Effects of Moisture on Canola
Water is essential for plant growth. Too much or too little
water at any particular growth stage reduces yield potential.
Canola plants obtain all their moisture needs from the soil.
Moisture factors that may limit yield include:
- spring soil-stored moisture
- the rate and duration of rainfall and/or irrigation during the
- the capability of the soil to absorb, store and make water
available for plants
Modifying some of these factors can improve moisture
availability and efficiency of water use.
Role in the Canola Plant
Water is the major component of the canola plant. It plays an
important role in nutrient absorption and transportation, formation
of new products, plant growth and plant response to abiotic
stresses. Carbohydrates, the products of photosynthesis, are moved
in water solution to storage organs. A major portion of the water
absorbed by the canola plant evaporates through the stomata (tiny
pores in the leaves). This evaporation process called transpiration
is essential for absorbing oxygen for photosynthesis. In addition,
water absorbs heat, cools the plant and prevents plant injury from
high temperatures. Water is also lost directly from the soil
surface by evaporation. The combined loss of moisture from the soil
and the plant is called evapotranspiration. Heat and wind increases
evapotranspiration by rapidly removing and changing the air
A firm moist seedbed provides uniform seed germination and rapid
seedling growth. Adequate soil moisture at the seedling and
elongation stage promotes the development of a strong, healthy
plant less subject to lodging with a maximum amount of leaf growth
by the end of June. Leaves provide the predominant source of food
for seed development. Therefore, water management practices that
increase leaf development and prolonged leaf life will also promote
Germination and Emergence
Moisture is essential for seed germination. Soil moisture and
temperature are the two most important factors controlling
germination, the start of root growth and emergence. Soil moisture
is critical as it affects how quickly water penetrates the seed.
Canola seed has to imbibe a high percentage of its weight in water
before germination begins. Cold temperatures and variable soil
water availability in early spring limit germination and subsequent
growth. In addition to cool temperatures in the spring, most
Canadian prairie soils are also exposed to rapid drying on the
surface when disturbed by any form of cultivation. Canola is sown
very shallow and germinates in the portion of soil that is subject
to the greatest drying effect. Germination and emergence of canola
is progressively delayed and reduced as soil water availability
decreases. As the soil dries it becomes more difficult for the seed
to obtain water from the soil particles. This is why semi-dry
seedbeds very often result in slow, uneven germination and more
abnormal seedlings. Coarse textured soils may dry or drain rapidly
resulting in reduced germination or root growth.
Growth chamber research on seedling emergence at low
temperatures of 8.5 to 10°C (day/night) and various levels of soil
moisture, showed canola seeds in soils with lower moisture had
slower and lower emergence (Figure 1).
Figure 1. Seedling Emergence for Brassica rapa (Tobin) at Various Soil Water
However, in the field, soil temperatures can be lower and
seedling diseases can further reduce rates of emergence.
Researchers at Agriculture and Agri-Food Canada Melfort, SK
Research Centre working with Grey Wooded and Dark Grey Wooded soils
in northern Saskatchewan, reported that clay soils with higher
moisture storage capacity had better and quicker emergence than
sandy loam soils with lower moisture storage capacity at 100% field
capacity (F.C.) (Table 1).
1. Emergence of Canola at Two Soil Moisture Levels
||% Emergence of B.
||6 Days After Seeding
||13 Days After Seeding
|Sylvania Sandy Loam
A loose or dry, cold seedbed will result in reduced and delayed
germination, reduced rate of seedling emergence and may inhibit
germination altogether until a rain occurs. Conserve soil moisture
during seedbed preparation. Maintain a firm seedbed to reduce the
loss of moisture to the surface. As long as sufficient moisture
remains to maintain a relative humidity in the soil pores of 60 to
75%, the canola seed coat will absorb moisture. There is little
capillary water movement in the seedbed.
If irrigation is available, centre pivot systems provide greater
flexibility as irrigation can be used to aid seedbed preparation.
Prepare the field for seeding then irrigate heavily enough to firm
the surface and wet the soil to the level of sub-surface moisture,
usually 1 to 2 cm (0.5 to 0.75"). Leave the field to become surface
dry, then seed. Do not irrigate between seeding and emergence due
to potential soil crusting. A single irrigation to promote
germination and emergence is usually all that is necessary until
the crop is in the four- to six-leaf stage. Avoid overirrigation
during this period which can reduce the rate of crop growth and
increase the level of seedling disease.
Elongation and Flowering
Adequate soil moisture:
- promotes root growth
- promotes a large abundant leaf area
- helps plants retain their leaves longer
- lengthens the flowering period
- increases the number of branches per plant, number of flower
forming pods, seeds per pod, seed weight, and seed yield
Research at Lethbridge, AB and Outlook, SK has shown that
adequate soil moisture from irrigation has a large influence on
plant growth and development in comparison to dryland canola (Table
Table 2. Average Effects of Water
on Yield Components and Yield of B.
napus - Outlook, SK Area
||Branchers Per Plant
||Pods Per Plant
||Seeds Per Pod
||Seed Weight g/100
Moisture stress during the early vegetative stages may reduce
leaf expansion and dry matter production (Figure 2).
