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close this book Nitrogen fixing trees for acid soils - A field manual
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Assessing soil acidity

Richard Fisher, E. M Hutton, Avilio A. Franco, Anthony Juo, Donald Kass, and Dale Evans

What Is an acid soil? Soil scientists use ranges of pH values to describe the acidity of soils. Soils in the pH range of 6.8 to 7.2 are considered neutral. Any soil with a pH of less than 6.8 is considered acidic, and any soil with a pH of more than 7.2 is considered alkaline. Soils with a pH of less than 35 or more than 10 rarely support plant growth Acid soils are described as "mildly acidic," "moderately acidic," and "strongly acidic" as pH values decrease. Mildly and moderately acid soils may not be detrimental to the growth of most plants.


Regions in the tropics in which soils are likely to be acid or low in bases such as calcium and magnesium.

Source: Caudle (1991).

The term "acid soil" is usually reserved for soils in which many types of plants have difficulty growing. This manual is concerned with these strongly acidic soils. They are characterized by a pH of less than 5.5 and one or more chemical problems that limit plant growth. Such problems may include (1) toxic levels of available aluminum, (2) toxic levels of available manganese, and (3) infertility due to insufficient levels of other elements important for plant growth, particularly calcium and phosphorus. Strongly acidic soil conditions limit the kinds of plants that can grow, the productivity of those plants, and the efficiency of fertilizers applied to increase plant productivity.

What is pH?

The acidity of a soil is assessed in terms of the acidity or alkalinity of the soil solution - the moisture in the soil - as measured in units of pH. The soil solution contains chemical elements in dissolved ionic form. Many of these function as essential plant nutrients, taken up from the soil solution by the roots of plants.

The acidity of a soil results from the relative presence or absence of acidic ions, such as hydrogen (H+), in the soil solution. Soil acidity increases with the increased presence of these ions and decreases with the increased presence of basic ions such as calcium (Ca++) and magnesium (Mg++).

The acidity of a soil solution is expressed on the pH scale as the negative logarithm of the hydrogen ion (H+) concentration. Because the pH scale is mathematically logarithmic, a pH change of one unit represents a ten-fold change in the acidity or alkalinity of the solution being measured. Thus a soil with pH 5 is ten times more acidic than a soil with pH 6. A soil with pH 4 is ten times more acidic than one with pH 5 and 100 times more acidic than a soil with pH 6.

How do soils become acidic?

Soils become acidic through the normal leaching action of rainfall over long periods of time. As rainwater moves down through the soil, it absorbs carbon dioxide from the soil atmosphere and forms weak carbonic acid. It also acquires weak organic acids as it encounters soil organic matter. This acidic solution attracts basic ions, such as calcium (Ca++), magnesium (Mg++), potassium (K+), and sodium (Na+), detaches them from the soil exchange complex, and leaches them from the rooting zone. As these basic ions are leached, they are replaced by acidic ions of hydrogen (H+) and aluminum (Al+++). Over long geologic periods, soils in warm climates with high rainfall become severely depleted of basic ions and strongly acidified. Many of these acid soils also have levels of available aluminum or other ions that limit plant growth.

At a pH of 5.5, a soil generally does not inhibit the growth of crops or trees because it contains little available (exchangeable) aluminum. As pH decreases to 5.1 or lower, the amount of available aluminum increases and begins to interfere with the uptake of calcium and phosphorus, adversely affecting plant growth.

The soil taxonomy classification of the United States Department of Agriculture labels most tropical soils with a pH of less than 5.5 as Oxisols or Ultisols. The Food and Agriculture Organization (FAO) calls these Ferralsols and Acrisols. The Oxisols, with aluminum saturations of 79 to 89 percent, are more harmful to leguminous trees and crops than are the Ultisols, with aluminum saturations of 49 to 64 percent. There are also some strongly acidic Entisols (called Arenosols by FAO), Inceptisols (classified as Cambisols, Plinthosols, and Gleysols by FAO), and Andisols (Andosols).

How does soil acidity affect the availability of nutrients?

Plant roots obtain nutrients from the soil solution, and that solution's chemical composition is affected by its pH. Nutrient availability is greatest in soils with a pH between 55 and 65. When the soil solution falls outside this range, plants often show signs of nutrient deficiencies.

In alkaline soils at a pH above 7.0, phosphorus, iron, zinc, boron, and copper become less available to plants. In acid soils at a pH below 5.0, phosphorus and molybdenum become less available and soil nitrification slows down. Some nutrients - such as calcium, magnesium, and potassium - may be lost, and high levels of available iron or aluminum may lead to the formation of insoluble phosphate compounds, dramatically reducing the level of phosphate available to plants.

The two most important indicators of acid soil conditions that are severe enough to limit plant growth are low pH and high levels of available aluminum. Indeed, aluminum toxicity and soil infertility are often associated. In soils with a pH of 5.1 or lower, aluminum levels often constitute more than 50 percent of the cation exchange complex. Manganese toxicity can also occur in a soil with a pH of less than about 5.5, but manganese toxicity is not as common as aluminum toxicity.

How do you measure soil acidity?

The best way to determine whether a soil is strongly acid is to consult a soil scientist. Failing this, a general soil survey map may be useful. Such a map may include specific information on soil pH and the degree of aluminum saturation. Otherwise, as a general "rule of thumb," soils classified as Oxisols or Ultisols are likely to be strongly acidic in tropical climates.

