Overwatering, Underwatering, and Fertigation Basics

1
Overwatering, Underwatering, and Fertigation
Basics

• Managing Water and Fertilizer Applications on
  Vegetable Crops

2
Water Sources for Drip Irrigation

• Ponds
• Lakes
• Streams
• Springs
• Wells
• Municipal

3
Water for Drip Irrigation Must Be Clean

• Any surface water source needs a sand filter.

4
Drip Tape Basics

• Requires 150-200 mesh filter.
• 5/8 in. dia. for 300-600 ft.
• 7/8 in. dia. for 600-1500 ft.

5
Monitor Drip Irrigation

• Pressure gauges are indicators of system
  performance.
• Pressure differences used to monitor filters.
• Pressure readings used to set pump speed.
• Pressure at drip tape no more than 15 psi.

6
Pressure Regulators

• Reduce pressure in manifold to desired (12-15
  psi) tape pressure.
• Drip tape may break at 30 psi.

7
Screen Filters

• Used as secondary filters.
• Over size because 2 dimensional.
• 2-3 psi drop across filter normal.
• 5-8 psi drop means clean the filter!

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Disk Filters

• Stack of grooved disks.
• More effective than screen filters.
• Can be used as primary filter on small acreages.

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Disk Filter Rings

• Grooves in rings filter particles from water.

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Sand Filters

• Used for surface water.
• Capacity must match flow rate.
• Use pressure change to time back flush.
• Fertilizer injected before filter.

11
Maintenance of Irrigation Filters

• Check screen and sand filters after each
  operation.
• Back flush sand filters.
• Soak or brush clean screen and disk filters.
• Use pressure gauges at filters inlet and outlet
  to tell when filter needs cleaning (5-10 psi).
• Flush main, submain and lateral lines regularly.
• Use automatic back flushing devices on drip lines.

12
Drip Irrigation System Maintenance

• Chlorine is used to
• kill bacteria.
• decompose organic matter.
• oxidize soluble minerals.
• Apply chlorine ahead of first filter.
• flush lines immediately after chlorination.
• Acid treatments.
• Lower pH and dissolve Mn, Fe, Ca.

13
Water Management and Schedule

• Available water key to crop growth.
• Relationship between plant-soil-water
• Soil that contains plants roots is water
  reservoir
• Field Capacity - water stored in soil 12-24 hrs
  after saturation.
• Permanent Wilting Point water no longer
  available to plant.
• Available Water Holding Capacity -difference
  between Field Capacity and Wilting Point.

14
Water Management

• Irrigation scheduling determines
• How often to irrigate? Decide the allowable soil
  depletion.
• Soil water holding capacity.
• Rate of crop use.
• How much to apply? Irrigate to replace crop use.
• Crop growth stage.
• Weather conditions (sun, temperature, humidity,
  wind).
• Estimate daily crop use (pan or SMT readings.
• Verify irrigation amount applied.

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Determining Allowable Soil Depletion

• A general recommendation is to
• maintain SMT in the 10-20 cb range.
• maintain Available Moisture in the 80-85 or
  15-20 depletion range. Keeping the SMT low
  ensures the plants have adequate water.
• Peak ETc usually 0.2 0.3 in./day.
• 5,430 8,146 gal/acre/day.
• Usually 33-50 of an acre is drip irrigated.
• Limit individual irrigations to less than 0.5
  acre inch.

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Soil Moisture Sensors

• Place first in middle of crop root zone.
• Place second at bottom of root zone.
• Read sensors every day in early morning.
• Irrigation should occur when sensor exceeds a
  predetermined tension reading.

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Soil Moisture Sensors Used To

• Soil moisture tension measured in centibars.
• Centibar values for different soil types indicate
  different percents available water.

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Soil Moisture Sensors Used To

• Time irrigation.
• Prevent overwatering.
• Prevent underwatering.
• Maximize crop quality.
• Ensure optimum yield.

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Tensiometer Readings
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Soil Moisture Level Chart

• Tensiometer readings can be charted for each
  crop. The information charted is a record of our
  ability to provide moisture to the growing crop.

