Many Kansas soils require periodic applications of ag lime, or other liming materials, for optimum crop production. Liming has several beneficial effects: 1) it reduces harmful or potentially toxic conditions which can develop in acid soils, especially aluminum toxicity; 2) it increases the availability of some nutrients; 3) it replaces the supply of calcium and magnesium essential for plant growth (these nutrients are depleted as soils become acid); 4) it ensures favorable conditions for the activity of certain herbicides such as atrazine; and 5) it provides a suitable environment for microbial activity.
Crops are impacted differently by soil acidity. In Kansas, wheat is one of the most acid-tolerant crops we grow, while alfalfa and sweet clover are two of the least acid tolerant. The measured impact of soil pH and acidity on corn, soybeans and wheat yield is summarized in the table below:
Table 1. Impact of soil acidity on corn, soybean and wheat yields
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|
Soil pH |
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|
|
4.5 |
5.0 |
5.5 |
6.0 |
6.5 |
|
Crop |
Percent yield |
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|
Corn |
-- |
89 |
94 |
97 |
100 |
|
Soybeans |
-- |
82 |
90 |
96 |
100 |
|
Wheat |
37 |
97 |
97 |
100 |
97 |
The impact of pH on yield is only part of the story though. Weed control, nutrient availability and the health and diversity of the soil microbial community are also important considerations. So with this as a basic introduction, let’s take an in-depth look at soil acidity: what it is, how we measure it, factors that cause soils to become acid, and how we can correct the problem.
What is an acid?
By definition, an acid is a proton or hydrogen donor. Soils are considered acidic when the water surrounding the soil particles contains a high concentration of H+ ions. In soils, the ions or molecules in the soil water, also called soil solution, are in equilibrium with the ions or molecules held on the soil surfaces. That means that when a large portion of the cation exchange capacity (CEC) is satisfied by acids such as hydrogen (H+) or aluminum (Al+3 reacts with water to form AlOH+2 which donates H+ making it an acid), the soil can donate acidity (hydrogen) to the water which surrounds the soil particles, and the soil is acting as an acid. But when the majority of the cations on the CEC are basic cations, the soil solution will have a low H+ concentration, and the soil will be weakly acidic or neutral.
The main cations, or positively charged ions, found in most Kansas soils are: Hydrogen (H+); Aluminum (Al+3); Calcium (Ca+2); Magnesium (Mg+2), Potassium (K+); Sodium (Na+); and Ammonium (NH4+). Hydrogen and aluminum are acidic cations and the others are basic cations.
How do we measure acidity? The pH scale
The concentration of acidity in a solution is commonly expressed using the pH scale, the negative log of the hydrogen ion concentration (activity) in solution. The pH scale runs from 1 (very acid) to 14 (very basic). The midpoint, pH 7 is neutral, with the concentration of acids in solution being equal to the concentration of bases. At pH 7, the acidity, or H+ concentration in solution would be 0.0000001M (molar), and the OH-, or base concentration would also be 0.0000001M so the solution would be neutral. Since the pH scale is logarithmic, a change of one pH unit is a 10 fold change in the H+ concentration, or acidity level, in the soil solution. So as pH drops from 7 to 6, the H+ concentration increases from 0.0000001M to 0.000001M. As the pH drops further to 5, the H+ concentration increases another 10x to 0.00001M, now 100 times as high as at pH 7.
Measuring the acidity of soil
The relative acidity of a soil is commonly measured by estimating the pH, or hydrogen ion activity/concentration in the soil solution. This is normally done by collecting a soil sample, mixing the soil with a standard volume of water and measuring the pH of the soil:water mixture with a pH meter. The pH value obtained reflects the chemical environment in the soil water where plant roots reside. It also tells us something about the relative concentration of acidic and basic cations present on the cation exchange capacity, or CEC of the soil, and the percent base saturation. More on that in a minute.