Figure 2. Dry Matter (DM) Production
and Leaf Area Index (LAI)
Plants under early season moisture stress will usually recover
normal growth with rainfall or irrigation. Stressed plants have the
ability to recover leaf area, form flowers, set pods and fill seeds
when the water becomes available but with hastened development
rates, early maturity and lower yields. The worst time to
experience drought stress on canola is during stem elongation or
Heavy rain or sprinkler irrigation during flowering may cause
flower damage, reduce pollination and yield. However, most
irrigation and dryland growers have not reported this to be a
serious problem. In general, canola produces more flowers than its
photosynthetic machinery can sustain and when a portion of the
flowers are affected by heavy rain or irrigation, later formed
flowers can compensate. Since flowering can extend up to 30 days,
it is almost impossible to avoid irrigation during this period, if
adequate soil moisture levels are to be maintained. With water use
of around 7 mm (0.28") per day, the crop would need 210 mm (8") of
water during flowering. However, the soil moisture storage capacity
of most soils is well below this level. Irrigating during the
flowering stage is particularly important since a water deficiency
will result in reduced dry matter production, fewer pods, early
leaf loss and reduced yields (Figure 3).
Figure 3. Effect of Water Stress
on Brassica Plants
Moisture stress during flowering or ripening stages results in
large yield losses. Leaves wilt and die sooner causing reduced
branching, pods per plant, pod length, seed size and seeds per pod.
Seed oil content drops while protein content increases. Moisture
stress may greatly slow or stop root growth affecting further soil
water intake. If moisture stress is severe, recently formed pods
may abort. The flowering period and maturity are shortened,
especially when combined with high temperatures. An excessively low
relative humidity with high temperatures can result in pollen
germination and seed fertilization failure. Moisture stress
combined with higher temperature from flowering to maturity
significantly decreases the number of pods, number of seeds, seed
weight, oil content and yield (Table 3).
Table 3. Effects of
Temperature and Water Stress Applied from the End of Flowering
until Maturity on Yield, Yield Components and Oil Content for B. napus
|Day/Night Temperature Effect (% relative to
irrigated low temperature)
Agriculture and Agri-Food Canada researchers at Lethbridge, AB
maintained available soil moisture above 50% in different
treatments up to budding, early pod formation and ripening for B.
rapa in years of below average precipitation (Table 4). The
greatest yields resulted from adequate fertility and soil moisture
throughout the growing season. With increasing available moisture,
Table 4. Effects of
Irrigation Levels on Canola Yield
|Irrigate to stem elongation
|Irrigate to early pod formation
|Irrigate to pod ripening*
* First seed turning brown
Oil content and seed weight of canola increase with adequate
water. Therefore, apply irrigation to avoid available soil moisture
from the root zone dropping below 50% until the earliest pods begin
to ripen. On late maturing fields, terminate irrigation prior to
this growth stage to hasten maturity and reduce the risk of frost
The exact timing and amounts of irrigation required will depend
on the soil moisture at seeding, rate of water use, weather
conditions, rainfall and type of irrigation system used. Adequate
soil moisture will tend to lengthen days to maturity of canola by
up to 10 days.
In many cases it may not be possible to provide adequate soil
moisture. Rationed irrigation water or the use of a sideroll
sprinkler system may limit the amount of water that can be applied
and yield will be less than optimum. If a side-roll sprinkler
system is used, crop height can limit the system's use and reduce
yield because of moisture stress during ripening. In these cases,
provide a high level of moisture to the crop just before growth
prohibits further wheel movement. The best results, when water is
limited, will usually be obtained with irrigation just prior to or
at early flowering.
Long periods of drought will reduce yields on dryland more than
frequent short periods of drought, especially on coarse textured
soils and shallow soils with low water storage capacity.
Excess Soil Moisture
Canola roots require a good mix of water and air in the soil.
When water exceeds the soil's water holding capacity or where
impermeable subsoil slows water infiltration, water logging,
flooding or ponding may occur. Canola is quite susceptible to water
logging and shows a yield reduction after only three days. Wet
soils slow down or stop gas exchange between the soil and
atmosphere, causing an oxygen deficiency. A higher temperature
causes higher respiration rates in roots and soil micro-organisms
and therefore soil oxygen is consumed more quickly. Lack of oxygen
reduces root respiration and growth. Soil texture also affects the
time that critical levels of soil oxygen are reached. This is due
to the oxygen carrying capacity of soils. Coarser textured soils
can hold more oxygen, increasing the amount of time required to
reduce levels to a critical point. Water logging reduces nutrient
uptake and in very poorly aerated soils plants can die. A high
water table can also reduce the supply of oxygen to the roots,
restricting their growth. Symptoms include older leaves turning
purple and senescing more rapidly. Soil air supply must be
maintained by ensuring that the soil has good aggregation and
The amount of yield loss will depend on the growth stage at the
time of water logging, the duration of water logging and the
temperature (Figure 4).
Figure 4. Effect of Water
Logging on Yield
Water logging for seven days at the rosette growth stage can
reduce plant height, while number of branches and seeds per pod
decrease with three days or more of flooding. Water logging for
three days or more during flowering reduces the number of pods per
branch as well as seeds per pod. Higher temperature during water
logging reduces plant growth and dry matter production. Water
logging for seven days at seed filling decreases individual seed
weight and oil content. High temperatures combined with water
logging increases the detrimental effects on canola yield.
Water Use and Yield
The water required by a canola crop varies from spring to fall,
year to year, and location to location because of the influences of
humidity, temperature, wind and light. Crop evapotranspiration is
primarily influenced by the stage of growth and amount of ground
cover as shown (Figure 5) for B. rapa
varieties when adequately supplied with water in southern
Figure 5. Water Use by B. rapa
B. napus crops have a similar water
use curve but it extends to or past the end of August. When the
crop is young and not covering the ground, water use per day will
be low. Vegetative and root growth results in a gradual increase in
water use. Increasing temperature also contributes to the increase
in water use. When the crop is actively growing, providing full
ground cover, and adequately supplied with soil moisture,
evapotranspiration will be at a maximum for the current weather
conditions. Flowering occurs during the peak use period. Peak
moisture use occurs during hot, dry weather and can be expected to
reach 8 mm (0.32") per day or more. However, the weather conditions
necessary for such high water use are not likely to be prolonged
over long time periods. As the crop ripens, its ability to transmit
water from the soil declines and water use decreases.
