Soil Science Lab Experts

Intro Soils – Lab 4 Soil pH: Acidity and Liming

o Lecture and Text Materials: Soil Acidity (Chapter 9) with Review Questions also included from Soil Alkalinity (Chapter 10) and Soil Organic Matter (Chapter 12)

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Lab 4 – Soil pH: Soil Acidity and Liming pH Soil pH is considered a master soil variable due to its wide ranging effects on other soil variables. Whether a soil is neutral, acidic, or alkaline is a measure of the relative concentration of hydrogen (H+) and hydroxide (OH-) ions. pH is technically the negative algorithm of the hydrogen ion concentration: pH = - log10 [H+] (Equation 1). Thus, for example, when the concentration of H+ ions in a solution is 0.0004 M of 10-4 M the pH of the solution is 4. It is important to note these values are on a log scale, meaning that every unit on the pH scale is a ten-fold change; soil with a pH of 5 is ten-times more acidic than a soil with pH of 6. pH values below 7 are considered acidic, pH values above 7 are considered alkaline, with pH 7 being neutral. Soils have pH generally range from 4 to 9 (Figure 1, Text Figure 9.2).

Figure 1 (Text Figure 9.2). pH scale including common items and ranges for various types of soil. Many soil properties are tied to soil pH, but most importantly nutrients are generally either more or less plant available with more acidic or alkaline pH values. Many nutrients, especially aluminum and iron, are relatively unimportant to plant growth due to low nutrient requirements for productivity at neutral pH ranges, but can become toxic at acidic pH. As a general rule, most plant nutrients are most available at ranges of 6-7 where they remain soluble and in plant available form. Additionally, the soil microbial

community works most efficiently in the more neutral pH values (6-8) rather than the extremes; so for productivity purposes for nutrient cycling, residue decomposition, root nodulations, herbicide breakdown, and other microbial activities, it is important to keep soils in this neutral range. Soil Acidity Most soil activities either consume or produce H+ ions. Soil parent material as well as weathering conditions especially climate are a major determinants of soils potential to become acidic in addition to human influences like adding nitrogen based fertilizers. In highly weathered soils with lower CEC values, activities that produce H+ tend to outpace their counterparts and create soil acidity. Weathering leaches base cations, i.e. calcium, magnesium, potassium, and sodium, from the soil profile leaving behind aluminum (iron to an extent) and hydrogen on the exchange complex which lowers the soil pH, can create toxic plant levels of aluminum in soil solution, and makes other soil nutrients less available for plant uptake. Soil acidification is somewhat of a natural process from many soil activities, (1) as soils weather they lose base cations leaving aluminum an acid producer on the exchange, (2) carbonic acid is created when soil respiration produces carbon dioxide, (3) as nutrients including nitrogen, sulfur and iron, are reduced H+ ions are produced, (4) the deprotonation of pH dependent charges on soil organic matter produces acidity, (5) when plants take up cations for growth and production they tend to exude H+ ions to maintain ionic balance, as well as cause an overall loss of base cations from crop removal during production, and finally (6) soils gain acidity through the deposition of acidic products via precipitation. Alfisols and Ultisols generally are acidic in nature due to their highly weathered nature and thus lack of base cations; forest soils generally tend to also be acidic due to the nature of the organic matter from leaf and conifer deposition. In agronomic settings, soils utilized in production agriculture also tend to be acidic. The addition of nitrogen based fertilizers create large amounts of soil acidity through the nitrification process. Ammonium and ammonia based fertilizers added to soil are microbially transformed into nitrate through nitrification for plant availability and in the process create acidity (2 H+ ions are produced for every NH4+ ion added to the soil). The addition of nitrogen-based fertilizers are a necessity for crop production and exceed the amounts generally seen in routine nitrogen cycle and must be counteracted with soil amendments to maintain relatively neutral soil pH. Soils have the capacity to buffer or resist large changes in pH. Many activities in soil can either be consumers or producers of H+ or OH- ions depending on soil conditions. Most of these activities are reversible and are weak acids, so depending on amount of product or reactant more or less acidity can be created or consumed. These properties greatly enhance the soils ability to buffer itself against change. Further adding to the buffering capacity of soils are the various pools of acidity. There are three pools of acidity in soils, active, exchangeable, and residual. The active acidity is the smallest pool of acidity and is the hydrogen ion concentration out in soil solution; this pool of acidity is also the easiest to counteract with soil amendments. The exchangeable acidity readily exchangeable aluminum and hydrogen on the soil exchange complex and the residual acidity is the acid producing cations tightly bound to the soil colloids. As active acidity is counteracted, the exchangeable and residual pools release additional ions to keep the soil solution at equilibrium; this activity contributes to the buffering capacity of soils (Text Figure 9.9 and Lecture Material Slide 15). Aluminum and hydrogen are the acid producing cations while the base cations, which do not promote acidity, include calcium, magnesium, potassium, and sodium. All of these same cations contribute to CEC, the more base cations there are in the soil to counteract the acidifying cations, the stronger the buffering capacity the soils have. Hence why soil pH is also an indirect indicator of the amount of weathering that has occurred in a soil and the amount of CEC available. The acid saturation percentage (the percentage of the CEC held by acid producers, Al3+ and H+ ions) as well as the base saturation