In most Midwestern states, soil pH is commonly measured by making a 1:1 mixture by volume of soil and water. This slurry is stirred, allowed to react and settle for 10 to 20 minutes. Then a combination pH and reference electrode is inserted into solution above the soil in the sample container and the pH is measured. This sounds simple, and it is. But the procedure needs to be followed precisely to get an accurate and reproducible value. A good technician using a well-calibrated pH meter can run the procedure multiple times on subsamples taken from the same soil sample and generally get similar results, within +/- 0.1 pH units.
Measuring pH is fairly simple and reliable. The results we get from a soils lab are fairly reproducible, though environmental factors like dry soil conditions, can cause changes in soil pH. So what does measuring soil pH tell us? Measuring water pH in soils will tell us if the soil is acidic, and whether it needs lime. But unfortunately, soil pH is only part of the story. Soil pH alone won’t tell us how much lime will be needed to accomplish a desired change in pH.
Soil acidity, pH, and base saturation
Rain water moving through the soil will leach ions in the soil solution -- such as soil acidity -- below the root zone. But this leaching won’t change the soil pH much, because the soil will quickly re-establish the equilibrium between the solid soil and the soil solution. This process is buffering, or a resistance to change, and maintains a relatively constant environment for plant roots and other organisms residing in soils. The greater the soil’s CEC, the more highly buffered the soil system is. Sandy soils generally have a relatively low CEC, so they are relatively weakly buffered and change pH fairly quickly. Silt loam or silty clay loam soils have higher CEC and are more highly buffered; thus, these soils will change pH more slowly. Soils in a given region will have a general relationship between the relative concentration of acid and basic cations on the CEC (% base saturation) and soil pH. An example of this relationship is shown in Figure 1.
Figure 1. The relationship between percent base saturation and soil pH in a typical Midwestern soil when calcium is the dominant basic cation.
This is the general relationship found when calcium is the dominant basic cation on the exchange sites, as is the case in most Midwestern soils. The actual shape of this curve for a given soil will change depending on the clay and organic matter content, the type of clays present, and the total cation exchange capacity. So the shape isn’t all that important. What is important to remember is that as the base saturation approaches 0, the pH in most soils will approach 4. It is very unusual to find soils with pH levels below 4. The most common places that a soil pH below 4 is observed are areas where acid mine drainage accumulates, or drainage across a coal seam surfaces. The free acids present, normally sulfuric acids, can be strong enough to lower the pH below 4 in those cases. Another place one can see pH below 4 is in areas where the soil contains sulfides. This is most common in certain marine sediments, but not found here in Kansas.
In some areas of Kansas, particularly where irrigation with marginal quality water containing sodium has been practiced, sodium has become a significant portion of the exchangeable cations. Only when sodium is present in significant quantities, >15%, will soil pH above 8.5 occur. If soil pH's higher than 8.5 are found in routine soil testing, an exchangeable sodium test should be requested. In a normal calcium dominant system, calcium carbonate forms limiting the upper pH to around 8.2
Why do soils become acid?
Soil acidification is a natural process, with a number of sources of acidity being added to most soils naturally. Some soils are acidic because of the composition of the parent material from which they were formed. Other soils become acid by a number of natural processes over long periods of time. Crop production with the use of nitrogen fertilizers has accelerated the soil acidification process, making liming a much more common and important part of soil management today. The net result is that hydrogen and aluminum (acidic cations) replace calcium, magnesium, and potassium (basic cations) on the soil cation exchange complex.
Natural sources of soil acidity
Rainfall. Soils will naturally become acidic in humid regions because rainfall is a natural source of acidity. Natural rain, without any pollution from man, has a pH of about 5.4. This is due to the CO2 content of the atmosphere dissolving in the rain to form a very dilute concentration of carbonic acid, H2CO3. As CO2 levels rise in the atmosphere, the pH of rain water decreases. The actual amount of acidity supplied through rainfall each year is very, very small -- normally less than 1 pound of acidity per acre per year. But over long periods of times, centuries in geologic time, this will cause soils in a humid climate, where rainfall exceeds the amount of water use by plants, to become acidic. Most of the soils in humid parts of the United States, east of the Mississippi are naturally slightly acidic. This includes much of the eastern half of Kansas.
