20.4: Soil Acidity

Background

Figure 20.1. Soil pH and acid-base status. Note: Soils at pH 7.5–8 frequently contain fine particles of lime (calcium carbonate). Soils above pH 8.5–9 usually have excess sodium (sodic, also called alkali, soils).

A soil’s pH (or acidity status) is critical information because it influences nutrient chemistry and availability, and directly influences plant growth. Many soils, especially in humid regions, were acidic before they were ever farmed. Leaching of bases from soils and the acids produced during organic matter decomposition combined to make these soils naturally acidic. As soils were brought into production and organic matter decomposed (mineralized), more acids were formed. In addition, the most commonly used N fertilizers acidify soil as their ammonium is either converted to nitrate or is taken up by plants. Generally 4–7 pounds of lime are required to neutralize the acid formed from each pound of N applied to soils. Fertilizers that supply all their N in the form of nitrate, however, do not acidify the soil. In fact, applying calcium nitrate or potassium nitrate can slightly raise soil pH.

Plants have evolved in specific environments, which in turn influence their needs as agricultural crops. For example, alfalfa originated in a semiarid region where soil pH was high; alfalfa requires a pH in the range of 6.5–6.8 or higher (see Figure 20.1 for common soil pH levels). But blueberries, which evolved under acidic conditions, require a low pH to provide needed iron (iron is more soluble at low pH). Other crops, such as peanuts, watermelons and sweet potatoes, do best in moderately acid soils in the range of pH 5–6. Most other agricultural plants do best in the range of pH 6–7.5.

Several problems may cause poor growth of acid-sensitive plants in low pH soils. Three common problems:

• aluminum and manganese are more soluble and can be toxic to plants;
• calcium, magnesium, potassium, phosphorus or molybdenum (especially needed for nitrogen fixation by legumes) may be deficient; and
• decomposition of soil organic matter is slowed and causes decreased mineralization of nitrogen.

The problems caused by soil acidity are usually less severe, and the optimum pH is lower, if the soil is well supplied with organic matter. Organic matter helps to make aluminum less toxic, and, of course, humus increases the soil’s CEC. Also, soil pH will not change as rapidly in soils that are high in organic matter. Soil acidification is a natural process that is accelerated by acids produced in soil by most nitrogen fertilizers. Soil organic matter slows down acidification and buffers the soil’s pH because it holds the acid hydrogen tightly. Therefore, more acid is needed to decrease the pH by a given amount when a lot of organic matter is present. Of course, the reverse is also true: more lime is needed to raise the pH of high-organic-matter soils by a given amount (see “Soil Acidity” box).

Limestone application helps create a more hospitable soil for acid-sensitive plants in many ways:

• by neutralizing acids
• by adding calcium in large quantities (because limestone is calcium carbonate, CaCO₃)
• by adding magnesium in large quantities if dolomitic limestone is used (containing carbonates of both calcium and magnesium)
• by making molybdenum and phosphorus more available
• by helping to maintain added phosphorus in an available form
• by enhancing bacterial activity, including the rhizobia that fix nitrogen in legumes
• by making aluminum and manganese less soluble

Almost all the acid in acidic soils is held in reserve on the solids, with an extremely small amount active in the soil water. If all that we needed to neutralize was the acid in the soil water, a few handfuls of lime per acre would be enough to do the job, even in a very acid soil. However, tons of lime per acre are needed to raise the pH. The explanation for this is that almost all of the acid that must be neutralized in soils is “reserve acidity” associated with either organic matter or aluminum. And as the acid (H⁺) is removed from organic matter, new CEC sites are created, increasing the soil’s ability to hold cations such as calcium and potassium. (It also works in reverse as soils are acidified: H⁺ strongly attaches to what had been CEC sites, removing their ability to hold onto calcium, magnesium, potassium and ammonium.)

pH Management

Increasing the pH of acidic soils is usually accomplished by adding ground or crushed limestone. Three pieces of information are used to determine the amount of lime that’s needed.

Soil Sampling For pH

Traditionally soils have been sampled to 6 inches or deeper, depending on the depth of plowing. But for farmers using conservation tillage, especially no-till, the top few inches can become acidic while the zone below is largely unaffected. Over time, acidity will work its way deeper. But it is important to catch a significant pH decline early, when it’s easy to correct. Therefore, in no-till fields it’s best to follow pH changes in the top 2 or 3 inches.

Conversely, old soils in tropical regions often have high acidity in the lower soil regions, and a sample from deeper depths may be warranted.

