3.2: Soil pH and Liming

pH

This term is pronounced simply like the two letters p and h. The lowercase “p” is actually a mathematical symbol for “inverse logarithm”. The capital H is the symbol for the molar concentration of Hydrogen ions in a liter of a liquid, like water.

There is an analogy made by Henry Foth that “the pH of a soil is like the temperature of an animal. Both tests are easily made and provide basic information useful in diagnosing what is likely to be the disease or the problem”. I go further and suggest that knowing a soil’s pH, C.E.C, and % base saturation are all tools used by the organic and sustainable agriculturalist to manage soils well, apply nutrients conscientiously, and increase the health of soils and the bottom line of the farm or garden. It is important to know about soil pH because it affects all the nutrients in the soil and how well the plant can use them or take them up.

If you remember high school chemistry, water is made up of two Hydrogen atoms and one Oxygen atom. Because of this, water is commonly known as H₂O. It is also depicted as H--O--H where the “--“dashes are weak (ionic) bonds that hold the whole water molecule together.

pH in Soil Solutions

In almost all solutions, some water molecules break apart and result in ions of hydrogen floating around footloose and free. These are atoms that have a slight electrical charge because one of the hydrogen ions has generously lent an electron (it has a little negative or minus sign charge) to the other hydrogen ion so that it could stay bonded with the Oxygen atom. Now the arrangement looks like this: OH- is now separate from an H+

The H+ is called a hydrogen ion. (Occasionally, it is referred to as a proton)

The OH- is called a hydroxyl ion. The name tells you exactly what it is; hydrogen and oxygen = hydro-oxy ion.

When the pH of a soil solution is 7, it means that equal numbers of OH- (hydroxyl) and H+ (hydrogen) ions are present. This is a neutral solution. Specifically, there is one hydrogen ion (H+) in every 10,000,000 (ten million) moles per liter of water (note the seven zeroes). There is also one hydroxyl ion (OH-) in every 10,000,000 moles per liter of water. This is considered pure or neutral water.

A pH of 6 means there are 10 times more hydrogen (H+) ions than a pH of 7 (one in every 1,000,000) (note the six zeros). There are also, then 10 times fewer OH- (hydroxyl) ions in this solution. This is called an “acidic” solution. Whenever there are more H+s than OH-s, then the solution is acidic.

A pH of 8 is just the opposite; it means there are 10x more OH- ions than a neutral state and 10x fewer H+ ions. There is only one H+ in every 100,000,000 moles/liter (note 8 zeros). This is considered a “base” solution. Whenever there are more OH-s than H+s in a solution, it is considered “basic”.

When there is a pH of 9 there would be one H+ ion in a solution with nine zeros.

Exercise

If the pH is 5, how many zeros follow the 1 in the number of moles/liter of water?

Answer

5. There is one H+ for every 100,000 moles/litter of water in a pH 5 solution.

Cations

What does it matter if there are a lot of H+ ions in a soil solution compared to OH- ions?

The range of soil pH is usually 4 to 10 (the entire scale goes from 0 to 14). In the United States, farm regions' soils are typically in the 5.5 to 8.5 range. A pH between 6.0 and 6.8 is considered ideal for most crops. At this slightly acidic pH, there are enough free nutrient cations in the solution on which the plant can feed.

What is a cation (pronounced as cat – eye – on)?

Cations are ions that carry a positive charge, just like the hydrogen ion (H+). The typical nutrient cations needed by the plant are calcium (Ca ++), magnesium (Mg ++), and potassium (K+).

In acidic soils, for those with lots of H+ ions, the hydrogen ion may replace the other positively charged nutrient ions on the soil particle surfaces. This means the positively charged nutrient ions (cations) are bumped off the soil particle and are floating around, free, in the soil solution, ready to be absorbed by the plant cells. This is explained below.

To help explain this better, look at chemistry in terms of relationships. In the illustration above, the soil particle is perfectly happy in its relationship with potassium (K). There are a few H+ ions floating around the water film surrounding the soil particle, but the soil does not notice and is not interested. Then, the soil becomes a bit more acidic, meaning there are significantly more H+ ions flirting with the soil. The soil just cannot keep its eyes off all those handsome H+ ions. Finally, the soil lets go of K+ and instead bonds with an H+. A sad, but true story. But there is a happy ending. Now K+ is free to fulfill its true purpose; K+ is taken up by a plant root to become used as a nutrient. Just think, if the soil had not been slightly acidic, the K+ would have remained unavailable to the plant.

A soil with a pH of 6.9 is like soil with hardly any free H+ ions floating around. But a soil with a pH of 6.5 has significantly more H+ ions that attract the attention of the soil particle and prompt it to let go of its bond with the cation ion it is currently dating. Everyone is happy in the end, the plant gets the cation and the soil gets its hydrogen ion.

Soil Acidity Background

Figure \(\PageIndex{2}\): Soil pH and acid-base status.

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. The 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 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. 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 \(\PageIndex{2}\) for common soil pH levels). 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 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. 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 is 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 is easy to correct. Therefore, in no-till fields, it is 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. However, the pH value does not 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 is 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. 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 \(\PageIndex{3}\): 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. 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 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 above, 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.