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Home , Humus, Liming (soil), Organic matter, Soil carbon, Soil organic matter, Soil pH

Management a self-study course from MSU Extension Continuing Education Series 8 Module No. 8 CCA 1.5 NM pH and CEU Organic

by Ann McCauley, Soil Scientist; Clain Jones, Extension Specialist; and Jeff Jacobsen, College of Dean

Introduction This module is the eighth in a series of Extension materials designed to provide Extension agents, Certified Crop Advisers (CCAs), consultants, and producers with pertinent information on nutrient management issues. To make the learning ‘active,’ and to provide credits to Certified Crop Advisers, a quiz accompanies this module. In addition, realizing that there are many other good information sources, including previously developed Extension materials, books, web sites, and professionals in the field, we have provided a list of additional resources and contacts for those wanting more in-depth information about soil pH and . This module covers the following Rocky Mountain CCA Nutrient Management Competency Areas with the focus on soil pH and organic matter: soil reactions and soil amendments, and reports and management recommendations.

Objectives After reading this module, the reader should: 1. Know what soil pH is and how it is calculated 2. Understand how soil pH affects nutrient availability in the soil 3. Learn techniques for managing soil pH 4. Know the processes of cycling 5. Understand the role of soil organic matter in nutrient and organic management 4449-8 May 2009 Background nutrient storage and soil aggregation. SOM As noted throughout Nutrient levels have declined over the last century Management Modules 2-7, soil pH in some as a result of over- and organic matter strongly affect soil and the conversion of grasslands functions and nutrient availability. to tilled farmland. This reduction has Specifically, pH influences chemical lead to decreased soil fertility, increased solubility and availability of plant essential fertilization needs, and increased soil , pesticide performance, and in some areas. Furthermore, organic matter . Although SOM has been recognized for its role in soil pH is relatively similar in Montana the carbon (C) cycle as a sink for (CO ) and other and Wyoming (pH 7-8), it is known to 2 vary from 4.5 to 8.5, causing considerable emissions to the and is a key fertility and production problems at indicator of . these extremes. Therefore, to understand plant nutrient availability and optimal Soil pH growing conditions for specific crops, it Soil pH is a measure of the soil is important to understand solution’s acidity and . By and interacting factors that affect soil pH. definition, pH is the ‘negative + Soil organic matter (SOM) serves of the concentration [H ]’, + multiple functions in the soil, including i.e.,pH = -log [H ]. Soils are referred to as being acidic, neutral, or alkaline (or basic), Examples of Common depending on their pH values on a scale from approximately 0 to 14 (Figure 1). A Soils pH scale and alkalis pH of 7 is neutral (pure ), less than 7 is acidic, and greater than 7 is alkaline. 0 Battery Because pH is a logarithmic function, each 1 unit on the pH scale is ten times less acidic Stomach acid (more alkaline) than the unit below it. For Acid sulfate 2 Lemon juice example, a solution with a pH of 6 has a 10 soils Acidic times greater concentration of H+ than 3 Vinegar a solution with a pH of 7, and a 100 times soils 4 higher concentration than a pH 8 solution. Soil pH is influenced by both acid and 5 water -forming ions in the soil. Common acid-forming cations (positively charged Humid 6 dissolved ions) are hydrogen (H+), arable soils aluminum (Al3+), and iron (Fe2+ or Fe3+), 7 Pure water Neutral Calcareous whereas common base-forming cations soils 8 include (Ca2+), (Mg2+), Sea