Diagnosing and managing acid soils

Acidic soils can restrict plant growth. Learn how to recognize and manage these soils on your farm

Soils with a pH ranging between 6.0 and 8.0 are suitable for most crops on the Prairies. Soils with a pH range between 6.5 to 7.5 are considered to be near neutral. Soil pH between 6.0 to 5.6, 5.5 to 5.1 and < 5.0 are considered to be moderately acidic, strongly acidic and very strongly acidic, respectively.

When soil pH declines below 6.0, the physical, chemical, and biological properties of soils are gradually affected and yield of crops will decline. Crops can vary considerably in their tolerance to the various components of soil acidity. The damage caused by soil acidity to crops is often complex.

Effects of Soil Acidity

Soil acidity can have negative direct and indirect effects on crop growth and yield. Acid soils usually contain soluble forms of aluminum (Al) and manganese (Mn). As soils become more acidic, the soil pH decreases, and this increases the concentration of hydrogen (H+) ions in soil. As soils become more acidic, this causes aluminum and manganese to become more soluble in soil; they will gradually increase to levels toxic to plants.

Aluminum toxicity will restrict root growth and tie up phosphorus (P), reducing crop uptake of P. The indirect effect of restricted root growth is a reduced uptake of water and nutrients which further restricts plant growth.

Manganese toxicity will result in visual symptoms, including black necrotic spots or streaks on leaves of cereal crops. Manganese toxicity can cause chlorosis on leaf margins and cupping of leaves of canola and legume crops. Toxicity of aluminum and manganese can reduce yields of most crops when grown on strongly acid soils (pH < 5.5). Recent research has shown that higher concentrations of H+ ions can be directly toxic to plants.

The other major negative effect of soil acidity is on the survival and growth soil microorganisms. Of particular concern is the survival of rhizobium bacteria, which live in association with legume roots to fix nitrogen. The rhizobium bacteria that live in association with alfalfa, sweet clover and pulse crops such as pea are especially sensitive to acidity.

In acidic soils, microbial activity is reduced. This affects nutrient cycling, such as the mineralization of soil organic matter. This can reduce the mineralization and release of nitrogen, phosphorus, sulphur and other nutrients from organic matter.

Location of acidic soils

The majority of acid soils occur in the gray and dark gray soil zones of Alberta, Saskatchewan and Manitoba. These soils formed under boreal forest vegetation. The effect of climate and vegetation caused the formation of soils that tend to be slightly to strongly acidic. Acidic soils can also occur in localized areas in fields throughout the Prairies, and tend to occur in lower relief areas of fields where water tends to accumulate and the soils tend to be more leached, reducing the soil pH.

Farming practices are contributing to the decline of soil pH. Nitrogen (N) and sulphur (S) fertilizers acidify the soil and over many years of application cause the slow decline. For example, anhydrous ammonia (NH3), urea [CO(NH2)2] and other ammonium (NH4+) fertilizers react in the soil in a process called nitrification to form nitrate (NO3−), and in the process release H+ ions. So, as we continue to use significant amounts of N and S fertilizers, the pH of our agricultural soils will gradually decline and become more acidic.

Diagnosing soil acidity

Poor yields of more sensitive crops may indicate acidic soil problems. Soil sampling and analysis are the first steps to correctly diagnose and confirm a soil acidity problem. Visual crop symptoms alone are not enough to diagnose a problem.

Fields of concern must be carefully soil sampled. Uniquely different areas of a field should be sampled separately. Fields should be divided into areas based on soil type, topography and differences in crop growth. Each area must be sampled separately. Often, soil pH will vary with topography, so on land with more rolling topography, the lower, mid and upper slope areas should be sampled separately. Often different areas of a field may be more acidic and require higher rates of lime than other areas, and some areas may not require any lime at all.

