All plants vary in their water requirements according to their size and growth stage as well as the length of their maturity and time of year of maximum growth. Possibly no other major crop varies in its sensitivity to water stress based on growth stage than potatoes. Irrigation recommendations at key production periods are based on the S-shaped growth curves of roots, vines and tubers. Soil moisture requirements are related to different growth stages.
1. Pre-planting
to Planting:
A pre-plant irrigation is often recommended for two reasons. First,
soil moisture should be about 70 to 80% field capacity. This will bear-saturate
the field, allowing some room for rains. This level amounts to around a quarter
of the allowable deficit of the soil. Soil moisture should be acceptable to
support the developing roots after planting and reach emergence. Another
benefit from a “pre-irrigation” is the breaking down of clods and clumps for
better planting.
2. Planting to
Pre-emergence (Sprouting):
Soil moisture in the top foot of soil should be 65 to 80%. No
irrigation is recommended during this production period. First, seed-pieces at
a recommended size 30-60g, have sufficient water to support the sprout until
emergence. Irrigating during this period would raise the soil moisture and
lower soil aeration to a level that would support several pathogens, most
notable bacterial soft rot or black leg (Erwinia carotovora), and stem and
stolon canker (Rhizoctonia solani). Excess moisture will also decrease tuber
respiration, putting the seed-piece under metabolic stress. Water deficit,
too-dry soil, will decrease the healing of the cut surfaces of seed-pieces,
inhibit root growth and increase susceptibility to soil pathogens such as
Fusarium and Rhizoctonia. In short, pre-plant irrigation and seed-piece water
are more than sufficient to carry the sprouting tuber.
3. Emergence to
Tuber Initiation (Early Vine Growth):
This is the log phase of vine growth. Roots are in the second half
of their growth. During this period, the vine grows very rapidly, as much as
doubling the canopy every week. With rapidly increasing foliage every week,
irrigation starts low and gradually increases every week. A soil moisture of 70
to 80% is preferred, less than 65% FC would be considered a deficit. Water
deficiency at this point would inhibit canopy and root growth, and indirectly
weed control by less ground cover. An excess would retard root branching
(development) by water-logging root hairs and promote nitrogen leaching. In
short, with an increase in foliage and thereby transpiration, irrigation should
begin and gradually increase as the canopy grows.
4. Tuber
Initiation to Full Bloom (Vegetative or Stage II. Tuberization):
In determinate varieties full bloom marks the end of vine growth,
while in indeterminate varieties full bloom starts a noticeable slow-down of
vine growth, some branching still occurs. The first set of tubers are being
initiated and these are in a slow-growth, development stage, the lag phase of
tuber growth. Irrigation becomes increasingly important and water stress
becomes less tolerable, transpiration reaches its highest rate. Irrigation
increases per week on sandy type soils. Water deficit would dramatically
increase tuber malformations and sugar-ends. It can also weaken plants,
promoting early blight. Common scab (Streptomyces scabies) attack is promoted
and the longer the deficit, the greater the attack and more pronounced and
enlarged the blemishes. In areas and with varieties prone to common scab,
maintaining soil moisture at 90 to 95% is suggested if possible. Excessive
water will increase brown centre and hollow heart of larger tubers, and promote
early dying of the vine. Too much loose water, swampiness, can also promote
late blight, and weaken plants promoting early blight. In short, soil moisture
levels must be increased and therefore irrigation is increased. Note, also that
this stage of the plant often corresponds with June and July and the hottest of
weather. The length of this period is also related to variety, weather and
cultural practices. It may be prolonged by excessive nitrogen.
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