Canola yields will be highest when there is adequate soil
moisture throughout the growing season. Adequate soil moisture is
defined as maintaining 50% or more of the available soil moisture
in the root zone. The actual root zone of canola will vary from 5
to 6 cm (2 to 2.4") deep at emergence, to at least 14 cm (6") deep
during flowering and seed production. Under irrigation, soil
moisture is managed to a depth of one metre (39"). Not all of the
available soil moisture is equally available to the plant.
Generally, when soil moisture is maintained at 75% of available
soil moisture, yields are better than at the 50% use level. Yields
are reduced if more than 50% of the available soil moisture in the
root zone is used before soil moisture is restored (toward field
capacity) by rainfall or irrigation (Figure 6).
Figure 6. Crop Response to
Additional soil moisture will result in no further increase in
yield and may cause yield reductions through poor soil aeration
and/or increased lodging and diseases.
Canola plants require a threshold amount of water before any
yield is obtained. Beyond that threshold increasing amounts of
water will result in higher yields. Usually 25 mm (1") of water
will result in about 150 to 200 kg/ha (2.75 to 3.60 bu/ac) of yield
depending on the soil type with good growing conditions and
adequate fertility. Three lines are presented in Figure 7
representing a range of crop growing conditions from poor to
excellent. In some research studies, under ideal conditions, yields
have ranged up to 392 kg/ha (7 bu/ac) per inch of water.
Figure 7. Moisture Use and Canola
Yield (Southern Alberta 1994-98)
A study at the Agriculture and Agri-Food Canada Research Centre
in Lethbridge, AB showed that a B. rapa
crop used the following amounts of water under adequate soil
moisture and fertility (Table 5).
5. Water Use By B. rapa
||Water Use (mm)
||Water Use (")
Under the same conditions a higher yielding, later-maturing
B. napus crop would use from 450 to 550
mm (18 to 22") of water in a growing season.
With sufficient soil moisture and fertility during the growing
season to produce maximum yields, crops in Black and Grey Wooded
soils will require about 325 to 350 mm (13 to 14") of water. In
most of the Thin Black, Black and Grey Wooded soil zones, the
rainfall during the growing season usually exceeds 250 mm (10").
Additional moisture stored in the soil will result in higher
yields. Therefore, in these areas, canola can be grown successfully
as a stubble crop.
Canola crops grown in cooler, more humid areas require less
moisture to produce the same crop yield than warmer, drier areas.
Heat and wind increase water use while low temperatures and less
wind reduce water use. If the air is moist or on cloudy days,
moisture use is low. Potential water use is highest in the
southwest prairies, especially in the brown and dark brown soil
zones. These areas usually have more sunlight, higher temperatures,
lower humidity and more wind than the Black or Grey-Wooded soil
zones. The Black soil zones have better moisture conditions and
crop yield, not because of higher precipitation, but as a result of
lower temperatures and slower wind speeds.
In the Dark Brown and Brown soil zones, rainfall combined with
stored soil moisture is rarely sufficient to furnish the optimum
amounts of water required by the crop during the growing season. In
these soil zones, use canola where the soil moisture profile is
fully recharged. A fully recharged soil profile is common on
summerfallow, however, use caution on stubble. Seed early as the
plants develop a deeper rooting system to utilize soil moisture.
Early seeding also minimizes the risk of damage at flowering from
high temperatures in these soil zones. Increasing and conserving
stored soil moisture is important in these soil zones for higher
Under conditions in much of the irrigated areas, the net annual
irrigation requirement for maximum canola yields will be 250 to 350
mm (10" to 14") of water. The exact timing and amount of irrigation
required will depend on the soil moisture at seeding, rate of use,
rainfall, and type of irrigation system.
Crop Water Use Comparisons
Canola and mustard crops use about the same amount of water as a
wheat crop. A three-year joint study by Agriculture & Agri-Food
Canada and the University of Manitoba in Winnipeg, MB on crop water
relations in the semi-arid prairie found that under natural
rainfall and imposed drought, total water use by all Brassica
oilseeds was similar to wheat (Table 6).
Table 6. Crop Yields, Water Use and
Water Use Efficiency (WUE) 1996-1998
|Imposed Drought Test (No Rain from Mid-June to
||Water Use (")
||Seed Yield (lb/ac)
|B. napus canola
|B. rapa canola
|Rain Fed Test
||Water Use (")
||Seed Yield (lb/ac)
|B. napus canola
|B. rapa canola
||Water Use (")
||Seed Yield (lb/ac)
|B. napus canola
|B. rapa canola
Values within a column followed by the same letter
were not statistically different (P=0.05).
In general, B. rapa canola used
significantly less water than wheat except under drought
conditions. This reflects the fact that B.
rapa matures much earlier than any other crop. Wheat has a
higher water use efficiency, with higher pounds per unit of water
consumed than canola or mustard. Canola and mustard utilize more
energy in producing oil than wheat does in producing starch. B. napus canola and mustard used the same
amount of water and had similar water use efficiencies. Expect
differences between canola and mustard varieties in drought
tolerance and water use.
The water use ranking of the various crops is: B. napus canola = mustard < spring wheat =
B. rapa canola < kabuli and desi
chickpea > lentil = pea. This ranking is the same for the each
crop's rooting depth ranking.
Canola and field peas are dicots and have a tap root system,
while wheat is a monocot with a fibrous root system (Figure 8).
