Unit 12 - Soil pH

Chapter 6 - Changing Soil pH

The ability to change soil pH has improved the productive capacity of many soils.

The pH of the soil is dependent on the quantity of hydrogen ions in the soil solution. If we want to raise soil pH, we need to increase the quantity of OH- ions in solution. However, when more H+ ions are removed from the solution, they are replaced by hydrogen ions that were held on the cation exchange sites.

This ability of the soil to withstand rapid changes in pH is important for plant growth. However, it means that the total amount of bases needed to raise the pH is dependent on the total amount of hydrogen ions held on the reserve. This is referred to as buffering capacity.

This diagram shows a convenient way to think of the capacity of the soil to resist changes in pH (the soil's buffering capacity). The active acidity (soil solution hydrogen ions) is in the small outside spout. The reserve acidity is in the big tank. When we remove some active acidity by adding a base such as CaCO₃, we remove some hydrogen ions from the active acidity. However, the original soil acidity of the soil solution is very rapidly restored. So, in order to change soil pH, we need to change the percent base saturation.

It is common practice in soil testing labs to make direct measurements of lime requirements by using a buffer solution. A buffer with a pH of 7.5 is mixed with a known quantity of soil. The exchange acidity is replaced from the exchange sites, and the depression in pH from 7.5 is a measure of the total acidity.

Liming Soils

Raising soil pH requires a quantity of agricultural liming material that is determined by the amount of acidity in the soil and the quality of the liming material. Soil acidity is measured by soil testing; the quality of agricultural liming material is determined by its purity and particle size distribution.

An agricultural liming material is defined as a material containing calcium (Ca) and/or magnesium (Mg) compounds capable of neutralizing soil acidity. These materials include: limestone (both calcitic and dolomitic), burned lime, slaked lime, marl, shells, and by-products such as sugar beet sludge, and sludge from water treatment plants.

Fluid lime is a term that is generally used to describe the concept of suspending liming materials of various types in either water or fertilizer solutions. Frequently, the liming material in fluid lime is finely ground agricultural limestone with a high neutralizing value. Advantages include rapid availability and application with existing fluid fertilizer equipment. Drawbacks are low rates of application and relatively high cost for the lime applied.

Lime Neutralizes Soil Acidity

When added to the soil, calcium and/or magnesium dissolved from the liming materials displaces hydrogen (H+) from the clay particles. Remember it is the hydrogen ion (H+) that makes soils acid. The displaced hydrogen then reacts with carbonate.

Carbonate dissolved from the limestone materials forms carbonic acid. Carbonic acid is not stable in soils and quickly forms carbon dioxide and water. With this chemical process, the hydrogen (H+) has been converted from an ion on a clay particle to a neutral molecule of water, thereby reducing soil acidity.

The chemical reaction for this process is

H+©Clay + CaCO₃ (Limestone) ——> Ca++©Clay + H₂CO₃ (Carbonic Acid)

Then: H₂CO₃ ——> H₂O (Water) + CO₂ (Carbon Dioxide)

Everything that contains calcium or magnesium is not necessarily a liming material. Gypsum, for example, is calcium sulfate (CaSO₄ • H₂O). When added to the soil, the calcium in the gypsum can displace the hydrogen on a clay particle. The hydrogen, however, would remain in the soil solution and the pH would not change because of the absence of carbonate.

Limestone Purity - Agricultural liming materials must, by law, meet minimum quality standards. The limestone recommendation on most soil test reports is based on the use of a liming material that is equivalent in neutralizing power to pure calcium carbonate -- that is, a material with a calcium carbonate equivalent (CCE) of 100%. Adjustments must be made to the recommended amount of any liming material if the material is not equivalent to pure calcium carbonate limestone in neutralizing power (higher or lower than 100% CCE) so that you actually apply the correct amount of liming material to neutralize the acidity in your soil. In Minnesota the chemical purity of the limestone material is expressed in terms of Total Neutralizing Power (TNP) which is the same as CCE.