water potassium (K+) and sodium (Na+). Most Sodic soils 9 agricultural soils in Montana and Wyoming have basic conditions with average pH 10 Milk of magnesia values ranging from 7 to 8 (Jacobsen, 11 Alkaline unpub. data; Belden, unpub. data). This is primarily due to the presence of base 12 cations associated with carbonates and Bleach found naturally in soils 13 Sodium hydroxide and . Due to relatively 14 low precipitation amounts, there is little of base cations, resulting in a Figure 1. The pH scale (From Nutrient Manager, 1996). relatively high degree of base saturation 2 Module 8 • Soil pH and Organic Matter and pH values greater than 7. In contrast, 7.6 acid conditions occur in soil having parent 7.5 7.6 material high in elements such as silica 7.6 (rhyolite and granite), high levels of 6.6 7.8 8.4 with low buffering capacities (ability to 7.5 resist pH change), and in regions with 7.2 7.9 high amounts of precipitation. An increase 7.6 in precipitation causes increased leaching 8.1 7.7 7.1 7.3 of base cations and the soil pH is lowered. 7.6 7.5 In Montana and Wyoming, acidic soils 7.4 are most commonly found west of the Figure continental divide or in high elevation 2. areas (increased precipitation), in areas 7.9 Average 7.5 where soils were formed from acid- soil pH values for 7.5 forming parent material, forest soils, various Montana mining sites containing pyritic (iron and and Wyoming elemental ) , and a few 7.7 other isolated locations. Figure 2 shows counties. Values average soil pH values for select counties were calculated from a minimum of 100 77..3 7.5 in Montana and Wyoming. 8.0 soil samples sent to Nu t r i e n t Availability either the Montana State University Soil Exchange Capacity Testing Laboratory (currently Soil, Plant, Water Analytical Cation and anion exchange capacities Laboratory) in Bozeman or the University of Wyoming Soil are directly affected by soil pH as described Testing Laboratory in Laramie, respectively. (Jacobsen, unpub. in Nutrient Management Module 2. data; Belden, unpub. data). Briefly, exchange capacity is the soil’s lower (i.e., less basic, higher concentration ability to retain and supply nutrients to of H+), more H+ ions are available a crop. Because most soils throughout to “exchange” base cations, thereby Montana and Wyoming have a net negative removing them from exchange sites and charge, the soil’s cation exchange capacity releasing them to the soil solution (soil (CEC) is higher than the anion exchange water). As a result, exchanged nutrients capacity (AEC). Soils with high CECs are are either taken up by the plant or lost able to bind more cations such as Ca2+ through leaching or erosion. or K+ to the exchange sites (locations at which ions bind) of and organic Nutrient Availability matter surfaces. A high CEC soil As described above, plant nutrient will also have a greater buffering capacity, availability is greatly influenced by soil increasing the soil’s ability to resist pH. Figure 5 in Nutrient Management changes in pH. Soils with high amounts 2 shows optimal availability for many of clay and/or organic matter will typically nutrients at corresponding pH levels. have higher CEC and buffering capacities With the exception of P, which is most than more silty or sandy soils. available within a pH range of 6 to 7, Since H+ is a cation, it will compete macronutrients (N, K, Ca, Mg, and S) are with other cations for exchange sites. more available within a pH range of 6.5 When the soil pH is high (i.e., more basic, to 8, while the majority of low concentration of H+), more base (B, Cu, Fe, Mn, Ni, and Zn) are more cations will be on the particle exchange available within a pH range of 5 to 7. sites and thus be less susceptible to Outside of these optimal ranges, nutrients leaching. However, when the soil pH is are available to at lesser amounts.

