Crop Production Guidelines |
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By: Hector Valenzuela
HITAHR, University of Hawaii
INTRODUCTION
Drip irrigation technology has gone a long way since it was first
developed in England in the 1940s. Widespread use of this
technology began in the 1960s after polyethylene plastics were
used to make the drip tubes. Drip irrigation is especially
appropriate for the production of capital intensive crops such as
vegetables, fruits, and ornamentals. However the use of drip
irrigation requires a high initial capital investment as well as
greater management skills than for more conventional irrigation
systems. The incorporation of drip irrigation with plastic mulch
culture of vegetables result in greater water and fertilizer use
efficiency, and has resulted in increased yields of muskmelon,
cucumber, eggplant, pepper, squash, tomato, watermelon, among
other vegetable crops. For example, bell pepper yields in Puerto
Rico were 12 MT/Acre with drip and no mulch, vs. 19 MT/Acre with
drip and plastic mulch culture (Crespo-Ruiz et al., 1988).
Irrigation efficiency with drip systems ranges from 75-95%
compared to 25-50% for surface (furrow); 70-80% solid set
sprinklers and 65-75% for portable sprinkler systems (Smajstrla
et al., 1988).
Important areas of concern to design a successful drip irrigation
system include: 1) Is it adapted to the crop you are growing? 2)
Water sources; 3) Major components of the drip system, and
Installation; 4) System maintenance; and 5) How much and when to
irrigate?
IS IT ADAPTED TO THE CROPS I GROW?
Disposable drip systems are compatible with vegetable crops that
are grown as annuals, in rows, and which do not require flooding.
The main concern is the affordability of the drip system for the
specific crop to be grown. Drip systems are justified for crops
of high market value. Sprinkler systems are preferred for use in
leafy crops, especially during Summer months, because wetting of
the foliage provides for evaporative cooling during the warmers
hours of the day. The release of latent heat from water under
sprinkle irrigation is also used during the Winter months in the
continental U.S. to protect crops from freezing injury. Crops
easily adapted to drip irrigation include broccoli, cabbage,
cauliflower, cucumber, eggplant, muskmelon, pepper, squash,
strawberry, tomato, and watermelon.
WATER SOURCES
Water quality tests are conducted to determine contaminant and
precipitate presence in the irrigation water. Main quality
factors include salinity, iron, sulfur, and calcium levels. This
information may also be available from the local Municipality.
Well water, with screen filters, is normally adequate for drip
systems. Use sand filters for surface water sources such as
streams, ponds, or rivers. Use a sand separator to separate sand
particles from surface stream waters. Consider the size of the
area which will be under irrigation based on the volume of
irrigation water available in that location. The drip system
under peak irrigation demands, should match pump and irrigation
volume capacities.
INSTALLATION AND MAJOR SYSTEM COMPONENTS
Due to the high capital investment, and to the many technical
features involved in the installation of a drip system, it is
recommended that growers seek professional assistance from
irrigation dealers. Locally these services are provided by such
outfits as Brewer Environmental and by Wisdom Inc. The main
components will include 1) Delivery system (mainline,
sub-mainline, feeder tubes, and drip tube); 2) Filters; 3)
Pressure regulators; and 4) Valves. Emitters normally function at
a pressure of 10 psi to deliver from 0.5-2 gallons per minute.
The flow rate of the drip line has to match the particular soil
type. Sandy soils require drip lines with a higher flow rate to
increase the lateral wetting pattern. Turbulent flow tapes are a
recent development in tape types and offer several advantages
over earlier types. The system should be designed to meet peak
water demands of the crops to be irrigated. For example tomatoes
and peppers may require peak levels of 0.4 acre inches per day.
Install a water meter to record water used, and as an indicator
when clogging or other irrigation problems occur.