Actual liming material required=(soil test limestone recommendation ÷ CCE of liming material) x 100

Example: Soil test recommendation for limestone: Apply 4,000 lb calcium carbonate equivalent
A Liming material label: Calcium carbonate equivalent (CCE)=80%
Actual liming material required: (4,000 ÷ 80) x 100=5,000 lb liming material

Minnesota, liming materials are analyzed and sold on the basis of Effective Neutralizing Power (ENP). The analysis label lists pounds of ENP per ton of liming material. Similarly, lime recommendations are now provided in terms of lb. of ENP per acre. The ENP is calculated from an analysis of carbonate content and a measure of particle sizes. In the past, recommendations were made in terms of tons per acre. Specific recommendations in terms of lb. ENP per acre for major crops in Minnesota are listed in Fact Sheet FS-5956, Lime Needs in Minnesota Lime Needs in Minnesota Fact Sheet.

Fineness of Limestone- A liming material must be finely ground to be effective. This is important because (1) limestone's solubility increases as it is ground finer, and (2) limestone affects only a very small volume of soil around each particle. Therefore, more finely ground limestone has more particles and, if there is adequate mixing, neutralizes more of the soil with which it comes into close contact.

Therefore, it would seem desirable to use only limestone that is smaller. However, this decision must be balanced against the high cost of grinding limestone to finer than 100 mesh. A compromise must be reached so that the material is fine enough to be effective agronomically but still economically sound.

The Fineness Index (FI) is determined in the laboratory by measuring the percentage of the liming material that passes through sieves of various sizes. Three sieve sizes (8 mesh, 20 mesh, 60 mesh) are used.

The FI is determined from the following equation: FI=(% passing 8 mesh but remaining on 20 mesh) x .2 + (% passing 20 mesh but remaining on 60 mesh) x .6 + (% passing 60 mesh) x 1.0

The larger particles dissolve more slowly in soils and provide for an increase in pH over a longer period of time. In general, Aglime is a mixture of particles of various sizes. This mixture provides for both a rapid increase in soil pH and maintenance of this increase for a period of time. When purity, moisture content, and fineness index have been determined, the % ENP is calculated from the following equation:

% ENP=% TNP x FI x % Dry Matter

When selecting a liming material, there generally is little advantage in exceeding the minimum standards for fine-sized material. In emergency situations where a very rapid change in soil pH is required, paying extra for a finer material may be warranted; however, planning ahead by using a less expensive material and giving it time to react normally generally will give better and more economical results.

The spreading ability of the aglime is one other factor to consider when choosing a liming material. The liming material selected should spread easily so that it's possible to achieve a uniform application over the entire field. Aglime spreads easily. On the other hand, liming materials that contain relatively large amount of water (sugar beet lime; water softening lime) are more difficult to spread uniformly over the field. This lack of uniform spreading could cause production problems for several years after application. If applied to supply equivalent amounts of ENP per acre, all liming materials should have an equal effect on crop yield. So, the decision on source to use should be based primarily on cost.

The determination of the ENP of various liming materials has economic implications for the grower. With the ENP determined it is possible to compare various liming materials on a cost basis. The following equation can be used to calculate this cost:
Cost/lb. ENP=Price/ton of Material ($) ÷ lb. of ENP per ton

Mg in Aglime -

Most of the Aglime quarried in Minnesota contains both calcium and magnesium. Both of these nutrients are essential for crop production. Calcium requirements of crops are low and Minnesota soils contain ample amounts of this nutrient. There are some who believe that, when lime is needed, only calcitic lime should be used. This belief originates from a concept which suggests that there is an ideal ratio of calcium to magnesium in soils and any deviation from this ratio will cause problems with crop production.

Several field trials have been conducted to test the validity of this concept. The results are clear—the ratio of calcium to magnesium in soils has not had any effect on crop yield in the northern Corn Belt. The calcium to magnesium ratio is not important in Minnesota soils. However, the supply of magnesium can affect production. Magnesium will be needed in a fertilizer program if the soil test for magnesium is low. The use of dolomitic lime is one of the easiest and most cost effective ways to add magnesium to soils.

As soil acidity increases the need to apply a liming material to any crop and the expected returns of applying it become greater. If soil acidity is limiting crop production, other production inputs such as fertilizer, seed, pesticides, machinery costs, and labor will not realize the kind of return that would be possible on a well-limed soil.

Wood Ashes as Liming Material

How safe is it to use wood ashes as a liming material? Wood ashes provide a good source of potassium for plant growth. It also is an alkaline material that will reduce the acidity of the soil. It is not as effective as limestone but with repeated use, it can drastically raise the pH value of a soil, especially if the soil is primarily sandy in texture. Often at the soil testing lab, when a soil sample has a pH value in the range of 8.0 to 11.0, it often came from the garden of a homeowner who was routinely adding wood ashes during the winter as they became available from the wood stove.