Module 8 • Soil pH and Organic Matter 3 With the exception of (Mo), alfalfa (a legume) grows best in soils with availability decreases as pH levels greater than 6, conditions in soil pH values approach 8 due to cations which their associated -fixing being more strongly bound to the soil and grow well too. The optimal pH not as readily exchangeable. Metals (Cu, range shown for potatoes is also reflective Fe, Mn, Ni, and Zn) are very tightly bound of a relationship; the to the soil at high pH and are therefore bacteria causing common scab infection more available at low pH levels than high are more prevalent as pH rises (Walworth, pH levels. This can cause potential metal 1998). Therefore, the optimal pH range toxicities for crops in acid soils. Conversely, for growing potatoes is 5.0 to 5.5 because ‘base’ cations (Ca, K, Mg) are more weakly the risk of common scab infection is bound to the soil and are prone to leaching minimized at lower pH (due to lower at low pH. microbial activity). When soil pH is In addition to the effects of pH on extreme, either too acidic or alkaline, pH nutrient availability, individual plants and modifications may be needed to obtain soil also vary in their tolerance optimal growing conditions for specific to alkaline and/or acid soil conditions. crops. Neutral conditions appear to be best for crop growth. However, optimum pH Mana g i n g So i l pH conditions for individual crops will vary (Table 1). Soil microorganism activity Alkaline Soils is also greatest near neutral conditions, In modifying soil pH, the addition of but pH ranges vary for each type of amendments, , practices, microorganism. soil organic matter levels, and Specifically, should all be considered. A common very acid soils amendment used to acidify alkaline soils is Table 1. Optimal pH (less than 5) sulfur (S) (Slaton et al., 2001). Elemental cause microbial sulfur (S0) is oxidized by microbes to ranges for common crops 2- + activity and produce sulfate (SO4 ) and H , causing

in Montana and Wyoming numbers to be a lower pH. Ferrous sulfate (FeSO4) and (Havlin et al., 1999). considerably aluminum sulfate (Al2(SO4)3) can also lower than in be used to lower pH, not due to SO4, but Crop Soil pH more neutral because of the addition of acidic cations Alfalfa 6.2-7.5 soils. Moreover, (Fe2+, Al3+) (see Q & A #1). Application rates studies for these amendments will vary depending Barley 5.5-7.0 have shown upon product properties (particle size, Dry Bean 6.0-7.5 that certain oxidation rate) and soil conditions (original Corn 5.5-7.0 ‘specialized’ pH, buffering capacity, minerals present). micro- (CaCO ), common Oat 5.5-7.0 3 organisms, throughout many Montana and Wyoming Pea 6.0-7.0 such as soils, consistently buffers soil to pH values

Potato 5.0-5.5 nitrifying near 8. For soils high in CaCO3, larger beet 6.5-8.0 bacteria quantities of amendments will need to be (convert applied to lower pH, generally making pH modifications uneconomical. + to nitrate) Ammonium (NH4 )-based fertilizers and nitrogen-fixing bacteria associated and soil organic matter (SOM) acidify with many legumes, generally perform soil by producing H+ ions, thus lowering + poorly when soil pH falls below 6 (Haby, soil pH. NH4 -based fertilizers, such as 1993; Sylvia et al., 1998). For example, (46-0-0) and ammonium phosphates