Table 1. Approximate water requirement to grow an acre of
selected vegetable crops in furrow or surface irrigation (z)
Crop |
Acre inches |
Gallons |
Length of growth cycle (days) |
Bean, snap |
18 |
488,772 |
90 |
Broccoli |
30 |
815,620 |
150 |
Cabbage, Chinese |
24 |
651,696 |
90 |
Cabbage, Head |
15 |
407,810 |
90 |
Carrots |
18 |
488,772 |
120 |
Cauliflower |
15 |
407,810 |
90-120 |
Celery |
30 |
815,620 |
120 |
Cucumber |
20 |
550,000 |
120 |
Eggplant |
30 |
815,620 |
150 |
Ginger root |
40 |
1,086,160 |
300-365 |
Lettuce, leafy |
18 |
488,772 |
50-60 |
Lettuce head |
18 |
488,772 |
90 |
Muskmelon |
30 |
815,620 |
120 |
Onion bulb |
30 |
815,620 |
120 |
Pepper, Bell |
18 |
488,772 |
150 |
Potato |
30 |
815,620 |
120 |
Squash |
550,000- |
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(summer/winter) |
20-30 |
815,620 |
80-180 |
Strawberry |
48 |
1,303,392 |
365 |
Sweetcorn |
18 |
488,772 |
90 |
Sweetpotato |
30 |
815,620 |
150 |
Tomato |
18 |
488,772 |
120 |
Watermelon |
30 |
815,620 |
120 |
Yukio Nakagawa, Univ. Hawaii Internal Manuscript. December, 1962;
and Anon. 1975. Horticultural opportunities of Molokai Ranch
Parcels in Hoolehua, Kaunakakai, and Kawela. Univ. Hawaii,
Internal Manuscript. Thanks to Dr. Kenneth Takeda for providing
this references.
SYSTEM MAINTENANCE
Factors involved in drip maintenance include: a) Daily inspection
of filters; 2) Back flushing of sand filters; 3) Leaking of drip
tubes; 4) Prevent mineral precipitation by dissolving with
phosphoric acid; 5) Clean from bacteria, and algae with 2 ppm
chlorine regular maintenance rinses or 30 ppm target treatments
to clean slime clogged lines. Irrigation water acidification with
phosphoric, sulfuric, hydrochloric or other acids may be
necessary to reduce mineral precipitation.
HOW MUCH TO IRRIGATE?
Very few studies have been conducted in Hawaii to evaluate the
water use of specific vegetable crops. Water use rates have been
estimated based on studies conducted in temperate areas and on
the few studies conducted locally. Furthermore, reported water
rates for traditionally surface irrigated crops (Table 1) should
now be calibrated to allow for the different watering patterns,
and for the greater water use efficiency of drip irrigation
systems.
CALCULATION OF WATER DEMAND IN DRIP SYSTEMS BASED ON
KNOWN IRRIGATION LEVELS FOR FURROW IRRIGATION
Let's use tomato as an example. Table 1 indicates that tomato
requires about 18 acre inches (488,772 gallons) during a 120 day
growing cycle. The number of gallons thus applied per growing
cycle for tomatoes per square foot of soil =
488,772 gallons per acre
----------------------------------------------- = 11.2 gallons
per square ft.
43560 square feet per acre
If the crop is grown on 6-foot centers, it results on a total of
7260 row feet per acre. Lateral water movement from the drip line
is about 15 inches on each side for heavy soils. The total wetted
width in the row is then 30 inches or 2.5 feet. The total
irrigated area is then (7260 ft)*(2.5 ft)= 18,150 sq ft. The
number of gallons required to irrigate this area would then be:
(11.2 gallons per square ft)*18,150 sq ft= 203,280 gallons/Acre.
Notice that this value is less than half of the rates required to
irrigate tomatoes when the entire field is wetted. When making
calculations of water use based on rows feet per acre remember
that values vary depending on the number of tractor rows
(normally placed every 5 or 6 beds), on the particular efficiency
of your drip system (normally between 80-90%) and that water may
also be used at pre-planting, and for rinsing of agrichemical
tanks, to flush the drip tubes, etc. Water use will also vary
between locations, planting season, cultivars, incidence of pest
attack, need for leaching of salts, and other management
practices.
HOW OFTEN TO IRRIGATE?
Available water holding capacity is about 1 inch per foot for
sandy soils and about 1.5-2 inches per foot of soil in heavier
soils. The fraction of water taken by the plant then depends on
the root volume and on the soil water holding capacity (leaching
faster on sandy soils and remaining longer in heavier sandy loams
or clay loams). Irrigations are usually scheduled when 50% of the
available soil water has been depleted, with exact levels
depending on the particular crop.
To continue with our example with tomato on 6 foot center beds,
what would be the allowable water depletion from the soil between
irrigations? Lets assume that these are young tomatoes with an
effective root zone 10 inches deep, and that the soil water
capacity is 1.5 inches per foot.