(percentage of the CEC held by non-acid producing cations, Ca2+, Mg2+, K+, Na+) are also important values to know and understand when evaluating CEC values. The higher the base saturation and the lower the acid saturation the better for soil productivity. If acid saturation exceeds 15-20% of the total CEC, aluminum toxicity can occur and soil amendments are generally recommended to counteract that acidity. Soil pH will also have a great effect on pH dependent charges on soil colloids including clays and soil organic matter. Even with the capacity to buffer the system, highly weathered soils with lower CEC and agronomic soils over time tend to be acidic necessitating amelioration using soil amendments. Counteracting Soil Acidity – Lime Generally speaking, to improve soil acidity one needs to increase the pH of the soil from acid to more neutral pH by altering the ratio of H+ and OH- ions in the soil profile. On agricultural soils, this improvement tends to come in the form of soil amendment like limestone or lime for short. Liming as whole is less of a precise science than fertilizer additions as this amendment is working to overcome the soil buffering capacity and to change the chemical nature of the entire rooting zone for the plant. For these reasons, it generally takes large quantities of these materials to force a change in soil conditions, usually in the tons per acre quantities. Liming agents for these reasons need to be relatively inexpensive, readily available, as well as be safe and easy to handle. Several compounds fall under the generic term ‘ag lime’ and are listed in table 1. The main characteristic of a liming product is that is provides large quantities of base cations to counteract the acid producing cations on the exchange complex. Calcium carbonate (CaCO3) is the mainstay for ag lime products. The neutralizing capacity of all other liming products is routinely compared to calcium carbonate on a percentage basis which is the calcium carbonate equivalent (CCE). Dolomitic lime, (CaMg(CO3)2), is often used in areas that are deficient in magnesium as a source of the cation for plant nutrition. Wood ashes can also be used as a liming material and are often used in homeowner or small garden settings. Table 1 (Text Table 9.4) includes the chemical formula, calcium carbonate equivalent, as well as some comments on the product.

Table 1 (Text Table 9.4). Common liming materials and their compositions.

Again, these soil amendments are added to the soil to increase the pH by changing the rooting zone environment to make nutrients more available and limit other elemental toxicities for maximum plant and microbial production. First, lime readily counteracts the small pool of active acidity with the increase in base cations to produce carbon dioxide and water. Next, in the largest, most important change, base cations (Ca2+ and/or Mg2+) in mass flow action replace Al3+ and H+ on the exchange complex and send them into the soil solution. With water, Al3+ will ultimately precipitate as the insoluble gibbsite (Al(OH)3). Ultimately, the goal is to raise the pH of the soil system to the target pH recommended for a particular crop which generally range between 6 and 7 where most plant nutrients are most available. The calcium and/or magnesium from the liming materials added also serve as a base cation for plant nutrition during the growing season. Liming requirements and their calculations vary depending on soil test methods and state and testing facility guidelines. Ideal pH and thus liming needs are also specific to plants with some requiring more acidic or neutral pH to maximize yield. Testing facilities take two different measurements to gauge the need to lime soils. A soil pH with water and a buffer soil pH. Briefly, pH is determined using a pH electrode routinely called a pH meter. The meter is placed in a solution of soil and water (1:1 or 1:3 ratio) or soil and buffer. The meter has a standard reference electrode where the difference in activity of the H+ in the soil and the reference create an electrometric potential which is converted into the pH scale. The soil water pH (pHwater) is a measure of the active acidity in the soil solution. This measurement can act as a guide in determining whether lime is needed or not. The exchangeable and reserve acidity, the most important pool, is determined using a buffer (pHbuffer). The buffer pH helps determine how much capacity the soil has to resist change in pH, or buffer the soil system. The buffer is meant to resist change, so if the soil has the capacity to change the pH of that buffer by considerable margins, the soil will require more lime to produce a change in soil pH. The reasoning behind this is based on CEC and

