Growing Irrigated Potatoes (continued)
AE-1040 (Revised) March 1999
Back to Start
Irrigation Management
Harvest Considerations
Groundwater Protection
Economic Analysis
Additional Sources of Information
The potato is sensitive to moisture stress over much of the
growing season. High yields and high quality are achieved by
maintaining relatively high soil moisture levels. The average
seasonal water use for potatoes is near 18 inches, which is
provided by stored soil moisture, rain and irrigation. The
average daily water use, cumulative water use and rooting depth
are illustrated in Figure 4. Water use rates begin at about 0.02
inches per day when the crop emerges and increases to over 0.25
inches per day when the potato canopy completely shades the
ground. As the potatoes achieve full tuberization, water use will
decrease. The frequency and amount of irrigation will depend on
the water holding capacity of the soil, the crop growth stage,
and the prevailing weather conditions.
Figure 4. Seasonal
evapotranspiration and irrigation management criteria for
potatoes. (Adapted from Stegman, NDSU Research Reports) (7KB b&w graph)
Water Holding Capacities of Soil
The water holding capacity of the soil has a great influence
over when and how often irrigation is required. Different
textures of soil have different available soil water holding
capacities. The variation in water holding capacities of general
soil textures is shown in Table 5. Note that the more water a
soil has available for the plant the less frequent the
irrigations should be. For example, if the potatoes are using
0.25 inches of water per day, you want to irrigate when the root
zone moisture depletion reaches 40%. For a sandy loam soil (using
an 18 inch rooting depth) that holds 1.5 inches of water per
foot, you would irrigate every three to four days. For a fine
sandy loam that holds 1.9 inches per foot, you would irrigate
every four to five days but apply a larger amount of water than
on the sandy loam soil.
Table 5. Range
of available soil moisture holding capacities for various
soil textures. (Inches/Inch are the inches of water per inch of
soil depth,
Inches/Foot are the inches of water per foot of soil depth).
-----------------------------------------------------------
Available Moisture
------------------------------
Soil Texture Inches/Inch Inches/Foot
-----------------------------------------------------------
Coarse Sand and Gravel 0.02 to 0.06 0.2 to 0.7
Sand 0.04 to 0.09 0.5 to 1.1
Loamy Sand 0.06 to 0.12 0.7 to 1.4
Sandy Loam 0.11 to 0.15 1.3 to 1.8
Fine Sandy Loam 0.14 to 0.18 1.7 to 2.2
Loam and Silt Loam 0.17 to 0.23 2.0 to 2.8
Clay Loam and Silty Clay Loam 0.14 to 0.21 1.7 to 2.5
Silty Clay and Clay 0.13 to 0.18 1.6 to 2.2
-----------------------------------------------------------
To show how the number of days between irrigation events are
calculated for the sandy loam soil: multiply the 1.5 foot rooting
depth by 1.5 inches per foot water holding capacity. This gives a
total of 2.25 inches of available water in the root zone. Next
multiply the 2.25 inches by .40 (40% depletion) which gives 0.9
inches of water that can be depleted. Now divide the 0.9 inches
by the 0.25 inches that is being used each day, and that gives
4.5 days. This calculation assumes you are starting with a root
zone that is at its maximum water holding capacity, generally
referred to as its "field capacity."
Effective Rooting Depth
Potatoes are a shallow rooted crop. Typically, roots grow
laterally 10 to 18 inches and downward to a depth of about 3 feet
(Figure 5). Root extension to this depth is usually completed
within 30 days after plant emergence. Root distribution is
heavily concentrated near the soil surface. About 90 percent of
the roots will be found in the top 2 feet. Irrigation timing and
amount should be based on the soil moisture depletion in the top
12-18 inches of coarse textured soil profiles and in the top 18
to 24 inches of finer textured soil profiles.
Figure 5. Potato root
development under a hill of potatoes (36" row spacing). (9KB b&w illustration)
Water Management
During the 1998 growing season there were approximately
130,000 acres of potatoes in North Dakota. Of these, around
35,000 acres were irrigated. Center pivot sprinkler systems are
used on over 99% of the irrigated potato acres in North Dakota.