The irrigated soil volume is:
( 10 inches or 0.85 ft root zone)*(2.5 ft wide wetted zone)*(7260
feet per acre) =
15,064 cubic feet per acre
The amount of water stored in this irrigate soil volume (1.5
inches per foot= ca 13%) is:
(0.13)*(15,064 cubic feet per acre)*(7.48 gallons per cubit foot
of water)
= 14,648 gallons per acre
Irrigations should then be conducted, at say, 50% allowable
depletion, that is:
(0.50)*(14,648 gallons) = 7,324 gallons
WHEN TO IRRIGATE
From our example with tomatoes on 6 foot centers we now know that
our total irrigation demand for the crop cycle are 203,280
gallons per Acre. We also determined that at a growth stage when
root depth is 10 inches irrigations are recommended at 50% of
allowable depletion (7,324 gallons). Water budgets are utilized
to determine when to irrigate next. A formula is used to
determine the current levels of available soil water. Current
soil water content= (the previous level) + (effective rainfall) +
(irrigation water) - (crop evapotranspiration (ET)). ET may be
expressed as acre inches or as gallons per irrigated plot.
Following with our example, if daily ET = 0.10 inches per day per
acre then
Daily irrigation requirement (ET)=
(0.10 inches)*(27,152 gallons per acre-inch)=
= 2715 gallons per acre.
We determined below that 50% allowable depletion occurs at 7,324
gallons per acre. Therefore we should be able to irrigate every
2-3 days. After 2 days the water levels lost through ET would be
37% of allowable depletion and 56% after 3 days.
ET rates which range from < 0.10 during the winter to over
0.15 inches/day during the summer, can be estimated by using an
open pan or may also be available from your local county
extension office. Dr. I.P. Wu at UHM has developed a simple
evaporation pan which would be of practical use to local
producers.
TENSIOMETERS
Neutron probes and tensiometers, available commercially, provide
a simpler method to determine irrigation schedules. Typically,
two tensiometers are used per irrigation block, one at 12-inch
and the second at 6-inch soil depth. A tensiometer reading of 0
indicates soil water saturation. As an example, the drip system
is turned on when the 12-inch tensiometer reads 20 to 30, and
then turned off when the 6-inch one reads 10 or below. These
values, however, have to be calibrated to match the particular
crop and soil characteristics in the farm. Studies in Florida,
however, indicate that irrigation scheduling based on pan
evaporation data was as effective as scheduling based on
tensiometers (Smajstrla and Locasio, 1990). For large planting
blocks a combination of the tensiometer and pan evaporation
techniques would provide the most sound irrigation management
program.
FERTILIZER MANAGEMENT CONSIDERATIONS
If drip irrigation is incorporated into a plastic mulch system,
the fertilizer is incorporated on the beds prior to mulch
placement. This approach, however, may lead to soluble salt
injury, especially to the young seedlings. An alternative is to
broadcast on the bed 30-40% of the total N and K, prior to
planting and to place the remainder of the N and K through the
drip tubes, a practice termed fertigation. For small planting
blocks, of up to an acre, a "hozon" venturi injector
may be used to siphon soluble fertilizer from a bucket, say, at a
1:16 ratio (gallons soluble fertilizer:gallons irrigation water).
Dosatron injectors use an hydraulic device to partition
fertilizer solution at several dilution rates, and are effective
to
Table 2. Evapotranspiration Values Reported for Vegetable Crops
from various locations
Crop |
ET (inches) |
Location |
Bean, snap |
9.69 |
Georgia, N. Dakota |
Bean, snap |
8.95 |
Missouri (1956) |
Broccoli |
19.7 |
Arizona (1973) |
Cabbage |
19.7 |
Arizona (1973) |
Cauliflower |
19.7 |
Arizona (1973) |
Cucumber |
11.3 |
Missouri (1956) |
Carrots |
16.6 |
Arizona (1973) |
Lettuce, Head |
8.5 |
Arizona (1973) |
Muskmelon |
19.1 |
Arizona (1973) |
Muskmelon |
14.67 |
Missouri |
Onions |
23.3 |
Arizona |
Pea, green |
10.98 |
N. Dakota (1952) |
Potatoes |
19.75 |
N. Dakota (1952) |
Potatoes |
24.3 |
Arizona (1973) |
Tomatoes |
19.24 |
Florida |
Tomatoes |
17.56 |
Georgia |
Tomatoes |
20.4 |
Missouri (1956) |
Tomatoes |
26.8 |
California |
Sweetpotato |
16.88 |
Missouri (1956) |
Sweetcorn |
16 |
Florida (80 days) |
Sweetcorn |
19.6 |
Arizona |
Sweetcorn |
19.8 |
Georgia |
Sweetcorn |
13 |
Missouri (1956) |
Sweetcorn |
25.2 |
California |
Vegetables, small |
10.55 |
N. Dakota (1952) |
Vegetables, small |
3.22 |
Florida (30 days) |
Vegetables small |
6.99 |
Florida (60 days) |
Vegetables general |
9.7 |
Florida (80 days) |
Vegetables general |
12.84 |
Florida (100 days) |
Watermelons |
19.24 |
Florida |
1. Data From G. Marlow, Univ. Florida, Some ET values reported
for vegetables grown at field capacity, VC499-21.