ultimately soil texture. Generally, soils with greater amounts of clay have higher CEC and thus base saturation, and contain more cations in the system to buffer change and will require larger amounts of lime to change the soil pH. More coarse textured soils high in sand are just the opposite with lower amounts of clay, lower CEC and thus less base cations in the system to buffer pH and require less lime to produce a change in the soil pH. Depending on several factors including typical soil organic matter levels, typical parent materials, and CEC, different buffers have been designed specifically for use in soil testing facilities. Two common buffers used for liming estimated are SMP (Shoemaker, McLean, and Pratt) and Adams-Evans which is used in most soil testing facilities in TN and is the basis of the recommendations from the University of Tennessee soil testing facility. As mentioned previously, each state has varying recommendations for lime applications based on previous research as well as knowledge of the soil systems in that area. The University of Tennessee Agricultural Extension Service utilizes regression equations combining the water and buffer pH as well as target pH for the various crops in TN to create easy to use approximations in tabular tables to recommend lime additions (Table 2). For instance, for corn production (middle, b section) with a target pH of 6.5 (middle, b section), with a soil water pH of 6.0 (left side column) and buffer pH of 7.4 (top row) a farmer would need to add ~ 2 tons of lime with greater than 75% CCE.

Table 2. UT Ag Experiment Station Lime Recommendations (Essington, ‘Soil and Water Chemistry: An Integrated Approach’)

A popular private soil and tissue testing facility in our area, A&L Laboratories in Memphis, TN, utilizes the following regression equation to calculate lime recommendations for soil test results (personal communication, Ruiz, A&L, Memphis):

Lime = { 1250 + ((pH goal - 0.3) - pH) * 1820)) + ((6.95 - buffer pH) * 5260)

For example: Soil pH= 5.0 Buffer pH= 6.7 pH goal= 5.3

Lime = { 1250 + ((5.3- 0.3) – 5.0) * 1820)) + ((6.95 – 6.7) * 5260)

Lime = {1250 + 0 + 1315} = 2565 lbs. lime recommended/acre or ~ 1.3 tons/acre

Other Quality Factors for Lime Application Several other factors besides overall quantity of lime are included in the quantification of lime requirements and include calcium carbonate equivalent, depth of incorporation, and size of the lime product applied. These characteristics are ultimately utilized to calculate how much of a particular liming product will be required. Calcium carbonate is the standard for ag lime and other products ability to neutralize soil acidity are referenced to this standard using a percentage called calcium carbonate equivalent (CCE). Pure calcium carbonate or limestone is the standard and has a CCE of 100% while other products may have more or less neutralizing capabilities with CCE of above or below 100% (Table 1). It is important to check the CCE of all liming materials as they can have a range of values and thus effectiveness. A CCE of less than 100 generally also indicates impurities in the product which increases the total amount of amendment needed to meet recommendations. The speed at which limestone reacts in a soil to neutralize acidity is largely determined by particle size. Smaller particles have more surface area to contact soil acidity, thereby producing more rapid change in pH. Crushed limestone is screened through a series of sieves to determine its particle size range. Sieve size (mesh) indicates the number of wires per linear inch, thus a larger sieve number (more wires) yields smaller particle size in the lime product. The percentage of product in a sample of the liming product that fits mesh size is used to calculate efficiency ratings for the various liming products. The smaller the particle size, the higher the efficiency. Each state utilizes its own verbiage and classifications for liming materials, but in Tennessee particle size efficiency and relative neutralizing values (RNV) are utilized. For instance: Table 2 lists the particle size breakdown for a liming material; the table includes size range (various mesh ranges), the percentage of that size range for each category, the efficiency factor for each size range and finally the particle efficiency for each size rage (% x Efficiency Factor). The summation of those particle efficiencies is the total particle size efficiency of your liming product. The relative neutralizing value (RNV) is simply the particle size efficiency for the product multiplied by the CCE. So, for instance, if this liming product had a CCE of 90%, the RNV would be 88.4 (particle size efficiency) x 0.90 (CCE) = 80. Table 2. Example Particle Size Breakdown of potential Liming Material – Total Particle Size Efficiency and Relative Neutralizing Value

Size Range Percentage Size Range Efficiency Factor Particle Efficiency

Coarser than 10 Mesh 5 0.33 1.6

10 – 40 Mesh 20 0.73 14.6

40 – 60 Mesh 40 0.93 37.2

Finer than 60 Mesh 35 1.0 35.0

Total Particle Size Efficiency 88.4%

Relative Neutralizing Value (PSE x CCE) 80

The Tennessee Liming Materials Act requires liming materials sold in the state meet several requirement: (1) minimum calcium carbonate equivalent of 75, (2) ground so that at least 85 percent passes through a 10-mesh sieve and at least 50 percent passes through a 40-mesh sieve, and (3) liming materials sold must have a relative neutralizing value (RNV) of 65 or greater. All of these values can be