If you cannot consult a soil scientist or a reliable soil map, you may need to collect soil samples and have them analyzed. Take separate samples at depths of 0 to 20 cm, 20 to 50 cm, and 50 to 100 cm below the soil surface. The subsoil is normally the best indicator of acidity because the surface soil (at 0 to 20 cm) is often affected by recent management. Altogether, you will need about 10 separate samples at each depth for each distinct soil area in your site. Mix together the 10 samples for a specific area and depth and take a small subsample of the mixture.

You may be able to send your samples to a soil laboratory for analysis of pH and available aluminum levels. Alternatively, you can analyze the pH levels of your soil samples using a portable pH meter, colorimetric test kit, or test strips. Mix each subsample with an equal volume of pure water (rain water is preferable to tap water if deionized water is not available). After mixing, allow the soil particles to settle for a few minutes and then measure the pH of the solution above the soil particles.

Equipment for measuring pH is available by mail order or from retail outlets that specialize in agricultural or scientific supplies. Colorimetric pH test kits are fairly inexpensive but are less precise than electrochemical instruments. There is no simple field test for available aluminum, but if the pH is below 5.0, then available aluminum is likely to be high.

An alternative to analyzing the soil is to observe plant growth as an indicator of soil conditions. What kinds of plants are growing in the soil? The presence of plants that tolerate acid soils such as imperata grass, bracken ferns, and Stylosanthes species-is an indication of acid soil conditions. If crops are growing well, the soil is probably not highly acid. If, on the other hand, there are problems with beans, cotton, or maize crops, then soil acidity may be the culprit. Phaseolus beans (not cowpea types) are particularly sensitive to aluminum toxicity if they are growing well, aluminum may not be a problem.

How do you Interpret the results of a soil analysis?

An analysis of soil nutrients is often expressed in terms of milliequivalents per 100 g of soil (meq/100 g). An equivalent expression is cmol charge/kg. Values given as milliequivalents per 100 g of soil may be converted to parts per million (ppm) as follows:

1 meq/100 g of K+ (potassium) = 391 ppm

1 meq/100 g of Al+++ (aluminum) = 90 ppm

1 meq/100 g of Mg++ (magnesium) = 122 ppm

1 meq/100 g of Ca++ (calcium) = 200 ppm

1 meq/100 g of Na+ (sodium) = 230 ppm.

Phosphorus content is usually expressed as parts per million. Most field and vegetable crops will respond to additions of phosphorus and potassium fertilizers when soil phosphorus (sodium bicarbonate [NaHCO3]-extractable) is in the range of 8 to 15 ppm and exchangeable soil potassium is in the range of 60 to 100 ppm. Soil phosphorus above 25 ppm is considered adequate for maize. One important measure that can be obtained from soil test results is the percent of aluminum saturation. This value compares the amount of exchangeable aluminum in the soil with the sum of aluminum plus exchangeable bases, as in the formula:

Al / (Ca + Mg + K + Na + Al) x 100 = % Al saturation

In most cases, not all of these elements need to be analyzed. As a minimum for calculating percent aluminum saturation, the content of aluminum, calcium, and magnesium should be determined.

Plant species and varieties differ in the amount of aluminum saturation they can tolerate: above that limit, plant growth is reduced. Generally, cowpea-type beans, males, rice, and cassava have high tolerance to aluminum (70-100% saturation), whereas phaseolus-type beans, sorghum, soybeans, and wheat have low to moderate tolerance (0-70%), and cotton and maize have low tolerance (0-40%). Some nitrogen fixing tree species are known to tolerate high levels of aluminum in the soil, but the critical level for many species is not known. Controlled experiments are required to provide this information for a number of tree species and, in some cases, for particular varieties and provenances.

If plants show stunting, crinkled leaves, or leaves with small brown spots, manganese toxicity may be suspected. To determine manganese toxicity, apply a 5 percent hydrogen peroxide solution to a soil sample: if the solution fizzes (makes bubbles), manganese toxicity may be a problem.

How to take a soil sample

1. Use the right tools. A soil tube or soil auger are the best tools for taking soil samples. If these tools are not available a clean shovel may be used. Using a shovel, dig a hole to a depth of 20 cm, cut a slice of soil about 1 cm thick from the side of the hole, and trim the slice to a strip 2 to 3 cm wide. Continue digging to a depth of 50 and 100 cm and take samples at each depth.

2. Take a representative sample from each area of the field that is significantly different from the others in its soil, fertility, or vegetation. In each distinct soil area, take about 10 separate samples at each depth. Use a zig-zag pattern like the one in the illustration. Avoid crop rows,wet spots and other atypical areas.

3. Mix together the 10 samples for a specific area and depth in a plastic bag or plastic bucket, and extract about one-haH Iiter of the mixture for the sample. Place the sample in a container supplied by a soil testing laboratory, or in a plastic bag, and number and label it. Include information on the previous crop, the next crop, manure-use history, drainage status of the field, depth of tillage, and other special problems or conditions that may affect plant growth. Do the same for each area sampled.

4. When to sample: sample several months before planting. Wait four months to sample land cleared by burning to avoid the liming effect of ash. Sandy soils should be sampled every two to three years, clay and silt soils every four years.


How to take a soil sample

Source: Caudle (1991).