Figure 8. Picture of Root Systems
of Canola (Cyclone), Wheat (Katepwa) and Field Pea (Grande) at
Swift Current, SK in 1998
Canola and field pea root density was about 65% of wheat root
density in the top 60 cm (24") of the soil profile. The total root
length density of canola and field peas in the upper square metre
of the soil profile were 131 m (429') and 166 m (544') compared to
248 m (814') in wheat. Canola had the highest root length density
in the 100 to 120 cm (40" to 48") layer. This research showed that
field peas had the shallowest root system and canola the deepest
root system in this soil type.
The root system is elastic and depends on soil type, moisture
content, temperature, salinity and physical structure. Therefore,
similar root density profiles will occur in mediumtextured soils
with moisture throughout the profile at seeding in the Brown soil
zone. Canola, mustard and wheat rely on water from their roots to
maintain a favourable water balance so these crops more often
experience water stress than do pulses. Pulse crops show less water
stress as their plant tissues are more elastic and can lose water
and still maintain a favourable water balance. Wheat is better at
osmotic adjustment (maintains more "suction" in its leaves) than
canola or mustard so it can transport sufficient water from its
roots. The deep and conductive roots of canola and mustard are key
to the plant's ability to bring water to the leaves to maintain
water status. Canola or mustard grown on fallow can extract soil
moisture at depth to reduce the effects of a dry period. Under
stubble, fall and early spring rains and snowmelt will often re-wet
the soil profile to a depth of up to 2'- much less than the depth
Brassica crops can root. Below that, the
soil can be bone dry. Brassica crops
cannot grow roots through extended zones of dry soil and if the
crop is limited to water near the soil surface there is an
increased risk of lower yields unless there are timely rains in
June and July. Consequently, canola and mustard are the poorest
adapted to a dry rooting zone. Therefore, these crops in dryland
areas are best suited for production on fallow where the soil
profile is recharged with moisture at seeding.
Early and late seeded canola or mustard use about the same
amount of water, but early seeded fields produce higher yields due
to more efficient water use. Mid-May sown canola or mustard uses
more water per day. However, as flowering and seed filling occur
during hotter drier conditions the crops mature earlier so that
total water use is the same as an early seeded crop.
Soil Moisture Storage Capacity
Not all of the yearly precipitation is stored in the soil and
available to plants. Precipitation may be stored in the root zone,
drained below the root zone, used by weeds and volunteer plants,
evaporated from the soil surface, blown away as snow or lost as
The amount of water that can be stored varies widely among soils
depending upon the number and size of pore spaces they contain, and
the depth to layers of soil difficult for water to penetrate. The
number and size of pore spaces in a soil depend on its texture,
organic matter content and structure.
Effect of Soil Texture on Moisture
Soil texture refers to the size and amount of the mineral
particles of sand, silt and clay present in the soil. The diameter
of individual particles of sand range from 0.05 to 1 mm (0.002 to
0.04"), silt from 0.002 to 0.05 mm (0.00008 to 0.002") and clay
less than 0.002 mm (0.00008"). The proportions of sand, silt and
clay determine the soil texture (Figure 9).
Figure 9. Soil Texture
The 13 soil textures can be grouped into:
- very coarse-sandy, loamy sands
- coarse-sandy loam
- medium-loam, sandy clay loam, sandy clay, clay loam
- fine-silt loam, silty clay, loam, silt
- very fine-clay, silty clay, heavy clay
Figure 10 shows three soils of different textures-sandy loam,
loam, and heavy clay-each holding water at or near field capacity.
Field capacity is the amount (%) of water that a soil can hold
Figure 10. Volume of Soil
Components in Three Soils
Note that the total amount of pore space (water and air) is
greater as the percentage of clay increases. In clay soils the clay
particles are smaller, have more total surface area, and contain
many very small individual pore spaces. In sandy soils, the sand
particles are larger, have less total surface area and fewer larger
individual pore spaces. Clay soils are able to hold more water than
sandy soils because of the smaller individual pores and greater
total pore space (Figure 11).
Figure 11. Relationship of
Field Capacity, Wilting Point, Available Water and Unavailable
Water to Soil Texture
The small amount of pore space and many pores are so large that
water readily drains from sandy soils. Clay particles are very
small with a large number of fine pore spaces, which retain
moisture. Organic matter increases the soil's water-holding
capacity. After a rain or irrigation that saturates the soil, about
one-half of the total water will drain out of the soil until it
reaches field capacity if there is no restriction to drainage. For
a sandy soil, this drainage will be more than half the total water
held at saturation, and for a clay soil it will be less than half.
As the water drains downward through the soil, evaporates from the
soil surface, or is used by plants, air will first occupy the large
soil pores. As the soil dries by evaporation and plant use, the
intermediate and smaller size pores also become occupied by air. As
the soil dries further, increasing amounts of suction and energy
are required by plants to extract water from the soil. The water
content of the soil when the crop begins to wilt and not recover
overnight is the wilting point. The amount of soil water held
between field capacity and wilting point is the available soil
moisture. In general, only about half of the total water that a
soil can hold at field capacity is available soil moisture.
Effect of Soil Texture on
Evaporation from the soil surface mainly affects the water in
the top 10 to 13 cm (4 to 5"). To be effectively stored in the
soil, rainfall or irrigation must be heavy enough, or frequent
enough, to wet the soil below 10 to 13 cm (4 to 5"). Showers which
only dampen the surface will be lost to evaporation in a few days
or even sooner if winds are prevalent.
Soil texture and soil structure will regulate how much and how
fast water can infiltrate the soil. A sandy soil will have large
pore spaces through which water can move easily. The pore spaces in
clay are small, causing water to move slowly. Think of three glass
cylinders of soil, containing sand, sandy loam and clay loam soils
respectively, as shown in Figure 12.