Soil samples that are moderately or strongly acidic should then have a “lime requirement test” to determine the amount of lime required to raise the soil pH to 6.0 or 6.5. Lime rates depend on the amount of pH change that is needed and must take into account the soil buffering capacity. Buffering capacity is the amount of lime required to change pH a given amount. Sandy soils have low buffering capacity and require less lime to modify soil pH versus soils with higher clay content, which have a high buffering capacity. Once the rates of lime are determined, then the cost to purchase, transport and apply the lime can be estimated to assess the economics of liming.

How does lime 
change soil pH?

The most common product used to modify acid soils is lime, which is calcium carbonate (CaCO3). Other calcium based products such as calcium hydroxide [Ca(OH)2] and calcium oxide (CaO) can also be used as liming materials.

When calcium carbonate is added to an acidic soil it produces a gas (carbon dioxide) and leaves Ca+2 in the soil. The Ca+2 will exchange with exchangeable acidity on the soil exchange complex. The reaction continues with calcium carbonate until all the acidity is neutralized or all the calcium carbonate is used up. The reaction process occurs over many months to several years.

Other calcium-based products such as calcium chloride or calcium sulfate (gypsum) are neutral salts and cannot be used as liming materials and are ineffective in modifying acid soils.

Lime application

From the “lime requirement test,” the lab provides the rates required as “pure lime.” But sources of agricultural lime are not pure. They may only be 70 or 80 per cent calcium carbonate, which must be taken into consideration when determining the rate of product to apply. This is called the calcium carbonate equivalent (CCE).

The lime must also be very finely ground. The finer the liming material, the greater its surface area, resulting in faster reactivity with the soil. Fineness of the liming material must also be considered in calculating the actual application rate of the liming product.

Ideally, apply lime immediately after harvest to allow time for the lime to react for greatest benefit on soil pH before the next growing season. Lime should be spread very evenly over the soil surface and thoroughly incorporated into the soil. Water is required for the reaction process between the lime and soil. Lime will react more rapidly in a very moist soil versus a drier soil. It often takes a year or more before a response can be measured even under very good soil moisture conditions.

The reaction time will depend on the type of lime used, the fineness or coarseness of the lime material, and moisture conditions. Remember that liming materials differ widely in their neutralizing power due to variations in the percentage of calcium and magnesium content. Liming materials with a higher CCE will to neutralize soil acidity faster than those with a lower CCE.

Long term 
benefits of liming

The major benefit of liming is increased crop production. This also results in more root and plant fibre returned to the soil, which in turn will benefit soil organic matter levels in the long term. The toxicity issues of aluminum and manganese are minimized or eliminated. Production of legume crops such as alfalfa, sweet clover and pea can be greatly improved due to more favourable soil conditions for the nitrogen fixing rhizobium bacteria. Forage quality can also significantly improved.

The application of lime to acid soils will improve the biological, chemical, and physical properties of the soils. Liming will increase soil pH causing a more favourable environment for soil microbiological activity. This improves soil nutrient cycling and turnover of plant available nutrients from soil organic matter. Ultimately, lime can contribute to improved soil health.

Reduced soil acidity will increase the availability of plant nutrients, particularly phosphorus. In strongly acidic soils, phosphorus is retained in less available forms than on slightly acid and neutral soils. Therefore, a major benefit of liming acid soils is the increased utilization of residual phosphorus by crops.

The application of lime can also improve the physical properties of some soils. Notably, soil structure may be improved and soil crusting is less of an issue. This leads to improved emergence of small seeded crops such as canola.

The first steps?

If you think you have reduced crop yields due to acid soils, the place to start is to have problem fields soil sampled to determine soil pH. If a problem is identified, you may want to undertake more intensive field soil sampling, and have lime requirement soil tests completed. Then, undertake the process to find lime sources, to determine the cost of lime, the transportation costs and application costs, to decide if application is economically feasible. If the economics look questionable, consider lime application in some test strips first to assess potential benefits.

About the author

Columnist

Ross H. McKenzie, PhD, P. Ag., is a former agronomy research scientist. He conducted soil and crop research with Alberta Agriculture for 38 years. He has also been an adjunct professor at the University of Lethbridge since 1993, teaching four-year soil management and irrigation science courses.

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