4 Module 8 • Soil pH and Organic Matter (11-52-0 or 18-46-0), are oxidized by soil materials for neutralizing soil acidity microbes, producing H+ ions. Organic and is based on both purity and particle matter mineralization results in the size. Chemical purity is calculated as formation of organic and inorganic acids CCE and represents the that also provide H+ to the soil. However, sum of the calcium and the acidifying effects of fertilization magnesium carbonates # may be more than compensated for by in a material. Q&A 1 other practices. For instance, As CCE increases, a study by Jones et al. (2002) found that the acid neutralizing over a twenty-year period, fertilized and power in the (CaSO ) wasn’t cultivated soils in Montana experienced, increases. Particle size 4 on average, higher pH values (average is measured as the mesh mentioned as an difference of 0.3) than non-fertilized/non- size (number of screen cultivated soils. Possible explanations wires per inch) through amendment to lower for these results is that in the fertilized/ which ground lime will pH, yet it is often cultivated soils, practices such as crop fall; increasing mesh removal during harvest and tilling size corresponds with added to alkaline decrease SOM levels and subsequent acid smaller mesh openings. soils. Why? production. Additionally, tillage increases Fine sized lime (mesh The sulfur in CaSO4 (and surface and sub-surface soil mixing, size of 40 or greater) will FeSO4 and Al2(SO4)3) is already moving CaCO3 from the sub-surface closer react more effectively oxidized and will not react to the soil surface. and quickly in the soil, to form acidifying ions, so it While the addition of some organic whereas coarser sized does not lower soil pH. Rather, matter sources lower pH, not all sources lime will dissolve more gypsum is added to sodic soils are effective. Many sources within slowly and remain in the (high in Na+), which often have Montana and Wyoming are alkaline (“they soil for a longer period of pH levels greater than 8.5. are what they eat”) and may not effectively time. Many commercial Sodium (Na+) causes soils to acidify soils to the degree desired liming products are disperse, reducing - (sometimes the pH may even temporarily a mixture of particle holding capacity and aeration. increase). sizes to provide both The Ca2+ in gypsum will replace a rapid increase in pH Na+ from exchange sites, Acid Soils and maintenance of this + causing Na to be easily leached Although acidic soils are less common increase for a period of from the soil. in Montana and Wyoming than alkaline time (Rehm et al., 2002). soils, there are some areas in which soil acidity is problematic. For example, some Te s t i n g So i l pH soils near Great Falls, Mont. have pH levels Soil pH is measured to assess potential near 5.0. A common method for increasing nutrient deficiencies, crop suitability, soil pH is to lime soils with CaCO , CaO, 3 pH amendment needs, and to determine or Ca(OH) . The liming material reacts 2 proper testing methods for other soil with carbon dioxide and water in the nutrients, such as (P). soil to yield (HCO -), which 3 Soil sampling methods and laboratory + 3+ is able to take H and Al (acid-forming selection were described in Nutrient cations) out of solution, thereby raising Management Module 1. Soil pH is the soil pH. Companies supplying lime measured in soil slurries with soil to water amendments are required to state the ratios of 1:1 or 1:2, or in a saturated soil effective neutralizing value (ENV), calcium paste. Soil pH values are measured with carbonate equivalent (CCE), and particle a pH electrode placed into either the size on their label. ENV is a quality index slurry solution or paste. Though most used to express the effectiveness of liming soil testing laboratories utilize the soil to

Module 8 • Soil pH and Organic Matter 5 water or saturated paste methods, some Soil Organic Matter research proposes using KCl or CaCl2 solutions to mask the effects of naturally- r g an i c a t t e r y c l i n g occurring soluble salt concentrations on O M C Soil organic matter (SOM) is pH (Prasad and Power, 1997). By adding defined as the summation of plant a slight concentration of salts (KCl or and residues at various stages CaCl ), more exchangeable H+ ions are 2 of decomposition, cells and tissues of brought into solution, and the measured soil organisms, and well-decomposed pH is generally 0.5 to 1.0 units less than substances (Brady and Weil, 1999). Though water solutions. In addition, differing soil- living organisms aren’t considered within water methods produce slightly different this definition, their presence is critical pH values; a reading obtained from a 1:1 to the formation of SOM. Plant and soil:water ratio sample is generally 0.15 to (e.g., rodents, and 0.25 units higher than that of a saturated mites) all contribute to the movement and paste extract, but lower than a 1:2 dilution breakdown of organic material in the soil. (Gavlak et al., 1994). Therefore, it is Soil organic matter cycling consists important to be aware of the soil pH test of four main processes carried out by soil being used and to be consistent between (Figure 3): methods to ensure comparable data over 1) decomposition of organic residues; time. Soil testing laboratories usually denote the pH test method employed on 2) nutrient mineralization; the soil test report. 3) transfer of organic carbon and nutrients from one SOM pool to another; and 4) continual release of carbon dioxide

(CO2) through microbial respiration and chemical oxidation. The three main pools of SOM,