Figure 12. Water Movement in
Different Texture Soils
If a cup of water is poured on the surface of each soil it will
disappear into the sand first, into the loam next, and into the
clay last. After the water has stopped moving in the soil, the sand
will be wetted the deepest, the loam not so deep, and the clay the
least. This is because of the greater water holding capacity of the
clay loam. It has more pore space to hold water, even though the
individual pores are smaller. Sandy soils have a higher
infiltration rate over a longer period of time (Figure 13).
Figure 13. Change in Infiltration Rate
If rainfall or irrigation exceeds the rate at which the soil
allows infiltration, runoff will occur. Crop residues break up
raindrops and delay runoff, allowing penetration of water into the
Effect of Soil Structure on Soil
Soil structure has only a small effect on the ability of soil to
hold water. However, it controls water entry into the soil, thus
altering the "effective" water holding capacity. Soil structure
refers to the way in which mineral and organic particles are
arranged into granules or aggregates of different shapes, sizes and
volumes of pore spaces. Soil organic matter is involved in holding
soil particles together in aggregates. Soil micro-organisms are
central to this process with aggregates constantly being formed and
broken down again. Soil micro-organisms feed on organic matter,
producing binding agents that aggregate soil particles. When a
scarcity of organic matter occurs, micro-organisms in the soil
cause further destruction of aggregates by decomposing the binding
agents. Therefore, organic matter added to the soil in large
quantities will stimulate rapid growth of micro-organisms and
result in production of binding agents, greatly benefiting soil
particle aggregation. The plant residue must also be mixed
thoroughly in the soil to maximize the soil volume involved.
Consequently, plant roots are best for adding organic matter to the
soil in a form and location most beneficial to maintaining a
stable, fertile soil. The decomposing plant roots and
micro-organisms associated with them are continuous sources of soil
organic matter. Native prairie stands and forage crops provide more
water-stable aggregates than a continuous wheat or barley
Soil that is well aggregated has more pore space for air and
water. A desirable soil structure will have water-stable aggregates
that vary in size from 1 to 5 mm (0.04 to 0.2") with good pore
space within and between the granules. This allows air and water to
move freely into the soil and increase the ability of soil to hold
water. A soil with low organic matter and poor structure will have
an initial infiltration rate for rainfall of only one-tenth of a
soil with high organic matter and good structure. A well aggregated
soil has good tilth preventing it from becoming either too hard
with a crust or too loose. If sufficient organic matter is returned
to heavy clay soils they gradually develop a crumb-like structure,
which makes them easier to work and less susceptible to baking and
crusting. Soil surfaces that often bake or crust reduce water
intake and storage causing increased runoff and erosion after heavy
rains or irrigation. Wetting only a portion of the root zone depth
can result in reduced root growth and lower crop yields. Increasing
organic matter content in light sandy soils helps soil aggregates
hold together. When soil particles are bound together into
aggregates they stabilize the soil with a force strong enough to
resist breakdown by rainfall, wind erosion or runoff. Improved
aggregation results in less runoff and greater moisture
Suitability of Soils for Canola Based
on Moisture Holding Capacity
Medium-textured soils are most suitable to canola production
because of their favourable capacity for moisture infiltration,
water holding capacity and usually adequate drainage. These soils
usually have a better granular structure that allows them to be
firmly packed for a seedbed without baking and crusting. Such a
seedbed promotes rapid germination and uniform stands that strongly
compete with weeds.
Fine-textured soils can produce good canola crops when well
managed. In drier areas, clay soils with their higher water holding
capacity are better able to carry canola crops through short
periods of drought. However, clay soils are more prone to becoming
waterlogged than sandy loam or loam soils because they have smaller
air spaces and a slower rate of water movement. Clay soils tend to
remain wet and cold in the spring, which often results in slow
germination and uneven growth, and allows little competition to
early weed infestations. Fine-textured soils low in organic matter
often have poor structure and crust easily, thereby reducing
Sandy soils are usually not suitable for canola mainly because
of their low moisture holding capacity. The surface soil dries
rapidly in the dry prairie region. Sandy soils cannot store
sufficient moisture to support a canola crop through periods of
drought. However, sandy soils are not subject to crusting and under
irrigation, sandy textured soil can support canola crops.
Effect of Soil Fertility on Plant
Soil fertility promotes efficient soil moisture use. Fertilized
canola roots deeper and has a greater root volume. This increases
leaf development, prolongs leaf life and increases supply of food
for later pod and seed development. The amount of water required to
produce a kilogram or pound of dry matter is increased if the soil
is low in fertility (Table 7).
Table 7. Fertilizer Effect on Canola
Water Use Efficiency for 25 mm (1") of water - Fort Vermilion,
*These calculations are made on the basis of May to
August rainfall, plus an assumed amount of stored soil moisture as
the total water used by the crop.
The table illustrates a dramatic increase in moisture use
efficiency on a low fertility Grey Wooded soil. Adequate
fertilization usually can improve water use efficiency by up to 15%
on fallow land and by 30% on stubble land.
Soil Moisture Measurement
Manual examination and appearance can be used to assess soil
moisture content. Skill in assessing soil moisture by manual
examination is easily acquired, especially over a number of years
on the same fields. The only equipment needed is a shovel or auger
to obtain samples of the soil from the necessary depths. Table 8
describes the characteristics of soil after it has been squeezed
firmly in the hand. To obtain a useful assessment of soil moisture
with respect to the crop, examine the soil to three-quarters of the
rooting depth. For canola crops, the depth of rooting increases
from the seedling stage to full growth.