1. What is the soil pH? Knowing this and the needs of the crops you are growing will tell you whether lime is needed and what target pH you are shooting for. You need to use lime if the soil pH is much lower than the pH needs of the crop. But the pH value doesn’t tell you how much lime is needed.
2. What is the lime requirement needed to change the pH to the desired level? (The lime requirement is the amount of lime needed to neutralize the hydrogen, as well as the reactive aluminum, associated with organic matter as well as clays.) Soil testing laboratories use a number of different tests to estimate soil lime requirements. Most give the results in terms of tons per acre of agricultural grade limestone to reach the desired pH.
3. Is the limestone you use very different from the one assumed in the soil test report? The fineness and the amount of carbonate present govern the effectiveness of limestone, or, how much it will raise the soil’s pH. If the lime you will be using has an effective calcium carbonate equivalent that’s very different from the one used as the base in the report, the amount applied may need to be adjusted upward (if the lime is very coarse or has a high level of impurities) or downward (if the lime is very fine, is high in magnesium, and contains few impurities).

Soils with more clay and more organic matter need more lime to change their pH (see Figure 20.2). Although organic matter and clays buffer the soil against pH decreases, they also buffer against pH increases when you are trying to raise the pH with limestone. Most states recommend a soil pH of around 6.8 only for the most sensitive crops, such as alfalfa, and of about 6.2–6.5 for many of the clovers. As pointed out above, most of the commonly grown crops do well in the range of pH 6–7.5.

Note

Soil testing labs usually use the information you provide about your cropping intentions and integrate the three issues when recommending limestone application rates. (See the discussion under “pH Management” on the three pieces of information needed.) Laws govern the quality of limestone sold in each state. The limestone recommendations given by soil testing labs meet the minimum state standard.

Figure 20.2. Examples of approximate lime needed to reach pH 6.8. Modified from Peech (1961).

There are other liming materials in addition to limestone. One commonly used in some parts of the United States is wood ash. Ash from a modern airtight wood-burning stove may have a fairly high calcium carbonate content (80% or higher). However, ash that is mainly black—indicating incompletely burned wood—may have as little as 40% effective calcium carbonate equivalent. On the other hand, the char may provide other benefits to soil (see biochar discussion in Chapter 2). Lime sludge from wastewater treatment plants and fly ash sources may be available in some locations. Normally, minor sources like these are not locally available in sufficient quantities to put much of a dent in the lime needs of a region. Because they might carry unwanted contaminants to the farm, be sure that you test any new byproduct source of lime through an accredited laboratory for trace elements as well as metals and other potential toxins.

Liming and soil structure. Soil aggregation may show some improvements when applying calcium carbonate to soils that are relatively high in magnesium. The higher ionic strength of calcium pulls clay particles together better than magnesium. Conversely, when using dolomitic limestone, magnesium is added and may have the reverse effect (although the magnesium may be beneficial to the crop if it is deficient).

But liming is causing concerns when combined with no-till in the acidic cerrado (savanna) soils in Brazil, a productive region that has become a major global exporter of soybeans, beef and poultry. In these deeply weathered and highly oxidized soils, the structural degradation can be especially pronounced because the formation of aggregates under the naturally low pH of these soils results from high concentrations of Al³⁺ (which has high ionic strength) and dispersed organic matter. The negative charges on organic matter bind to the positive charges of oxides, and Al ions form bridges between organic matter and the minerals. However, liming raises the soil pH, which results in negative charges on soil particles and repulsion between them. In addition, high concentrations of calcium salts remove Al³⁺ from negatively charged sites within the soil, which reduces plant toxicity but also results in further dispersion of the clays and loss of aggregation. Under no-till, the dispersed clay can move with water to lower layers and cause dense and hard soils.

“Overliming” injury. Sometimes problems are created when soils are limed, especially when a very acidic soil has been quickly raised to high pH levels. Decreased crop growth because of “overliming” injury is usually associated with a lowered availability of phosphorus, potassium or boron, although zinc, copper and manganese deficiencies can be produced by liming acidic sandy soils. If there is a long history of the use of triazine herbicides, such as atrazine, liming may release these chemicals and kill sensitive crops.

Need to lower the soil’s pH? You may want to add acidity to the soil when growing plants that require a low pH. This is probably only economically possible for blueberries and is most easily done with elemental sulfur, which is converted into sulfuric acid by soil microorganisms over a few months to years (depending on the fineness of the material applied). For the examples in Figure 20.2, the amount of sulfur needed to drop the pH by one unit would be approximately 3/4 ton per acre for silty clay loams, 1/2 ton per acre for loams and silt loams, 600 pounds per acre for sandy loams, and 300 pounds per acre for sands. Sulfur should be applied the year before planting blueberries. Alum (aluminum sulfate) may also be used to acidify soils. About six times more alum than elemental sulfur is needed to achieve the same pH change. If your soil is calcareous—usually with a pH over 7.5 and naturally containing calcium carbonate—don’t even try to decrease the pH. Acidifying material will have no lasting effect on the pH because it will be fully neutralized by the soil’s lime.