21
Effective Crop Root Zone Depth
22
Soil Moisture Release Curves for Four Soil
Textures Found in Kentucky
23
Gallons/Minute/Acre Required for Various Drip
Lines
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Determining Irrigation Time and Amounts

• If crop Etc is 0.20 acre inches/day then crop
  used (0.2 x 27,154 gal/acre in. x .50 area
  covered by plastic) or 2,715 gal of water.
• If field has 6 ft rows and uses 0.42 gpm drip
  tape. Operating properly this is 30 gal/ac/min.
  Rate per hr. is 1,800 gal.
• 1.5 hrs application time (2,715 gal/acre / 1,800
  gal.)
• Use flow meter to ensure proper application.
• Check moisture sensor next morning and adjust
  application time.

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Flow Meter for Determining Water Applications.

• Sized to match system flow rate.
• Water applied is the difference between initial
  and final dial reading.

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Time Nitrogen Application to Match Crop Needs.

• Do not fertilize when mean weekly temperature is
  below 55 F.
• Apply fertilizer nitrogen once a week at rate to
  meet crop needs.
• Monitor the nitrate level of the soil (Cardy
  Meter).
• Monitor the nitrate level plant (Cardy meter).

27
Homemade Drip Fertigation Unit

• Dry fertilizer is placed in the unit.
• Water is forced through the unit.
• A screen filter if placed on the discharge side.

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Recommendations for Fresh Market Tomatoes

• Use 2 tensiometers.
• 1 at 6 inch depth.
• 1 at 12 inch depth.
• Start irrigation when 6 in. tensiometer reads 20
  cb.
• Maintain 12 in. tensiometer on 30 cb.
• During harvest use 30 cb and 40 cb respectively.

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Tomato Problems Yellow Shoulder Disorder and
Blotchy Ripening

• Studies indicate soils and plants with high K
  levels have less YSD.
• Keep soil pH in 6.2 6.5 range.
• Higher pH makes YSD worse.
• Organic matter gt3.4
• Soil nutrient levels indicate probability of YSD.
  Get a soil test!

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Desirable Soil Characteristics for Tomatoes.

• Extractable K gt 0.4 cmol/kg.
• Hartz ratio gt 0.35.
• K is at least 4 of CEC.
• Ca/Mg ratio (meq/100g) is at least 31.

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Web Site for Hartz Ratio
K / Mg
Web Site www.oardc.ohio-state.edu/tomato/ysd/new
hartzratio.htm
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Hartz Ratio
Enter values for Potassium (K), Magnesium (Mg),
Calcium (Ca), Cation Exchange Capacity and the
units used by the laboratory.
Calculate
33
2003 Quicksand Tomato Soil Test Results
34
Hartz Ratio
2003 Quicksand values for Potassium (K),
Magnesium (Mg), Calcium (Ca), Cation Exchange
Capacity.
Calculate
35
2002 Daviess County Tomato Soil Test Results
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Hartz Ratio
2002 Daviess County values for Potassium (K),
Magnesium (Mg), Calcium (Ca), Cation Exchange
Capacity.
Calculate
37
Tomato Field Hartz Ratio After Fertilizer
Application
Grower applied 125 lb of K2O/acre which is 104
pounds of K.
Calculate
38
Potassium Deficiency in Tomato

• Oldest leaves first
• Chlorosis of leaf tissue, margin necrosis.
• Veins remain green.
• Leaf may roll.
• Internal fruit discoloration.
• Yellow shoulders.
• Small root system supplies less K.
• More likely in wet year.
• Excess CaNO3 fertilizer.

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Recommendations for Fresh Market Peppers

• Apply 50 lb N preplant then fertigate with 6-8 lb
  N/ac/wk for 12 wks.
• Use 2 tensiometers.
• 1 at 6 inch depth.
• 1 at 18 inch depth.
• Start irrigation when 6 in. tensiometer reads 20
  cb.
• Maintain 18 in. on 30 cb.

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Recommendations for Fresh Market Cantaloupe

• Apply 50 lb N preplant then fertigate with 6-8 lb
  N/ac/wk for 8 wks.
• Use 2 tensiometers.
• 1 at 6 inch depth.
• 1 at 18 inch depth.
• During growth start irrigation when 6 in.
  tensiometer reads 25 cbs. 35 cbs at harvest.
• Maintain 18 in. on 30 cb until harvest then use
  40 cbs.