utilized to compare actual liming needs across difference liming materials based on their cost and cost to spread. Lime itself is relatively insoluble and thus requires water to move down into the soil profile to become active. This process can be faster with the finer, large surface area lime particles and slower with the larger particles. For this reason, some farmers utilize slow release products to lengthen the effective time the lime stays in the soil profile. The fall and early spring are good times to apply lime to the soil as the wetter winter months can help move that lime down into the soil profile where it can begin making a change to that soil exchange prior to planting. Lime is generally spread across the fields utilizing spreader trucks which in theory spread an even layer across the soil surface at the recommended application rate per acre. Generally speaking, lime has traditionally been added in a more liberal fashion than fertilizers due to its relative cost, ease of application and a more broad range and timeline for results. The use of precision agriculture techniques to more closely assess soil needs on a smaller scale has led to the utilization of variable rate lime. Lime actually is most effective if it can be incorporated into the soil profile, but modern conservation practices work to limit tillage and disturbance of the soil surface to build and maintain soil organic matter and soil structure. These same no-till practices tend to build up materials right at the soil surface actually intensifying soil acidity problems localized in the top few inches of no-till soils, but regular addition of lime in favorable conditions keeps this problem in check. Soil pH generally is most acidic at the soil surface and increases with soil depth as more base cations are still available deeper in the profile and surface applications of nitrogen tend to cycle in the upper soil layers. Most producers rely on ag professionals, ag retail dealers or certified crop advisors, to be very informed and knowledgeable about the ins and outs of all of the products they sell and recommend, but having a working knowledge of the recommendations and how they are produced is a valuable tool for producers and students alike. References abound for soil acidity, lime, liming recommendations, and general knowledge on the topic. A few listed below were helpful in preparing this laboratory exercise and may be useful as a review of the information: https://ag.tennessee.edu/spp/Pages/default.aspx http://utbfc.utk.edu/Content%20Folders/Forages/Fertilization/Publications/PB1096.pdf http://www.utextension.utk.edu/mtnpi/handouts/Fertility/Soil_pH_Explained.pdf https://extension.tennessee.edu/publications/Documents/PB1061.pdf http://publications.tamu.edu/SOIL_CONSERVATION_NUTRIENTS/PUB_soil_Managing%20Soil%20Acidity .pdf http://www.agry.purdue.edu/ext/forages/publications/ay267.htm http://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs142p2_051574.pdf

Intro Soils - Lab 4 –Assignment Questions Soil pH: Acidity and Liming Utilize Lab, Lecture and Text Materials: Soil Acidity (Ch 9) Review Questions also include: Soil Sodicity (Ch 10) and SOM (Ch 12)

1. Why is it important to maintain relatively neutral soil pH?

2. What are some of the natural sources of soil acidity? 3. How do nitrogen fertilizers produce soil acidity?

4. Farmer Brown’s CEC for his West Tennessee silty loam soil was 12 cmolc/kg soil. The acid saturation percentage (aluminum and hydrogen) was 30% of the total CEC. As a soil professional why might that value concern you? What issues might arise due to this high acid saturation percentage?

5. Explain how CEC and soil texture in general effects the buffering capacity in soils. For instance, Farmer John’s silty clay has a CEC of 25 cmolc/kg with soil water pH of 6.5 and Adams-Evans Buffer value of 7.0 while his loamy sand has a CEC of 8 cmolc/kg with soil water pH of 5.5 and Adams-Evans buffer value of 7.9. Explain how their difference in texture, clay percentage and thus CEC help shape those values. What effects might this also have on the amount of lime that will be required to alter the pH of each of those soils?

6. Why is it important to test both soil water pH as well as soil buffer pH? What pools of acidity do

each of those test, which one is most easily counteracted, and which one is the most important long term in maintaining neutral soil pH?

7. Farmer Jim is liming his row crop acreage and ended up with some extra lime and would like to

potentially use it on his alfalfa field, but does not have time to send it off for official analysis; Jim’s daughter is close and happens works in a soils lab on campus and reports back that his soil water pH is 5.8 and his Adams-Evans buffer pH is 7.4. Based on UT recommendations, approximately how much lime did his daughter recommend he add to his pasture?

8. Describe two additional lime quality metrics besides just the amount of product required

utilized to ultimately determine how much of a liming product will be needed to counteract soil acidity.

9. What are some defining characteristics of saline soils? (Hint: moisture, pH, nutrient deficiencies,

CEC, clays, etc.)

10. Why does irrigation in arid regions contribute to salinity issues?

11. What is dispersion? What role does the ion on the exchange site (i.e. sodium vs

calcium/magnesium) play in the tendency to disperse?

12. Describe the three major components of soil organic matter.

13. The nutritional requirement for the microbial community is important in the degradation

process. Explain the concept of a carbon to nitrogen ratio (C:N). Why is it important? What C:N ratios might enhance degradation and what rations might slow degradation?

14. Describe some agronomic management tools to help build soil organic matter.