Irrigation as practiced in North Dakota is called
"supplemental" irrigation because the water applied by
irrigation supplements the water received from rainfall.
The object of irrigation water management is to balance the
applied amount with the amount received from rain to maintain an
optimal growing environment for the potatoes.
Managing any irrigation system for the production of crops can
be divided into three basic categories: mechanical management,
irrigation system management associated with cultural practices,
and irrigation scheduling of water.
Mechanical Equipment
Center pivots are machines and thus require routine
maintenance to operate properly. Checking the operability of an
irrigation system before the irrigation season begins is
just as important as properly operating the system during the
season. Fixing small problems is less expensive than repairing a
major breakdown, especially with irrigated potatoes. Any downtime
of the center pivot during tuber bulking will affect both yield
and quality. Following are some of the most common problems
associated with center pivot downtime during the growing season.
During the fall, winter and spring, when the irrigation system
is not being used, rodents can damage electrical wiring in
control panels and motors. During the fall, they will make a nest
in the electrical control panels if they can gain entrance.
During the winter they will chew on electrical wiring which can
cause shorts and failure the following season. This can be
prevented by plugging all holes and maintaining a tight door seal
on all the electrical control panel doors and covers.
Rodents also do damage to electrical motors if they can get
access to the windings. Damage can be prevented by checking all
motor openings to see if they are properly screened to keep
rodents out. If a screen is damaged or missing, it should be
replaced with 1/4-inch mesh screen. This size screen can then be
left in place during operation without plugging with dust and
debris.
Before the irrigation season begins, the gearboxes on all the
towers should be checked for moisture accumulation and that each
contains the correct amount of proper weight oil. Lack of oil or
water in the oil are major causes of tower drive malfunction
during the season. Remove the gear box drain plug just long
enough to drain the condensed water then fill with oil.
Before the season begins and during the season if necessary,
remove and clean the sand trap on the last tower. Here is where
sand, scale and other debris collects. The amount and type of
debris in the sand trap can tell a lot about the operation of the
pivot.
Application Uniformity and Rate
The purpose of sprinkler irrigation is to make sure every square
foot of ground receives the same amount of water and at the same
rate. The most important irrigation management requirement is
that the sprinkler package on the center pivot apply water
uniformly along the entire length of the center pivot.
Poor distribution of water and, if chemigating, poor
distribution of nitrogen or pesticides can have a major impact on
yields. For potatoes, which are very responsive to water and
nitrogen, the uniformity of the applied water can have a large
impact on production. Some parts of the field will receive too
much water and/or nitrogen and some parts will receive too
little. The only way to check the uniformity is by performing a
can test.
A can test involves setting out a line of uniformly spaced
cans under the full length of the center pivot. After the pivot
passes over the cans, measure the amount caught in each can.
Record the distance from the pivot point and amount. Total all
the caught amounts and divide by the number of cans to determine
the average amount. Then compare the caught amounts to the
average to determine the areas where the pivot is over or under
applying water.
Along with the can test, the application rate of the
sprinklers should be checked. The application rate is the amount
of water applied to a particular point in the field as the center
pivot passes over it. Too high an application rate can lead to
runoff and washing of soil. For potatoes, this can mean washing
off the tops of the hills causing green top or water running down
the furrows and keeping the low spots wet. A continuously wet low
spot can hinder the movement of a tower that travels through it
by creating deep wheel tracks or cause the tower wheels to bog
down. Wet spots are also ideal areas for the development and
growth of fungus such as early and late blight.
Tips for Checking Center Pivot Sprinklers: While
the system is operating, walk down the entire length looking
closely at the operation of each sprinkler. Frequently, sprinkler
problems can be spotted and corrected. The most common problems
are plugging, not rotating properly, and broken parts. Generally
these problems are quite noticeable.
Check the uniformity of the pivot by placing at least one rain
gage (two would be better) under each of the last five spans of a
standard quarter section pivot. The rain gages should all be
exactly alike. Write down the amount in each rain gage after the
pivot has passed over and note the amount of variation from the
average.
If the tops of hills are being washed off or water is running
down the furrow into low spots, reduce the amount of water
applied by increasing the speed of the pivot. This may help, but
pay more attention to the water needs of the potatoes. If
increasing the speed of the center pivot doesn't help you may
have to change or alter the configuration of the sprinkler
package.
