Effects of Moisture

Effects of Moisture on Canola Growth

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 growing season
• 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.

Water's 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 surrounding plants.

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 seed development.

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 Contents 

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).

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.

Rosette, 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 2).

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 flowering.

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).

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, yield increased.

* 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 damage.

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 adequate drainage.

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 Alberta.

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 Available Moisture 

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).

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 yields.

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).

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 runoff.

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 Storage

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 Triangle 

The 13 soil textures can be grouped into:

1. very coarse-sandy, loamy sands
2. coarse-sandy loam
3. medium-loam, sandy clay loam, sandy clay, clay loam
4. fine-silt loam, silty clay, loam, silt
5. 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 against gravity.

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 Moisture Infiltration

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 with Time 

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 soil.

Effect of Soil Structure on Soil Moisture

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 rotation.

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 storage.

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 seedling emergence.

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 Moisture Use

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).

*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.

*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 Strategies

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 yield.

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.

*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 yields.

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 summerfallow.

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 areas.

Increasing Soil Organic Matter

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 level.

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.

Tillage Operations

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

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.

Weed, 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 available moisture.

Drainage

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.

Summary

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 maximum yields.

References