Chapter 6 - Acidity, Salinity, Solonetzic

Canola Response to Acidity, Salinity and Solonetzic Soil

Acidity, salinity and solonetzic soil problems can be major limiting factors in canola production. When severe, they are very visible and easily determined. At less limiting levels, they may not be as recognizable but still reduce growth and yield. This chapter discusses these soil problems and how to identify their extent, effects on canola plants and the steps to modify or reduce their limiting effects.

Soil pH and Acidity

Soil acidity is identified by a pH value. In practical terms, soils between pH 6.5 and 7.5 are considered neutral. Soils in the range 5.6 to 6.0 are moderately acid, 5.1 to 5.5 strongly acid and less than 5.1 are very strongly acid. Soils with a pH greater than 7.5 are called alkaline. A soil with a high degree of alkalinity (pH of 8.5 or higher) is called an alkali soil. The term alkali generally refers to high sodium soils but is frequently incorrectly used to refer to saline soils. Most saline soils are neutral to slightly alkaline (below pH 8.5). Canola will tolerate soils with a pH of up to 8.3 before yield reductions become serious. A majority of cultivated soils in western Canada are alkaline or neutral in reaction. However, large areas of soil with a pH of 6.0 or less occur naturally in Ontario, Saskatchewan, Alberta and northeast British Columbia. It has been estimated that medium to strongly acid soils (less than pH 6.0) occupy over 3 million hectares (7.4 million acres) in western Canada.

Soil pH affects the structure, chemical, and biological properties of soils and, therefore, crop yields. Crops also vary greatly in their tolerance to various components of acidity. A list of crops and the lowest pH value each will tolerate without yield reduction is shown in Table 1.

Table 1. Acidity Tolerance of Various Crops

Crop	pH
Alfalfa, sugar beets	6.5
Barley	6.0
Canola, wheat, corn, red clover	5.5
Potatoes, rye	5.0
Cranberries, oats	4.5

On strongly acid soils with a pH of less than 5.5, canola yields are often reduced substantially. Canola plant growth on acid soils can be limited by one or more factors, including:

• toxicity of hydrogen ions, aluminium, iron, or manganese
• deficiency of calcium, magnesium, potassium, phosphorus, boron, nitrogen, or molybdenum
• reduced organic matter breakdown and nutrient cycling by microflora
• reduced uptake by plant roots and inhibition of root growth

Acid soils often contain soluble forms of aluminium and manganese. As soil acidity increases below pH 5.5 (pH decreases), soluble aluminium and manganese increase to toxic levels. Aluminium toxicity restricts root growth and phosphorus uptake and translocation within the plant (Figure 1). While the primary effect of aluminium toxicity is at the root level, the more visible foliar injury is due to nutrient deficiency, drought (due to poor root penetration) or pathogens, which are more pronounced on aluminiumtoxic, acid soils. Manganese toxicity causes chlorosis on leaf margins and cupping of leaves in canola. Aluminium and manganese often reduce the yields of crops grown on acid soils.

Figure 1. Aluminium Toxicity Symptoms in Canola

Soil acidity reduces the availability of essential elements for plant growth such as phosphorus (P) and molybdenum (Mo) (Figure 2).

Figure 2. General Relationship Between pH of Mineral Soil and Availability of Various Elements*

Research has shown that canola yields on acid soils can be substantially increased by lime application. The application of lime to acid soils can affect biological, chemical and physical properties of soils. The increase in soil pH resulting from lime application provides a more favourable environment for soil microbiological activity that increases the rate of release of plant nutrients, particularly nitrogen. Reduced soil acidity following liming also increases the availability of several other plant nutrients, notably phosphorus. A crop takes up only about 20% of fertilizer phosphorus in the application year. The remainder is fixed in the soil in various degrees of availability for succeeding crops. Below pH 6.0, the fixed phosphorus is retained in less available forms than on slightly acid and neutral soils (pH 6.1 to 7.5). Phosphorus availability is reduced because of the formation of relatively insoluble compounds through reactions of phosphorus with iron and aluminium.

pH 7.0, fertilizer phosphorus availability is reduced because of reactions with calcium and magnesium. Therefore, one of the benefits of liming acid soils is the increased utilization of residual fertilizer phosphorus by crops. On slightly acid (pH 6.1 to 6.5) and moderately acid (pH 5.6 to 6.0) soils, liming will have a minor effect on canola yields (Figure 3).