Endguns
North Dakota state law prohibits spraying water onto a maintained
road. If an endgun is spraying a road, its on-off switches should
be adjusted at the pivot point.
Cultural Management
Irrigating Before Planting
Irrigated potatoes are often planted on sandy soils. It is not
uncommon to have a dry spring. If the soil is dry at planting
time, it is better to irrigate before planting than after.
Planting into soil with moisture will ensure better germination
than planting into dry soil and irrigating afterward.
Disease Control
Many of the diseases (early blight, late blight, soft
rot, etc.) that affect potatoes are caused by fungi. Water and
heat provide the optimum growing conditions for fungi. Center
pivot management can either hinder or help the conditions which
favor fungal growth.
Continuous wet spots in the field can be breeding grounds for
fungi. The pivot point of a center pivot is always wet due to the
slow speed of the pivot. This can be controlled by plugging the
sprinklers on the first span closest to the pivot point. The span
to the first tower on a pivot usually waters about 2 acres, so
even if all the sprinklers on the first span were plugged, only 2
acres would be lost. Usually, plugging the first three sprinklers
will be sufficient.
Weeds that are members of the nightshade family can be
carriers for the blight fungus, so kill all weeds around the
pivot point and wells or pumping stations within the field.
Irrigation Scheduling
Irrigation scheduling is the practice of using some method to
decide when to start an irrigation system and how much water to
apply. No matter what method is used, they all start with knowing
when and how much rain has been received on the field and then
using some mechanism to decide when to irrigate.
Growing season water use for potatoes is approximately 18
inches (rainfall plus irrigation) in North Dakota with an average
peak daily water use between 0.25 and 0.3 inches per day during
tuberization.
All irrigation scheduling methods can be classified as 1)
measuring and tracking the soil moisture levels in the root zone,
2) keeping track of the amount of crop water use each day and 3)
combining soil moisture measurement with crop water use.
Rainfall
Knowing how much rain has been received and on what day
is very important for irrigation water management. A rainfall
record book should be kept for each potato field. To accurately
measure rainfall received on a particular field, install two good
quality rain gages at opposite ends of the field in such a way
that they don't collect irrigation water.
Soil Moisture
Soil moisture levels in the root zone of potatoes can be
measured using a variety of instruments or by the
"feel" method. The feel method of soil moisture
determination is used on over 80% of the irrigated acres in the
US.
The soils under a center pivot are usually not uniform. Finer
textured (heavier) soils hold more water and won't reach critical
soil moisture deficient levels (when yield loss starts) until
after the potatoes on coarse textured soil have already started
to stress. Potatoes will use the same amount of water regardless
of soil type (if moisture is available), but coarse textured
soils hold less water. Therefore, the percent of soil moisture
deficiency under an irrigation system will vary with soil type.
Don't just check soil moisture levels where it is convenient
and near the road; use the coarsest soils in the field for
scheduling purposes.
The soils that have the lowest water holding capacity will
dictate the frequency and amount of irrigation. Potatoes have an
effective root depth of 2 feet, but the soil approximately 10
inches below the top of the hill will generally dry out soonest
because that is where most of the adventitious roots are located.
Using a soil probe to obtain samples from this depth in several
locations in the field will provide an accurate estimate of the
soil moisture status for the potatoes.
Potatoes thrive best when the soil moisture level in the root
zone is maintained between 60% and 80% of available water
capacity, which is the same as 20% to 40% depletion of available
water. Irrigation is initiated when soil moisture levels in the
coarsest soils in the field reach 60% of available water.
Irrigated potatoes are commonly grown on sandy soils. These
soils have a water holding capacity around 1 inch per foot of
soil depth. Because of the low water holding capacity, irrigators
generally put on small amounts of water (0.5 to 0.8 inches) every
two to three days, depending on rainfall amounts.
Potato Water Use Estimates
Daily water use estimates for potatoes can be obtained
by using the charts in NDSU Extension Service Circular, AE-792,
Irrigation Scheduling by the Checkbook Method, and electronically
by accessing the NDSU Extension Service computer system. During
the growing season, the electronic estimates are available on the
World Wide Web or through the NDSU Extension electronic bulletin
board (ExtNet).