8. Practical Interpretation Chart for Soil Moisture
|Feel or Appearance of Soils
|Available Soil Moisture % of Field Capacity
||Dry, loose, singlegrained, flows through fingers
||Dry, loose, flows through fingers
||Powdery, dry, sometimes slightly crusted but easily breaks down
into powdery condition
||Hard, baked, cracked, sometimes has loose crumbs on
|50 or less
||Still appears to be dry; will not form a ball with
||Still appears to be dry; will not form a ball*
||Somewhat crumbly but will hold together from pressure
||Somewhat pliable, will ball under pressure*
|50 to 75
||Still appears to be dry; will not form a ball with
||Tends to ball under pressure but seldom will hold together
||Forms a ball*, somewhat plastic; will sometimes slick slightly
||Forms a ball; will ribbon out between thumb and forefinger
|75 to field capacity
||Tends to stick together slightly, sometimes forms a very weak
ball under pressure
||Forms weak ball, breaks easily, will not slick
||Forms a ball and is very pliable; slicks readily if relatively
high in clay
||Easily ribbons out between fingers; has a slick feeling
|At field capacity
||Upon squeezing, no free water appears on the soil but wet
outline of ball is left on hand
||Same as coarse
||Same as coarse
||Same as coarse
|Above field capacity
||Free water appears when soil is bounced in hand
||Free water will be released with kneading
||Can squeeze out free water
||Puddles and free water forms on surface
*Ball is formed by squeezing a handful of soil very
firmly with fingers.
Thus, the depth of soil with respect to critical moisture
content is less in the early stages of growth than in midseason.
Take the shallowest samples in the 15 to 20 cm (6 to 8") depth.
In the lower rainfall areas of the drier Brown soil zone, the
risk of crop failure is greater and the critical depth of moist
soil is more important than in the other soil zones. Note that
sandy soils require the full rooting depth of 120 cm (4.8") to be
moist before recropping should be risked in all soil zones.
Soil Water Management
Canola crops on the Canadian prairies are frequently subjected
to temperature and moisture stress. The soil water supply during
the growing season is frequently insufficient to meet the potential
evapotranspiration needs of the crop, especially in dryland areas.
Crop productivity is directly proportional to the amount of water
transpired. The transpiration can be increased either by increasing
the water supply or by reducing evaporation. Therefore, any
management practice that improves water available for transpiration
either by conserving or by reducing evaporation, increases crop
On dryland, the manual assessment of soil moisture can help you
decide whether or not to recrop a stubble field. Table 9 provides a
general guide for recropping.
Table 9. Soil Moisture Reserves
Necessary for Recropping*
|Soil Moisture Reserves
||Black, Grey, Grey Wooded
||Peace River Region
|High: Soil moisture beyond 75 cm (30")
|Medium: Soil moisture to 45 to 75 cm (18 to 30")
||Recropping is to 45 to 75 cm (18 to 30") risky
except at high end of moisture range
|Low: Soil moisture to 15-45 cm (6 to 18")
||Recropping is not advisable
||Recropping is risky
||Recropping has some risk
||Recropping is risky in drier areas
|Very low: Little soil moisture below 15 cm (6")
||Recropping is not advisable
||Recropping is not advisable
||Recropping has some risk
||Recropping risk depends on subregion
*Assumes a loam soil. For clay soil, multiply
depths by 0.7. For sandy soil, multiply depths by 1.5.
Conserving Snow Moisture
Growers can influence the amount of water that enters the soil
between harvest and seeding time. Snowfall contributes about 25 to
35% of the total annual precipitation. As a rule of thumb, 25 cm
(10") of snow equals 25 mm (1") of rain. Use crop residue
management during the fall to help increase snow trapping and
reduce snowmelt runoff-which accounts for 85% of runoff from
agricultural lands. Minimize fall tillage and leave as much erect
stubble as possible. This helps conserve snow and increase stored
water in the spring. An extra 25 mm (1") of water stored through
moisture management will make a significant difference in crop
Standing stubble increases snow trapping compared to that on a
fallow field. The amount of snow trapped is directly proportional
to stubble height. Tall standing stubble also reduces wind speed,
solar radiation reaching the soil surface and keeps soil
temperature cooler than fallow reducing water lost by evaporation.
These changes in the microclimate are noticed early in the growing
season when the crop canopy is small and cannot yet affect
evaporation loss. Canola sown early on tall stubble has increased
water use efficiency and higher yields.
Researchers at the Agriculture and Agri-Food Canada Swift
Current, SK Research Centre and the University of Saskatchewan in
Saskatoon, SK, found that swathing fields at alternate heights each
round trapped more snow and increased soil moisture storage by up
to 45 mm (1.8") over bare summerfallow fields, and up to 25 mm (1")
over uniform standing stubble. The researchers have also developed
a modified swather cutter-bar that leaves tall narrow strips of
stubble each round. The modification of trash and stubble
management increased soil moisture about one and one-half times
over conventional stubble management. The increased soil moisture
storage through snow management on stubble fields with adequate
fertility has produced yields up to 95% of those on conventional
Use of Crop Residues
Provide a ground cover with crop residues on the soil surface to
improve soil water intake. Surface trash increases water
infiltration by breaking up raindrops and delaying runoff.
Keep crop residues on the soil surface for as long as possible.
The residue shades the soil, providing a reflective cover to reduce
the sun's energy that would otherwise evaporate water from the
soil. Crop residues also reduce the wind speed over the soil
surface reducing the loss of water vapour. However, residue causes
the soil to be slightly cooler in the spring, which could have a
negative effect on germination and emergence in more northern
Increasing Soil Organic
Increase soil organic matter content to improve soil structure
and water infiltration. Incorporate all crop residues. These
residues add to the organic matter reserve and help maintain it at
a higher level. Grass, grass-legume and legume stands with their
abundant rooting systems will increase the organic matter content
of the soil in the long term and improve soil aggregation. A
favourable aggregation will also benefit root development and
penetration allowing the root system to use soil moisture and
nutrients effectively. Manure assists in temporarily improving soil
structure and helps in maintaining soil organic matter at a higher
Use of Fertilizer
Fertilizer is essential to crop yields in extended rotations.