Other sources of acidity. In addition to the acidifying effects of rainfall, organic acids, similar to vinegar, are produced in the soil when plant residues and organic matter decompose. These weak acids react and combine with nutrients such as calcium, magnesium, and potassium and move down through and below the root zone with rainfall. Hydrogen is released from these organic acids and replaces basic cations, causing the soil in the leached zone to become even more acidic. Examples of this process are found where limestone rock is found just a few feet below an acid surface soil. The long-term leaching of these acids has dissolved sufficient limestone and caused a collapse of the surface soil, creating sinkholes.
Impacts of cropping on soil acidity
Crop removal. Calcium, magnesium, and potassium are essential nutrients for plant growth. Their uptake by plants, and subsequent removal through harvest, can have an acidifying effect on soils. The amount of these nutrients removed by cropping depends on: a) crop growth, b) the part of crop harvested, and c) stage of growth at harvest. Removal is greater for hay crops than for grain crops, as shown in Table 2.
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Table 2. Calcium, magnesium, and potassium content of common crops |
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Crop |
Production |
Calcium |
Magnesium |
Potassium |
|
Hay crops |
tons/acre |
lbs/acre |
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|
Alfalfa |
6 |
180 |
28 |
240 |
|
Red clover |
3 |
75 |
22 |
100 |
|
Grass |
3 |
22 |
10 |
110 |
|
Grain crops |
bu/acre |
|
|
|
|
Corn – grain |
150 |
2 |
8 |
25 |
|
Soybeans – seed |
50 |
8 |
8 |
45 |
|
Wheat – grain |
50 |
1 |
4 |
13 |
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Source: Feeds and Feeding. Frank B. Morrison. 22nd Edition. |
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Fertilizers. Nitrogen fertilizers have a greater acidifying effect on soils than any other source. Most commonly used nitrogen fertilizers contain ammonium nitrogen (urea is an ammonium forming material). Soil bacteria convert ammonium (NH4+) to nitrate (NO3-) through a biochemical process called nitrification. Hydrogen (H+) is released in this process, and the freed hydrogen ions cause an increase in acidity.
2 NH4+ + 4 O2 -----> 2 NO3- + 2 H2O + 4H+
Nitrification
Table 3 shows the calculated amount of ag lime needed to offset the acidity potential of several common nitrogen fertilizers. It is evident that applying more nitrogen fertilizer than a crop can take up is not only wasteful and expensive from the nitrogen standpoint but also increases the cost of a liming program and can be a pollution hazard.
The second potential acidifying effect from nitrogen comes from nitrate that is not taken up by the growing crop. Nitrates are very soluble and if not taken up by plants, will move downward with soil water and be carried below the root zone. The negatively charged nitrate ions take other positively charged ions, most likely calcium and magnesium since they are normally present in the largest quantities, with them and their removal in this manner has the same acidifying effect on soils as removal by a crop.
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Table 3. Amount of ag lime (pounds ECC) required to neutralize acidity created by nitrogen fertilizer |
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Fertilizer |
N concentration |
Pounds of ECC needed as lime to neutralize the acidity from 1 lb of actual N |
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Ammonium nitrate |
34% N |
3.6 |
|
Anhydrous ammonia |
82% N |
3.6 |
|
Urea |
46% N |
3.6 |
|
UAN solutions |
28-32% N |
3.6 |
|
Ammonium sulfate |
21% N |
7.2 |
|
Monoammonium phosphate |
11-12% N |
7.2 |
|
Diammonium phosphate |
18% N |
5.4 |
Summary
Soil acidity is a natural process which humans have accelerated with modern cropping systems. It has important impacts on soils, and the plants and organisms found there. Part 2 of this 3-part series will discuss how we determine lime needs in different cropping systems.
Dave Mengel, Soil Fertility Specialist
dmengel@ksu.edu
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