrients, and for various fertilizer products that can be used for fertigation.

Keep in mind the small portion of an acre available inside a high tunnel demands smaller amounts of fertil-izer. Fertigation amounts will be a fraction (literally) of most of the recommendations found in many publica-tions and labels for crop production. the high tunnel grower must be able to convert these amounts to fit the size of the high tunnel and the area devoted to each specific crop or irrigation zone within the high tunnel.

Available resources include http://www.aces.edu/anr/irrigation/microirrighandbook.php

ANR-1433

Tony Glover, County Extension Coordinator, Cullman County, and Robert Boozer, former Extension Horticulturist

For more information, call your county extension office. look in your telephone directory under your county’s name to find the number.

Published by the Alabama Cooperative extension system (Alabama A&m university and Auburn university), an equal opportunity educator and employer.

Web Only, New Nov 2012, Anr-1433

© 2012 by the Alabama Cooperative extension system. All rights reserved. www.aces.edu

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