Table 3. Reported Monthly Pan Evaporation Data for Several
Locations in Hawaii1
Location |
Big Island (Pahala) |
Maui
(20o48'-156o23') |
Oahu (Waianae) |
Oahu (Hele) |
Kauai (Kealia) |
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Elevation (ft) |
860 ft. |
665 ft. |
10 ft. |
700 ft. |
15 ft. |
Month |
Mean pan evaporation (inches per month) |
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January |
4.42 |
5 |
4.09 |
3 |
5.51 |
February |
4.3 |
5 |
4.37 |
2.94 |
4.79 |
March |
4.93 |
6 |
6.26 |
4.23 |
5.3 |
April |
5.36 |
7 |
6.81 |
3.74 |
5.13 |
May |
5.62 |
8 |
7.22 |
4.24 |
6.47 |
June |
5.7 |
10 |
7.94 |
3.98 |
6.71 |
July |
6.55 |
11 |
8.17 |
5.61 |
7.5 |
Aug. |
6.37 |
10 |
8 |
5.07 |
7.68 |
Sep. |
5.56 |
9 |
7.27 |
4.74 |
6.5 |
Oct. |
5.01 |
8 |
5.95 |
4.47 |
7.78 |
Nov. |
4.4 |
6 |
4.5 |
3.16 |
8.79 |
Dec. |
4.43 |
5 |
3.97 |
2.46 |
7.7 |
1 Anon. 1961. Pan evaporation data, State of Hawaii. Dept. Land
and Nat. Res., Honolulu. 54 pp. To calculate daily ET rates,
divide the monthly ET by the number of days in the month.
cover larger irrigation blocks. Bring the drip system to full
operating pressure prior to fertilizer injection. Nitrogen and K
may be placed through the drip lines but P, Ca, Mg, and
micronutrients are applied prior to planting. The frequency of N
and K injection depends on soil type but normally once per week
is sufficient (Cook and Sanders, 1991). Match the weekly
fertilizer rates with the particular crop growth stage. Lower
rates are applied early in the growth cycle, with rates peaking
during the fruit production phase.
All fertilizer sources used through the drip lines should be
highly water soluble. Common N sources include ammonium, calcium,
or potassium nitrate. K sources include potassium chloride and
potassium nitrate. When possible, purchase the highest analysis
liquid fertilizer, which will reduce the injection cycles.
Fertilizer applications should complement the nutrient levels
already available in the soil, as determined by previous soil
analysis determinations. The solubility of commonly used
fertilizers (in pounds of product per 100 gallons of water) is:
calcium nitrate, 851; potassium nitrate, 108; ammonium nitrate,
984; sodium nitrate, 608; urea, 651; diammonium phosphate, 358;
and nitrate of soda potash, 980. Most of these materials dissolve
best at pH of 5.8-7.8 (Sanders, 1989).
MANAGEMENT CONSIDERATIONS
Yields Based on ET
In Pulehu, Maui tomato marketable yields increased linearly with
evapotranspiration. Daily yields and water use efficiency peaked
20-30 days after the first harvest. This experiment used one
month transplants, and first harvest was conducted 60 days after
transplanting. The crop was picked every 4 days for 80 days.
Yields were 99 MT/Ha with 20 inches of irrigation (Sammis and Wu,
1986).
Bed Width
Preliminary research in West Florida indicates that bed with may
be reduced from 30-36 inches to 24 inches in drip irrigation
systems without a reduction in yields of cucumber, eggplant,
muskmelon, pepper and other vegetable crops. Potential benefits
of narrow beds include less polyethylene used, less energy used
for bed preparation, and increased linear bed feet per acre
(Maynard and Clark, 1990).