Increased nitrogen fertility increases both total water use and the
rate of water use. With higher nitrogen fertility levels canola
develops denser rooting systems with greater water extraction in
all soil layers. Adequate fertility allows canola plants, during
the early vegetative growth stage, to develop extensive roots for
full exploration of the soil profile. Adequate fertility increases
the crop's water use efficiency. An inadequately fertilized crop
does not use available water efficiently resulting in reduced
yields. Inadequate fertility reduces the drought tolerance of
canola by reducing the amount of soil water extracted and the water
use efficiency in producing seed.
Zero-till fields have greater soil moisture due to reduced
runoff, greater infiltration, reduced evaporation and increased
snow trapping compared to conventionaltill fields.
The depth and amount of soil disturbance by a tillage operation
influences soil moisture. Losses are proportional to the depth of
soil loosening. Turning the soil over by ploughing or mixing by
discing causes greater soil moisture losses than the actions of the
blade cultivator. Excessive spring cultivation also dries out the
surface soil to the depth of tillage, preventing shallow seeding to
moisture. High tillage speeds and excessive tillage dries out the
soil surface and pulverizes the soil structure. Pulverized fine
powdery surface soils are susceptible to erosion and crusting.
Crusting reduces water intake. Use only those operations necessary
for weed control, land levelling and seedbed preparation. Minimum
tillage cropping systems have similar yields to conventional
tillage. Use contour cultivation to lengthen the time that free
water remains on the surface, giving it a better chance to
penetrate the soil. Therefore, cultivation that leaves ridges on
the contour of sloping or hilly land will help control runoff and
increase the penetration of rain where it falls.
Summerfallow is a traditional means of conserving some of the
precipitation in one growing season to augment the plant-available
moisture during the next season. Research has found that
summerfallow does not conserve moisture very efficiently,
especially in the higher rainfall areas. Researchers in
Saskatchewan, Manitoba and Alberta have reported that during the
21-month fallow period in Brown and Dark Brown soils only 15 to 25%
of the precipitation is stored in the soil, while in the Black and
Grey-Wooded soils, often only 3 to 13% is stored. The rest is lost
as evaporation, runoff, blown off as snow, or drained out of the
root zone. Even though summerfallow is an inefficient means of
conserving soil moisture, the small amount stored may make the
difference between a paying crop and a crop failure, especially in
the arid regions of the Brown and Dark Brown soil zones. However, a
reduction of summerfallow is possible on most soils if combined
with better moisture management. In the higher rainfall, more humid
regions of the Black, Dark Grey and Grey-Wooded soil zones, there
may be little or no benefit from summerfallow other than perennial
weed control. In fact, negative results (through increased
salinization and/or loss of soluble nutrients from the root zone)
may occur in both the Brown and Black soil zones if water is
accumulated beyond field capacity. Use summerfallow sparingly in
these areas, preferably only to combat severe weed infestation, or
in cases of severe drought.
Insect and Disease Control
Weeds use water at a similar rate to canola plants. Their
control increases the moisture supply available to the crop.
Insects and diseases reduce the plant's ability to use water.
Control these pests to allow plants to more efficiently use the
When excessive water or ponding occurs, plant photosynthesis is
reduced. This limiting factor on some soils may be reduced by
carefully planned and installed drainage systems.
The amounts and duration of rainfall cannot be controlled and
may be a limiting factor to crop growth unless irrigation is
applied. The only water reservoir available to plants is that
stored in the rooting zone of the soil. In order to plan effective
use of water, you must understand the basic factors, which
determine the soil's capability to absorb, store and provide water
to plants. It is also essential to know the crop's water
requirements and the estimates of spring stored soil moisture and
growing season rainfall. Some soil factors affecting water storage
cannot be controlled and may limit water availability. Other soil
factors can be modified and managed through practices which
increase the availability and efficiency of use of precipitation.
Adopting these practices will allow growers to closely estimate a
potential yield and modify other production factors to achieve
Angadi, S.V., Cutforth, H.W. and B.G. McConkey.
2000. Effect of Stubble Microclimate on Canola Yield.
Saskatchewan Soils and Crops Proceedings 2000. Brandt, S.A. 1992.
Zero vs. conventional tillage and their effects on crop yield and
soil moisture. Can. J. Plant Sci. 72:679-688.
Borstlap, S. and Entz, M. H. 1994. Zero-tillage
influence on canola, field pea and wheat in a dry subhumid region:
Agronomic and physiological responses. Can. J. Plant Sci.
Champolivier, L. and A. Merrien. 1996. Effect
of water stress applied at different growth stages to Brassica
napus L. var Oleifera on yield components and seed quality. Eur. J.
Clarke, J.M. and G.M. Simpson. 1978. Influence
of irrigation and seeding rates on yield and yield components of
Brassica napus cv. Tower. Can. J. Plant Sci. 58:731-737.
Cutforth, H., McConkey, B., Miller, P., Brandt, S.,
Volkmar, K., Entz, M. and D. Ulrich. 1998. Yield and water
use of canola and mustard in waterlimited environments.
Saskatchewan Soils and Crops Proceedings Pp. 440-444.
Gardner, W.H. 1979. How water moves in the
soil. Crops and Soils Magazine.
Good, A.G. and Maclagan, J.L. 1993. Effects of
drought stress on the water relations in Brassica species. Can. J.
Plant Sci. 73:525-529.
Gutierrez Boem, F.H., Lavado, R.S. and C.A. Porcelli.