Figure 3. Effects of Liming on Canola Yields at 27 Alberta Sites on Soils with Different pH Ranges

However, liming may improve the physical properties of some medium and fine-textured soils (particularly Grey and Dark Grey Wooded soils). Research at the Agriculture and Agri-Food Canada Beaverlodge, AB Research Centre showed that liming improved the stability of soil aggregates and, therefore, soil structure or tilth of Grey Wooded soils. Soils with a stable soil structure are less prone to crusting following intense precipitation, are well aerated, have a high rate of water infiltration and result in good germination and stand establishment of small-seeded crops like canola.

On strongly acid (pH 5.1 to 5.5) and very strongly acid (pH less than 5.1) soils, liming reduces soluble aluminium and manganese to non-toxic levels and increases yield. In a three-year study (1993-95) at Beaverlodge, liming a pH 5.0 soil raised the pH to 6.5, increasing plant dry matter production and increasing canola yields by 37% in tilled and 17% in no-till soils. Aluminium stress predisposes plants to infection by brown girdling root rot (Rhizoctonia solani Kuhn) due to reduced root vigour. The increased growth resulting from liming was likely due to a decrease in brown girdling root rot, reduced weed populations and increased pH-related changes in soil fertility, and likely other factors.

Research shows that the benefits of a single application of lime can last for up to 30 years. Liming is a substantial investment, although by-product liming materials from municipal and industrial facilities may provide an alternative low-cost liming material for farmers. It is important to identify the extent and severity of an acid soil problem. The most reliable method of identifying an acid soil problem is through soil tests. With careful sampling of fields, soil tests can determine the extent and severity of soil acidity, the rate of lime required and provide an estimate of crop response to lime.

Soil Salinity

All soils contain soluble salts (those which dissolve in water). When the levels are sufficient to harm plants, the soils are "saline."

Salinity is the result of excess groundwater moving downward and laterally through the soil dissolving and transporting soluble salts. Large areas in Canada's prairie regions contain soil materials relatively high in soluble salts. The redistribution of these soluble salts by groundwater movement causes some areas to become excessively saline. When drainage is good, the salts are washed down through the soil and out of the root zone. When drainage is poor, as in low, flat or depressional areas, or in areas where roads or other construction interferes with normal drainage, the water table rises. When the water table rises to within 1 m (3') of the surface, water and salts can rise to the surface by capillary action. The water evaporates and the salts accumulate at the surface.

Salinity also occurs in saline seeps on hill slopes, often part way up the slope on very long and high hills. Rainfall on the upslope part of the hill (recharge area) moves down through the soil, picking up salts on the way. The water not used by crops moves down until it reaches an impermeable layer, which impedes its progress. The water then flows laterally (seeps) until it reaches a position lower down the slope (discharge area) where the water table is closer to the surface. There, seep water causes the water table to rise allowing the salt laden water to move up to the soil surface by capillary action. The seep phenomenon can take place over relatively short distances within the same field or over distances of several miles. Salinity levels vary widely across a saline seep. Salinity also varies from spring to fall. Salinity usually appears on the soil surface just after spring thaw.

The dominant salts in prairie "saline seeps" are calcium (Ca), magnesium (Mg), and sodium (Na) cations and sulphate (SO₄) anions. If Na levels are high or not balanced with the Ca and Mg, soil tilth can also be affected. The positively charged Na cations attach to the negatively charged clay particles in the soil, causing the soil to be sticky when wet and hard and impermeable when dry.