Checkbook Method. The checkbook method of
irrigation scheduling (AE-792) combines soil moisture measurement
with crop water use estimates. The soil moisture balance in the
root zone is treated as a bank account. Water added to the soil
in the form of rain or irrigation is considered a deposit, and
water removed by potatoes or drainage below the root zone are
considered withdrawals. With this method, the irrigator can track
the change in soil moisture in the root zone throughout the
season and do a better job of managing irrigation water.
ExtNet Crop Water Use Reports. ExtNet is the
computer network that connects the NDSU campus with all the
county extension offices. Access to ExtNet is available to anyone
with a computer with a modem for a $30 annual fee. The
application forms are available from NDSU Agricultural
Communications Computer Services. Two crop water use reports are
provided under the WEATHER section of ExtNet.
One report is a summary report of the potato water use for the
previous seven days. This report (called Weekly) is only
available from June 15 to September 30 each growing season. The
other report (Daily) is available year round and can be used to
obtain potato crop water use estimates from previous years. These
reports are explained in more detail in the bulletin WEATHER,
Software Users Guide 11, from NDSU.
Crop Water Use Maps. Color coded crop water
use maps and numerical tables for North Dakota are available on
the Internet via the World Wide Web (WWW). The maps and tables
are updated daily from June 15 to September 30 each growing
season. During the rest of the year, example maps and the table
of values for the previous growing season are shown. The maps are
shown after an emergence date for a particular crop is selected.
For potatoes, the six selectable emergence dates are seven days
apart and start on the first of May. They were created to provide
accurate estimates of crop water use for the eight major
irrigated crops in North Dakota. They are available at this
address:
http://www.ext.nodak.edu/weather/ndawn
The maps are created using daily weather data collected by the
North Dakota Agricultural Weather Network (NDAWN). Each weather
station on the network is automated and the weather data are
retrieved every day. The crop water use estimates are calculated
using the weather data from each automated weather station and
the numbers are shown on the maps. The crop water use estimates
for the eight crops use a reference ET amount (inches of water
use per day) calculated with the Jensen-Haise equation. The
reference ET amount is adjusted for each crop to produce the
estimated daily crop water use amount.
Harvest Considerations
Maturity and Vine Killing
Potatoes are considered mature when they have reached a good
marketable size, separate easily from the stolons, and can be
harvested with a minimum amount of tuber skinning. Tubers
generally do not mature until after the vines die. However, with
the use of fungicides and insecticides, plants usually remain
green until frost. It is usually necessary to kill and defoliate
the plants by either applying vine desiccants or by mechanical
means in order to harvest early and extend the harvest season.
Early termination of the plants is also used to control tuber
size or to supply early markets. Maturity is hastened by spraying
plants 10 days to three weeks before harvest with a chemical
desiccant. The desiccants currently labeled for use on potatoes
can be found in NDSU Extension Circular W-253 (revised), the
current year's Agricultural Weed Control Guide.
Irrigation for Bruise Control
Bruising of harvested potatoes is becoming an important
concern to the fresh pack and processing companies. Descriptive
terms such as blackspot, shatter, cracking and pressure bruising
are all used to identify potato tuber injury. Although most
bruising is caused by mechanical handling during harvest and
transportation, research has shown that irrigation can be used to
reduce bruising during the harvesting process. On sandy soils, a
soil moisture content between 60% and 80% of field capacity (40%
to 20% depletion) provides conditions for a desirable soil load
into the harvester with optimum separation of potatoes and soil
and minimum tuber damage. If the soil is dry before harvest, a
final irrigation should be applied at least one week prior to
harvest to raise the soil moisture content and also raise the
tuber hydration level. Air temperature also has a significant
effect on bruising. Harvesting when the tuber temperature is less
than 45 degrees Fahrenheit will increase shatter and cracking of
the tubers.
A large portion of the irrigated potato acreage is on sandy
soils which lie over relatively shallow aquifers (10 to 50 feet).