1996. Note on the effects of winter and spring
waterlogging on growth, chemical composition and yield of rapeseed.
Field Crops Res. 47:175-179.
Henry, J.L. and MacDonald, K.B. 1978. The
effects of soil and fertilizer nitrogen and moisture stress on
yield, oil, and protein content of rape. Can. J. Soil Sci.
Krogman, K.K. and Hobbs, E.H. 1975. Yield and
morphological response of rape (Brassica campestris L. cv. Span) to
irrigation and fertilizer treatments. Can. J. Plant Sci.
Kumar, A., Singh, D.P. and Phool Singh. 1994.
Influence of water stress on photosynthesis, transpiration,
water-use efficiency and yield of Brassica juncea L. Field Crops
Lafond, G.P., H. Leoppky, and D.A. Derksen.
1992. The effects of tillage systems and crop rotations on
the soil water conservation, seedling establishment and crop yield.
Can. J. Plant Sci. 72:103-115.
Lore, J. & Associates Ltd. 1990. Crop
response to moisture stress, flooding, and salinity, under dryland
conditions. Literature Review March 28, 1990.
McConkey, B.G., Ulrich, D.J. and D.A. Dyck.
1997. Snow Management and deep tillage for increasing crop
yield on a rolling landscape. Can. J. Soil Sci. 77:479-486.
Mendham, N.J. and P.A. Salisbury. 1995.
Physiology: crop development, growth and yield. In Kimber, D. and
D.I. McGregor (Eds.) Brassica oilseeds - Production and
utilization, CAB. Pp. 11-64.
Miller, M.H. 1983. Soil limitations to crop
productivity in Canada. Can. J. Plant Sci. 63:23-32.
Miller, P., Cutforth, H., McConkey, B., Ulrick, D.,
Brandt, S., Volkmar, K., and Entz, M. 1998. How thirsty
are canola and mustard. Semiarid Prairie Agricultural Research
Centre Research Newsletter No. 4, March 20, 1998.
Miller, P.R., McDonald, C.C., Derksen, D.A. and
Waddington, J. 2001. The adaptation of seven broadleaf
crops to the dry semiarid prairie. Can. J. Plant Sci. 81:29-43.
Miller, P.R., Johnson, A.M., Brandt, S.A., McDonald,
C.L., Derksen, D.A. and Waddington, J. 1998b. Comparing
the adaptation of sunola, canola and mustard to three soil climatic
zones of the Canadian prairies. Can. J. Plant Sci. 78:565-570.
Nielsen, D.C. 1997. Water use and yield of
canola under dryland conditions in the central great plains. J.
Prod. Agric. 10:307-313.
Nuttall. W. F. 1973. Influence of soil moisture
tension and amendments on yield, oil and protein content of Target
rape grown on Grey Wooded Soils in the greenhouse. Can. J. Soil
Oosterveld, M. and W. Nicholaichuk. 1983. Water
requirements, availability and development restraints for increased
crop production in Canada. Can. J. Plant Sci. 63:33-44.
Rao, M.S.S. and N.J. Mendham. 1991.
Soil-plant-water relations of oilseed rape (Brassica napus and B.
campestris). J. of Agric. Sci. Camb. 117:197-205.
Rao, S.C., and T.H. Dao. 1987. Soil water
effects on low-temperature seedling emergence of five Brassica
cultivars. Agron J. 79:517-519.
Richards, R.A. and N. Thurling. 1978. Variation
between and within Species of Rapeseed (Brassica campestris and B.
napus) in Response to Drought Stress. I. Sensitivity at Different
Stages of Development. Aust. J. Agric. Res. 29:469-477.
Richards, R.A. and N. Thurling. 1978. Variation
between and within Species of Rapeseed (Brassica campestris and B.
napus) in Response to Drought Stress. II. Sensitivity at Different
Stages of Development. Aust. J. Agric. Res. 29:479-490.
Rood, S.B. and D.J. Major. 1984. Influence of
plant density, nitrogen, water supply and pod or leaf removal on
growth of oilseed rape. Field Crops Res. 8: 323-331.
Thomas, P. M. Irrigated Canola Production. Alberta
Agriculture Agdex No. 149/561-1 1980.
Triboi-Blondel, A.M. and M. Renard. 1999.
Effects of temperature and water stress on fatty acid composition
of rapeseed oil. Proceedings 10th International Rapeseed Congress,
Canberra ACT, 1999.
Vigil, M.F., D.C. Nielsen, A.D. Halvorson, and B. Beard.
1993. Dryland canola production: variety selection,
nitrogen response, and water use in the central great plains. Proc.
Western Canola Development Meeting, Pp. 129-133.
Walton, G., Si, S., Tennant, D. and Bowden B.
1999. Environmental impact on canola yield and oil.
Proceedings 10th International Rapeseed Congress.
Canberra, ACT, 1999.
Wentz, D. 1997. Annual crops for recharge
control of saline seeps. Alberta Agriculture, Food & Rural
Development Agdex No. 518-14.
Wright, P.R., Morgan, J.M., Jessop, R.S. and A. Cass.
1995. Comparative adaptation of canola (Brassica napus)
and Indian mustard (B. juncea) to soil water deficits: yield and
yield components. Field Crops Res. 42:1-13.
Young, J.A., Evans, R.A., Roundy, B. and G. Cluff.
1983. Moisture Stress and Seed Germination. USDA
Agriculture Research Service. Agriculture Reviews and Manuals. ARM
- W- 36/April 1983.
Zheng, G.H., Gao, Y.P., Wilen, R.W. and L.V. Gusta.
1998. Canola seed germination and seedling emergence from
pre-hydrated and re-dried seeds subjected to salt and water
stresses at low temperatures. Ann. Appl. Biol. 132:339-348.