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).

Background

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 was decomposed (mineralized), more acids were formed. In addition, all the commonly used N fertilizers are acidic—needing from 4 to 7 pounds of agricultural limestone to neutralize the acid formed from each pound of N applied to soils.

SOIL ACIDITY

Background

  • pH 7 is neutral.
  • Soil with pH levels above 7 are alkaline; those of less than 7 are acidic.
  • The lower the pH, the more acidic is the soil.
  • Soils in humid regions tend to be acidic; those in semiarid and arid regions tend to be around neutral or alkaline.
  • Acidification is a natural process.
  • Most commercial nitrogen fertilizers are acid forming, but many manures are not.
  • Crops have different pH needs—probably related to nutrient availability or susceptibility to aluminum toxicity at low pH.
  • Organic acids on humus and aluminum on the CEC account for most of the acid in soils.

Management

  • Use limestone to raise the soil pH (if magnesium is also low, use a high-magnesium—or dolomitic— lime).
  • Mix lime thoroughly into the plow layer.
  • Spread lime well in advance of sensitive crops if at all possible.
  • If the lime requirement is high—some labs say greater than 2 tons; others say greater than 4 tons— consider splitting the application over two years.
  • Reducing soil pH (making soil more acid) for acidloving crops is done best with elemental sulfur (S).

Plants have evolved under 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 to 6.8 or higher (see figure 20.1 for common soil pH levels). On the other hand, 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 to 6. Most other agricultural plants do best in the range of pH 6 to 7.5.

Several problems may cause poor growth of acidsensitive plants in low pH soils. The following are three common ones:

  • 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. 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, such as the following:

  • 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; and
  • 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.

Soil testing labs usually use the information you provide about your cropping intentions and integrate the three issues (see the discussion under “pH Management” of the three pieces of information needed) when recommending limestone application rates. Laws govern the quality of limestone sold in each state. Soil testing labs give recommendations based on the use of ground limestone that meets the minimum state standard.

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:

  • 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. If the soil pH is much lower than the pH needs of the crop, you need to use lime. But the pH value doesn’t tell you how much lime is needed.
  • 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.) A number of different tests used by soil testing laboratories estimate soil lime requirements. Most give the results in terms of tons per acre of agricultural grade limestone to reach the desired pH.
  • 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—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).
Figure 20.2 Examples of approximate lime needed to reach pH 6.8. Modified from Peech [1961]

Soils with more clay and more organic matter need more lime to change their pH (see figure 20.2). Although organic matter buffers the soil against pH decreases, it also buffers 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 to 6.5 for many of the clovers. As pointed out above, most of the commonly grown crops do well in the range of pH 6.0 to 7.5.

There are other liming materials in addition to limestone. One commonly used in some parts of the U.S. 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. 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 any new by-product liming sources are field tested and thoroughly evaluated for metals before you use them.

“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? When growing plants that require a low pH, you may want to add acidity to the soil. This is probably only economically possible for blueberries and is most easily done with elemental sulfur (S), which is converted into an acid by soil microorganisms over a few months. For the examples in figure 20.2, the amount of S 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.