Sandy soils overlying shallow aquifers planted to a shallow
rooted, nitrogen loving crop like potatoes can pose a groundwater
contamination risk. The key to substantially minimizing the risk
to surficial aquifers is controlled management of inputs and good
crop rotation.
Results from many studies indicate that groundwater
contamination occurs when nitrogen fertilizer, pesticides, and
water are not well managed. Excessive application and poor timing
are regarded as the main causes. This type of management is not
economical, nor does it protect vulnerable water resources.
Irrigated agriculture offers significant opportunity for
controlled application of inputs.
The need to follow practices that utilize water and nitrogen
efficiently is particularly important for the safety of drinking
water supplies. Many rural water systems have their wells in the
same aquifers where potatoes are grown. Many people in these
areas are very concerned about their water supply and view
irrigated potato production with apprehension. Producers can
maintain yields and protect surficial aquifers by matching
pesticides, nitrogen and water to potato crop needs at different
growth stages.
An analysis of the economics of growing irrigated potatoes
must be related to the time frame of one's planning horizon. If
the irrigation system is already in place, then the pertinent
question is, What crop do I raise this year? This is an annual
decision that must be repeated every year. The decision criteria
is to determine which crop will result in the greatest return
over variable costs. It does not matter at that point what the
total cost of the land, machinery or irrigation system is. The
main concern is to raise a crop that fits into the crop rotation
and contributes the most income to pay for these fixed costs.
Obviously, the operator would choose a crop that results in the
most money left over after paying for variable costs.
However, if the decision time is prior to making the
investment in an irrigation system, then the pertinent question
becomes whether or not to invest in an irrigation system to
produce irrigated crops rather than dryland crops. At this point
the irrigation system can still be considered a variable cost
because the operator has not yet committed to the investment. In
this decision the operator is still looking to maximize return
over variable costs; however, the ownership costs of the
irrigation system should be included in the variable costs.
Enterprise Budgeting
Both situations require the information contained in
enterprise budgets to make an informed decision. To analyze this
investment, it is necessary to consider a complete rotation,
since few crops will be successful in a monoculture over several
years. Table 6 lists estimated costs and returns for several
crops grown under irrigation. The prices of both inputs and
products vary over time. For long run planning it is necessary to
use average prices and yields over the length of the planning
horizon.
Table 6. Economic cost budgets (1998) for a possible
growing rotation involving
potatoes on a per acre basis (Center pivot covering 130 acres of
irrigated land).
--------------------------------------------------------------
Potatoes Corn Drybeans
Marketable Yield 325 cwt 120 bu 2000 lbs.
Expected Price $4.15 $2.25 $0.18
MARKET INCOME $1,348.75 $270.00 $360.00
DIRECT (VARIABLE) COSTS
- Seed 220.00 29.12 28.00
- Herbicides 48.88 23.45 16.04
- Fungicides 124.90 0.00 0.00
- Insecticides 31.50 14.96 0.00
- Fertilizer 38.14 33.38 15.08
- Crop Insurance 27.42 11.20 13.05
- Fuel & Lubrication 13.79 11.29 9.12
- Repairs 34.43 13.70 12.74
- Irrigation Power 31.06 31.06 31.06
- Irrigation Repairs 9.97 9.97 9.97
- Drying 0.00 14.40 0.00
- Custom Hauling 97.50 0.00 0.00
- Miscellaneous 17.75 1.05 1.00
- Operating Interest 34.77 9.68 6.80
--------- --------- ---------
SUM OF LISTED DIRECT COSTS $730.11 $203.26 $142.86
RETURN OVER VARIABLE COSTS $618.64 $66.74 $217.14
INDIRECT (FIXED) COSTS
- Misc. Overhead 13.98 9.11 7.47
- Machinery Depreciation 54.54 26.58 21.08
- Machinery Investment 31.61 16.39 12.58
- Irrigation Depreciation 31.49 31.49 31.49
- Irrigation Investment 16.30 16.30 16.30
- Land Taxes 3.97 3.97 3.97
- Land Investment 26.25 26.25 26.25
--------- --------- ---------
SUM OF LISTED INDIRECT COSTS $178.14 $130.09 $119.14
SUM OF ALL LISTED COSTS $908.25 $333.35 $262.00
RETURN TO LABOR & MANAGEMENT $440.50 ($63.35) $98.00
--------------------------------------------------------------
3 year Average - $158.38
--------------------------------------------------------------
For more detailed information on crop budgets, consult the
Farm Management Planning Guide - Irrigated Crop Budgets
(Central or Western, North Dakota) available from the NDSU
Agricultural Economics Department, Fargo, ND.