Saline soils can be recognized by spotty growth of crops or by white crusts of salt, which accumulate on the soil surface usually in low-lying areas. Streaks of salt may be present in the soil even though white crusts may not appear on the surface. In many saline soils, it is not possible to see the salts and a laboratory soil analysis must be used to confirm their presence. Plants may develop a blue-green tinge. Salttolerant weeds such as Russian thistle, kochia, wild barley and goosefoot species are commonly found in areas of high salt concentration.

The process of salt buildup is reduced when plants intercept the upward flow of saline groundwater and reduce the amount that reaches the soil surface. When plants are not present, almost all of the water loss takes place by evaporation at the soil surface. Summerfallow is a major contributor to salinity as it encourages a build-up of a water table. Water not used by crops accumulates in the subsoil eventually coming to the surface laden with salts. These salts interfere with seed germination and crop establishment.

All irrigation water contains some salt. Over an extended period of irrigation where soil drainage is inadequate, salts accumulate in the soil and a salinity problem may develop. Water used in the major irrigation areas of Alberta and Saskatchewan is of high quality with little risk of salt buildup. Seepage or high water tables resulting from overirrigation cause most salinity under irrigation.

Effects of Salinity on Canola Growth and Yield

Plant species vary in their tolerance to salt affected soils. Some plants will tolerate high levels of salinity while others can tolerate little or no salinity. The relative growth of plants in the presence of salinity is called their salt tolerance. Salttolerant plants avoid toxicity by the sequestration of the Na+ and Cl- ions into either vacuoles or roots by a salt exclusion mechanism. Salt tolerances are usually given in terms of stage of plant growth over a range of electrical conductivity (EC) levels.

The degree of salinity or total soluble salt concentration in a soil is routinely measured by soil testing laboratories with a conductivity test. Electrical conductivity (EC) is the ability of a solution to transmit an electrical current. An electrical current is passed through a soil sample and measured. The units of conductivity are usually given in deciSiemens per metre (dS/m). Table 2 categorizes salinity into general ranges from non-saline to very severely saline. These values are used for plant selection for saline soils.

Table 2. Salinity Rating and Electrical Conductivity Value

Soil Depth (cm)	Non-Saline	Slightly Saline	Moderately Saline	Severely Saline	Very Severely Saline
0 to 60	<2 dS/m*	2 to 4 dS/m	4 to 8 dS/m	8 to 16 dS/m	>16 dS/m
60 to 120	<4 dS/m	4 to 8 dS/m	8 to 16 dS/m	16 to 24 dS/m	>24 dS/m

* dS/m = deciSiemens per metre

Excess soluble salts cause osmotic stress and ion toxicity in plant cells. Plants need both the water and the nutrients dissolved in it for proper growth. The sap in plant roots contains salt that attracts water into the plant via osmotic pressure. Dissolved salts in the soil increase the osmotic pressure of the soil solution. This decreases the rate at which water from the soil will enter the roots. If the soil solution becomes too concentrated, plants will slowly dehydrate, lose turgor and starve, even though the supply of water and nutrients in the soil may be quite high. If the salt content of the soil water becomes too saturated, water may actually be withdrawn from the roots. Osmotic stress also results from desiccation and, therefore, is a common component of both drought and salt stress. High concentration of certain salts in the soil may also be toxic because some plants may absorb an excess amount of the salt, reducing growth or causing death.

Canola is considered moderately tolerant to salt and sodium, and can tolerate moderate salinity up to levels of 5 to 6 dS/m. In a salinity study by the Agriculture and Agri- Food Canada Swift Current, SK Research Centre increasing levels of soil salinity reduced germination, emergence and emergence rate (Figure 4). This greenhouse study showed that the per cent emergence and plant survival was not significantly affected until salinity exceeded 5.6 dS/m. Above this value there was an increase in number of days from seeding to initial emergence. With severe salinity, after emergence began, there was a rapid number of new plants but over time the percentage that survived dropped off.