In general if the soil is already in the 5.8 to 6.5 pH range and no lime is recommended, it would be best to avoid putting wood ashes in that area. If the soil pH value is below 5.8 and there is a lime recommendation on the soil test report then put a dusting of the wood ashes over the soil surface and work it into the soil. Since wood ashes are low in calcium, compared to limestone, a supplemental source of calcium will be needed if the calcium level indicated on the soil test report is medium or low. Adding 1 or 2 pounds of gypsum (CaSO₄) per 100 square feet will the increase the low calcium levels.

Eggshells as Liming Material A study in Iowa looked at the use of eggshells as a source of lime. Aglime applications and eggshell applications increased the soil pH in a manner that corresponded to the rate of application. The increase in soil pH was more uniform with the aglime application than with the eggshell application. The variability of the eggshell product likely contributed to the differences in the increase of the soil pH compared with ag lime.

Amounts of Aglime to Apply

The need for lime is not uniform across Minnesota and recommendations will vary. Analyzing a soil sample for pH and buffer pH is the only way to arrive at an accurate lime recommendation. Soils should be sampled to a depth of 6 to 8 inches for this test. The recommendations will not be accurate if other sampling depths are used.

After the soil sample reaches the laboratory and is dried, a pH reading is taken. If the soil pH reading is less than 6.0, a buffer solution is added to the soil/water mixture and another pH reading is taken.

This second reading is referred to as the buffer pH and is used to determine the amount of lime to apply. The pH of the SMP buffer itself is 7.5. When it is added to an acid soil, the pH of the buffer will drop. The change in the pH of the buffer is directly related to the amount of lime needed. The buffer provides excess basic cations that drive the H+ ions off the exchange. In this way, the buffer's pH is lowered; the more it is lowered, the more hydrogen the soil had and the more lime that is needed.

The SMP buffer procedure is a quick way to determine the lime requirement. Without using the SMP buffer, knowing the pH and the cation exchange capacity is the only way to make accurate lime recommendations.

The buffer pH is not determined on soils with a soil pH of 6.0 or higher. The relative error of using the buffer is too high in this soil pH range. For these soils, standard recommendations are used to raise the soil pH to 6.5.

Lime recommendations for Minnesota are summarized in tables 1–4.Table 1, Table 2,&Table3&4 The area of the state must also be considered when these recommendations are used (see Lime Area Map) .

The same pH is not required for optimum growth of all crops. Crops grown in Minnesota are divided into 3 groups. These groups are as follows:

Group 1. alfalfa, alsike clover, apples, & asparagus
Group 2. birdsfoot trefoil, buckwheat, corn, edible beans, grapes, grass seed or hay, peas, red clover, barley, wheat, oats, raspberries, strawberries, sugar beets, sunflowers, sweet corn, & vegetable crops
Group 3. potatoes, grass sod, blueberries, & wild rice.

Lime should be applied to raise the soil pH to 6.5 for Group 1, 6.0 for Group 2, and the crops listed in Group 3 grow best in acid soils and no lime is needed.

Liming Materials and Application Procedures

Lime Application

Lime is generally applied to the surface of the soil by broadcasting. Lime is relatively insoluble and moves slowly, so without mixing the lime with the soil, much of the soil would still be acid. Therefore for maximum effectiveness, the aglime must be incorporated into the soil.

Aglime is generally spread on the soil with a truck or trailer using an end gate spreader.

Since minimum tillage mixes to only one-third of the normal tillage depth, liming material incorporation is not possible these situations. Therefore, the soil should be limed adequately before no-till crop production begins.

When a legume crop is used in a cropping rotation, lime should be applied 6 to 10 months before the new legume is seeded. This is to insure adequate time for the chemical reactions to occur and to change the soil pH.

In Minnesota, we often want to lower the soil pH around acid-loving ornamentals, thus making iron more available. The table below gives some recommended rates for sulfur applications. (Sulfur is slowly converted to sulfuric acid by soil microbes, thus acidifying the soil over a period of several months.) Remember that the pH and cation exchange capacity will determine the amount of material to apply.

Soil pH Chapters

Lab Units

© Terence H. Cooper & Regents of the University of Minnesota, 2007 The University of Minnesota is an equal opportunity educator and employer.