Assuming a three-year rotation with the crops shown in Table 6,
there are extreme differences in return over variable cost and
return to labor and management for each of the three crops. It is
obvious that of these crops, a producer would want to include
potatoes as often as possible in the rotation as long as the
yield could be maintained. In contrast, the corn budget projects
a loss of $63.35 per acre before putting a value on labor and
management. The logical question then becomes, Should corn be
grown at all? The assumption is that corn, or something like it,
is needed in the rotation to ensure that disease pressure does
not build to the point that the other more profitable crops can
not be produced.
In a long run analysis, the average return to labor and
management of all the crops in the rotation must be positive. A
rational decision-maker would want this value to be equal to or
greater than the fair market value of the labor and management
effort contributed by the owner-operator. Individual crops in the
rotation may not measure up to this goal, but as long as the
return over variable costs is positive for each crop, then they
are all helping to pay for the fixed costs. These budgets assume
that all labor and management is provided by the
owner-operator(s). Any labor that is hired should be included in
the variable or direct cost section of the budget.
Fixed Costs
Fixed costs are the expenses for each crop that will be
incurred regardless of production. They include overhead and
machinery ownership costs, which vary depending on the investment
value of the machinery needed for each crop. Other fixed costs
include the ownership costs of the irrigation development and the
land itself. These costs would be constant for all potential
crops to be grown. Irrigation ownership costs were calculated
using a center pivot on a quarter section of land (160 acres) and
irrigating 130 acres of that quarter. All the purchased equipment
and installation are new and required the following investment:
Center Pivot $39,000
Pump, Motor, Electrical $16,000
Well (at pivot point) $10,500
Pipe, valves, etc. $4,500
Land Preparation $4,000
---------
Total Investment $74,000
Renting Irrigated Land
Cash renting irrigated land involves determining a value for
the landowner's contribution. The landowner is contributing the
land and the irrigation system in a typical arrangement. A
minimum rent that will cover the landowner's contribution is the
sum of the ownership costs of the irrigation system and the land
costs. Since land does not depreciate, the applicable costs are
the land taxes and the dryland rental market rate. In this
example, the value of this contribution is $78 per acre.
Competition for irrigated land will push rents above this minimum
depending on the profitability of the crops to be raised.
Additonal Sources of Information
Potato Production and Pest Management in North Dakota and
Minnesota, NDSU Extension Bulletin Number 26 (revised, 1991).
Seed Potatoes, Red River Valley Potato Growers
Association, PO Box 301, East Grand Forks, MN, 56721.
Irrigation Handbook, Extension Agricultural and
Biosystems Engineering Dept., NDSU.
Potato Insect Control, NDSU Extension Bulletin Number
E-881.
Disease Control Guidelines for Seed Potato Selection,
Handling and Planting, NDSU Extension Bulletin Number PP-877.
Agricultural Weed Control Guide, NDSU Extension
Circular W-253.
Commercial Vegetable – Weed, Insect, Disease Control
Guide: Potatoes, University of Minnesota, AG-FO-1879-C,
Revised 1991.
Crop Production Guide, NDSU Yearly Publication (Current
Year)
Ring Rot of Potatoes, NDSU Extension Circular PP-507.
Field Crop Fungicide Guide, NDSU Extension Circular
PP-622 (updated annually).
Blackleg and Soft Rot of Potatoes, NDSU Extension
Circular PP-903.
Leaf Blight Diseases of Potato, NDSU Extension Circular
PP-1084.
Production Guides and Handbooks, Potato Processing
Companies
Irrigation Scheduling by the Checkbook Method, NDSU
Extension Bulletin, AE-792
Estimated Irrigated Crop Budgets (Central or Western) North
Dakota, Section IV of the Farm Management Planning Guide,
NDSU Agricultural Economics.
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AE-1040 (Revised) March 1999
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