Figure 4. Effects of Salinity (dS/m) on % Emergence, Days to Emerge and Plant Survival

Salinity under both moderate and severe conditions reduces average plant height, shoot and root biomass, number of leaves, leaf area, evapotranspiration, and crop yield at harvest. For example, the average harvest height in the above study averaged 167, 132 and 84 cm (66, 52 and 33"), respectively for the 0.6, 5.6 and 12.4 dS/m treatments. Root growth is also hindered at salinity above 6 dS/m. At 5.6 dS/m, canola yields in this study were reduced by 60% (Figure 5) and with severe salinity, canola yields were reduced by 80%. However, several other research studies have shown there was little effect on canola yields with salinity levels up to 10 dS/m.

Figure 5. Canola Yields (Expressed in % of Salt-Free Yields) under Moderate Root-Zone Salinity (5.6 dS/m)

Use soil tests to determine the extent of a salinity problem. Sample both affected and non-affected areas of the field. Perform analyses for electrical conductivity (EC), pH, cation base saturation, and calcium, magnesium, sodium and organic matter content. Soil tests can also identify possible future salinity problems.

Another type of soil problem occurs when sodium levels are high in relation to calcium and magnesium in the soil. These soils are very sticky and slippery when wet, and very hard, cloddy and prone to crusting when dry. The sodium adsorption ratio (SAR) is the ratio of sodium to the beneficial soil structural cations, calcium and magnesium. When the SAR value exceeds 13, the soil is "sodic." If the SAR exceeds 13 and the EC is greater than 4, it is considered a "salinesodic" soil. Use a soil test to determine the SAR of soils.

Sodium, even with no soil crusting, has been shown by an Argentina study to have a direct effect on canola emergence with a slight reduction in emergence at SAR values of 20 growing to a 60% reduction at SAR values above 34. At SAR values greater than 20, main stem growth and yield decreased slightly but an increase in the number of secondary branches compensated for this reduction. Reductions in number of stems, seeds per pod on the main stem, pods per stem and canola yields tend to decrease dramatically when SAR values exceed 34 (Figure 6).

Figure 6. Effects of Sodium Adsorption Ratio (SAR) on Yield of Canola

However, the most important effect of soil sodium is through increased crusting that reduces emergence. The amount of sodium required to affect emergence is much higher than that necessary for clay dispersion and crusting. The Argentina study showed SAR values of eight and even lower can encourage clay dispersion and crusting when raindrops impact the soil surface. Canola has a small seed and at germination the cotyledons are pushed through the soil to the surface, making emergence more difficult in crusted soils. Crusting is also affected by soil particle size distribution as a high proportion of fine particles enhances soil susceptibility to crusting. Very fine seedbeds resulting from excessive tillage increase the susceptibility of a soil to form a crust. Crust strength will greatly influence emergence (Figure 7).

Figure 7. Effect of Crust Strength (kPa) on % Seedling Emergence in B. napus

On soils with moderate salinity, use continuous cropping or at least lengthen the rotation to the maximum possible. Crops use the water rather than allowing it to move deeply into the soil or accumulate near the soil surface and contribute to salt movement.

Use shallow tillage and maintain all possible crop residues at the soil surface. Deep tillage may, in many cases, simply bring more salts to the surface and make problems worse. Seed canola shallow and early so that seeds may germinate when surface salt levels are temporarily lowered. Use of fertilizers, as recommended from soil test analysis, will produce better crops, which extract more moisture and assist in preventing further salinization. Subsurface drains can sometimes be used to drain water away and lower the water table, but this can be very expensive.

If soil salinity has already reached the stage where cereal and oilseed crops cannot be grown, consult qualified soil specialists on management procedures to combat and reclaim saline land.

