Agriculture Reference
In-Depth Information
to an increase in reducing sugars. Eldredge
et al
.
(1996) indicated that water stress during the
early bulking stage also resulted in sugar ends
on the variety Russet Burbank. For some var-
ieties, water stress, whether from too much or
not enough water, can lead to tuber malforma-
tions, such as knobs, growth cracks, and irregular
shape (Sparks, 1958; Kleinkopf, 1982; Holder
and Cary, 1984).
Excessive soil moisture may exacerbate the
development of diseases such as late blight (
P. i n -
festans
), blackleg (
Erwinia carotovora
var.
atroseptica
),
softrot (
Erwinia
spp.), and pinkrot (
Phytophthora
erythroseptica
), among others (Adams and Ste-
venson, 1990).
field capacity may equal
30
cbar (Ley
et al
.,
1994). Sandy loam soil ASW values of
65-
85%
should measure close to
25
to
15
cbar, respect-
ively, while a loam soil may measure
55
to
20
cbar,
respectively (Ley
et al
., 1994).
Common irrigation methods for potatoes
include sprinkler systems such as center pivots,
lateral/linear move systems, wheel- and hand-
lines/solid set, and big gun or boom hose reels,
as well as furrow/gravity systems, and drip-tape
or drip-hose systems. Most of these systems are
designed to be used on a regular basis as the pri-
mary water source for the potato crop; however,
the big gun and boom hose reel systems are
typically used to supplement fields that are
otherwise rain fed.
Irrigation and available soil water
recommendations
Soil water monitoring and
measurement
Maintaining moist soils in the plant's root zone
(~75% available soil water (ASW)) from
2
to
7
weeks after sprouts emerge significantly re-
duces common scab (
S. scabies
) (Davis
et al
., 1974).
Because plants may not be able to remove all soil
water, ASW is defined as the water within the
plant-extractable range, sufficient enough to
prevent plant wilt and death. The upper limit is
referred to as field capacity, while the lower limit
is called the wilting point. Stark and King (1997)
recommend maintaining ASW in the primary
root region (0-45 cm) between 65% and 85%
during the active growth stages of canopy estab-
lishment, tuberization, and rapid tuber bulk.
They also suggest an ASW of
70-
80% during
planting to hasten plant emergence. When vines
have senesced or are killed, they recommend de-
creasing ASW to
60-
65% to allow for proper
skin set and prevent tuber rot. Just prior to har-
vest, they suggest an ASW above 60% to reduce
bruising, which is exacerbated by tuber dehydra-
tion and cloddy soils.
Various irrigation scheduling instruments,
like tensiometers, use units of measure called
bars or centibars (cbar) (
1
bar =
100
cbar). Ten-
siometers measure soil water tension (suction),
which mimics the energy plants must exert to
extract water from soil. The relationship be-
tween ASW and soil water tension depends on
the soil texture. For coarse-textured soils, soil
water tension at field capacity or 100% ASW
may measure
10
cbar. In a fine-textured soil, the
Soil water is typically measured and monitored
with tensiometers, neutron probes, soil moisture
blocks, gravimetric sampling, the balance sheet
method, remote sensing, and soil feel and
appearance.
Tensiometers are constructed of airtight,
water-filled tubes that house porous ceramic tips
on one end. The ceramic tip is placed in the soil
at the desired depth and an aboveground
vacuum gauge displays water tension measure-
ments in the range of 0-
80
cbar (Morris, 2006).
Tensiometers are relatively inexpensive, common,
and useful; however, they must be maintained
and checked often. A common issue with tensi-
ometers is the loss of tension due to drying
periods and poor soil contact.
Neutron probes measure the hydrogen
associated with water (H
2
O) (Ley
et al
., 1994).
They can be very accurate, but are also expen-
sive and require training, radioactive material
licensing, and calibration to use. Growers who
rely on neutron probes often contract with a
commercial irrigation monitoring company that
owns and operates the probes.
Electrical resistance blocks or capacitance
probes measure the capacity of the soil to store
electricity (electrical resistance). Soil moisture
and electricity are correlated; wet soils hold
more electricity than dry soils. Electrical resist-
ance is low in wet soils, which make it easier for
the blocks to conduct electricity (Morris, 2006).