Moisture/Disease Interactions
A trial with drip irrigated potatoes in Guam indicated that the
drip treatments improved crop growth compared to non-irrigated
plots. However plots that received high irrigation levels showed
reduced yields due to greater root-knot nematode and soft rot
bacteria (Erwinia) infestations (Marutani and Cruz, 1989).
Frequency of drip irrigation also had an effect on spread of
crown root rot (Phytophthora) on bell peppers in N. Carolina
(Ristaino et al., 1992).
Table 4. Advantages and Disadvantages of Drip Irrigation and
Plastic Mulch Culture (From Marr et al., 1993; and Sanders, 1990.
)
Advantages
1. Reduces soil compaction in the bed.
2. Reduces soil erosion and fertilizer leaching
3. Reduces evaporation, especially when combined with plastic
mulch culture
4. Cleaner produce due to less direct contact with the soil.
5. Reduces weed pressure due to protection provided by plastic
mulch
6. Less water and fertilizer is used due to greater use
efficiency, resulting in reduced soluble salt injury
7. Reduces diseases because foliage remains dry
8. Reduces overall labor and operating costs
9. Field operations may continue during operation.
10. Can be used on different terrain and soil conditions
Disadvantages
1. Increased cost of removal and disposal of drip tubes and
plastic mulch
2. Greater initial capital investment
3. Increased management skills requred for correct operation.
4. Do not provide evaporative cooling during hot summer days.
5. Water filtration is required
6. Tube leaking may occur due to rodens, machinery, or insects.
REFERENCES
1. Clark, G.A. et al. 1990. Irrigation scheduling and management
of micro-irrigated tomatoes.
2. Cook, W.P. and D.C. Sanders. 1991. Nitrogen application
frequency for drip-irrigated tomatoes. HortScience 26:250-252.
3. Crespo-Ruiz, M. et al. 1988. Nutrient uptake and growth
characteristics of peppers under drip irrigation and plastic
mulch. J. Agric. Univ. P. Rico. 72:575-584.
4. Fereres, E. (ed.) 1992. Drip irrigation management. Univ.
Calif. Coop. Ext. Serv. Lf. 21259.
5. Hochmuth, G. 1991. Fertilizer management for drip irrigated
Vegetables in Florida. pp. 12-38. In: Proc. Drip Irri. Veg. Crops
Short Course, 25 July, 1991. Penn. State Univ.
6. Marr, C., W.J. Lamont, and D. Rogers. 1993. Drip irrigation
for Vegetables. Kansas State Univ. Coop. Ext. Serv.
7. Marutani, M. and F. Cruz. 1989. Influence of supplemental
irrigation on development of potatoes in the tropics.
HortScience. 24:920-923.
8. Maynard, D. and G.A. Clark. 1990. Bed width effects on
performance of micro-irrigated vegetables. Citrus and Veg. Mag.
Sept. 1990. pp. 15-17.
9. Ristaino, J.B., M.J. Hord and M.L. Gumpertz. 1992. Population
densities of Phytophthora capsici in field soils in relation to
drip irrigation, rainfall, and disease incidence. Plant Dis.
76:1017-1024.
10. Sammis, T.W. and I.P. Wu. 1986. Fresh market tomato yields as
affected by deficit irrigation using a micro-irrigation system.
Agric. Water Mgmt. 12:117-126.
11. Sanders, D.C. 1989. Fertigation-blessing or curse? Amer. Veg.
Grower. Feb., pp. 44-45.
12. Sanders, D.C. 1990. Using plastic mulches and drip irrigation
for vegetable production. N. Carolina Coop. Ext. Serv. Lf. 33.
13. Sanders, D. 1991. Component and design considerations for
drip irrigation. pp. 2-11. In: Proc. Drip Irri. Veg. Crops Short
Course, 25 July, 1991. Penn. State Univ.
14. Smajstrla, A.G. and S.J. Locasio. 1990. Irrigation scheduling
of drip-irrigated tomato using tensiometers and pan evaporation.
Proc. Fl. State Hort. Soc. 103:88-91.
15. Smajstrla, A.G. et al. Efficiency of Florida agricultural
irrigation systems. Univ. Fl. Coop. Ext. Serv. Bull. 247.
16. Wu, I.P. and H.M. Gitlin. 1973. Design of drip irrigation
lines. Univ. Hawaii Coop. Ext. Serv. Tech. Bull. 96.
17. Wu, I.P. and H.M. Gitlin. 1974. Lateral and submain design
for a drip irrigation system. The Engineer's Notebook- April
1974. Univ. Hawaii Coop. Ext. Serv. No. 13.