Solonetzic Soils

There are about 6 million hectares (15 million acres) of solonetzic land in western Canada. This includes about 4 to 5 million ha (10 to 12 million ac) in Alberta; 1.5 million ha (4 million ac) in Saskatchewan, and 10,000 ha (25,000 ac) in Manitoba.

Solonetzic soils, often called burnout, blow-out or gumbo soils, are characterized by a tough, impermeable, high sodium, clay hardpan from 5 to 30 cm (2 to 12") or more below the surface. This hardpan layer severely restricts root and water penetration below the topsoil. Solonetzic soils are formed on material that is naturally high in sodium salts or from materials that have been enriched with sodium salts through the upward movement of ground water. Aside from variations in depth of topsoil, these soils vary in the relative degree of formation of the hard pan. Some of these soils have a tough hardpan and are high in exchangeable sodium. Others have a hardpan that has been leached of sodium, is not as tough and can be penetrated by some roots and moisture.

Solonetzic soils usually occur in association with normal soils, that is, a field of good soil may contain patches of solonetzic soil. These patches may vary from 1 m (3.25') to several hundred metres (1,000') in diameter and may represent as little as 10% or as much as 90% of the soil in a particular field. Within a given field, good growth is often contrasted by thin, stunted growth on the solonetzic patches. Crops tend to develop an uneven or wavy appearance.

Moisture is generally a limiting factor to crop growth on solonetzic soils. The hardpan restricts root development and moisture use to the topsoil above the hardpan. The deeper the depth of topsoil, the greater the moisture storage available. On some of the weaker solonetzic hardpans, roots and moisture are able to penetrate.

Solonetzic soils usually have a pH between 5.5 and 6.5 in the surface layer, however, pH values may be 5.0 or lower. Canola yields are reduced when the soil pH is 5.5 or lower.

Seedbed preparation is relatively difficult on solonetzic soils with shallow topsoil as they dry out rapidly with cultivation. If tilled when wet the result is the formation of large, hard clods that are difficult to break down into a good seedbed. This condition inhibits moisture contact with the seed and results in poor germination. In addition, the surface soil structure tends to break down easily under rainfall, and dries to form a hard crust that restricts seedling emergence. Although the exact operations required to prepare a good seedbed will vary from year to year, working them during cool weather and when they are moderately dry generally gives the best results. Spring tillage should be shallow and minimal.

Crop residues are essential for the maintenance of tilth on solonetzic soils. Work all straw and stubble as well as available supplies of manure into the soil to increase organic matter content and improve surface structure and tilth. Use forage crops in rotation. Use blade implements, cultivators and rod weeders rather than disc implements, except where the trash cover is exceptionally heavy.

Provide surface drainage whenever possible to prevent temporary accumulation of water (ponding) before and after seeding. This will reduce uneven germination and allow more timely cultivation of the entire field. Seed as soon as the soil temperature and moisture conditions are favourable for germination.

Solonetzic soils, like most other soils, will respond to applications of commercial fertilizer. However, response to fertilizer as with any soil may be limited by a lack of moisture. Therefore, the optimum rates of fertilizer applications tend to be lower compared to normal soils because of the lower yield potential on solonetzic soil. Crops grown on solonetzic soils often suffer from drought in the middle to late growing season. Therefore, it is important that crops get an early vigorous start.

Deep ploughing and deep subsoiling (ripping) have increased production on some solonetzic soils. However, not all solonetzic soils are suitable for deep ploughing or ripping. Consult a qualified soils specialist before a soil is deep ploughed or deep subsoiled.

Summary

Canola crop growth and yield may be limited by acidity, salinity and solonetzic soil problems. Farmers must understand these factors to determine whether or not they may be limiting on their farms. Where these problems exist, use information on how they develop and affect canola production to modify and reduce their limitations. Consult with the local agricultural representative and provincial soil specialists to devise methods to combat these problems.

References