Plant determined by their time for complete Faunal and Micro- Biomass decomposition, are active (1-2 years), slow (15-100 years) and passive (500-5000 years) (Brady and Weil, 1999). Both active and slow SOM are biologically active, meaning they are CO2 CO 2 continually being decomposed by microorganisms, thereby releasing many organically-bound nutrients, such Decomposition CO as N, P, and other essential nutrients, Active SOM 2 CO back to the soil solution. Active SOM is 2 primarily composed of fresh plant and Slow SOM animal residues and will decompose fairly CO 2 rapidly. Active SOM that is not completely decomposed moves into slow or passive Passive SOM SOM pools. Slow SOM, consisting primarily of (cells and tissues of decomposed Figure 3. Organic matter cycle (modified from Brady material), is partially resistant to microbial and Weil, 1999). decomposition and will remain in the soil longer than active SOM. An intermediate SOM fraction falling within both active and slow pools is particulate organic

6 Module 8 • Soil pH and Organic Matter matter (POM), defined as fine particulate accumulation rates include SOM form, detritus (Brady and Weil, 1999). POM is and drainage, C:N ratios of more stable than other active SOM forms organic materials, climate, and cropping (i.e.,fresh plant residues), yet less than practices. passive SOM and serves as an important As previously long-term supply of nutrients (Wander et noted, varying SOM al., 1994). forms (i.e.,active or # In contrast to active and slow SOM, passive) accumulate Q&A 2 passive SOM, or , is not biologically and decompose at active and is the pool responsible for many different rates. For of the soil chemical and physical properties example, POM levels What is the associated with SOM and soil quality. can fluctuate relatively Representing approximately 35-50% of quickly with changes difference between total SOM, humus is a dark, complex in organic material mixture of organic substances modified practices, particularly from original organic , synthesized the adoption of no-till and soil organic by various soil organisms, and resistant to systems. Research has matter? further microbial decomposition (Prasad shown POM levels to Organic material is and Power, 1997). Because of this, humus increase in no-till systems plant or animal residue breaks down very slowly and may exist in compared to conventional that has not undergone soil for hundreds or even thousands of till systems (Albrecht et decomposition, as tissue years. Due to its chemical make-up and al., 1997; Wander et al., and structure are still intact reactivity, humus is a large contributor 1994), yet levels may and visually recognizable. to soils’ ability to retain nutrients on quickly decline following Soil organic matter is exchange sites. Humus also supplies the first tillage operation organic material that has organic chemicals to the soil solution or under certain climatic undergone decomposition that can serve as chelates and increase conditions (discussed and humification (process of metal availability to plants (Nutrient below). Humus content, transforming and converting Management Module 7 and discussed on the other hand, is organic residues to humus). below). Additionally, organic chemicals much more constant Soil organic matter is have been shown to inhibit precipitation and fluctuates very little. commonly defined as the of calcium phosphate minerals, possibly Since SOM tests do not amount of organic residue keeping P in soluble form for a differentiate between that will pass through a longer period of time (Grossl and Inskeep, SOM forms, changing 2-mm sieve (Brady and Weil, 1991). Physically, dissolved organic POM levels can cause 1999). chemicals act to ‘glue’ soil fluctuations to occur in together, enhancing aggregation and total SOM levels, even increasing overall soil aeration, water though humus content and retention, and resistance to remains the same. This can potentially erosion and crusting. The dark consistency give producers and farmers a false sense of of humus causes soils high in SOM to be long-term changes in SOM concentrations dark brown or black in color, increasing in the soil. the amount of solar radiation absorbed by Soils high in clay and are generally the soil and thus, soil temperature. higher in SOM content than sandy soils. This is attributed to restricted aeration SOM De c o m p o s i t i o n and Ac c u m u l at i o n in finer-textured soils, reducing the SOM content is equal to the net rate of organic matter oxidation, and difference between the amount of SOM the binding of humus to clay particles, accumulated and the amount decomposed. further protecting it from decomposition. Factors affecting SOM decomposition and Additionally, plant growth is usually

Module 8 • Soil pH and Organic Matter 7 conditions for soil microorganisms. High temperature and precipitation results in increased decomposition rates and a rapid release of nutrients to the soil (Figure 4). Some of the most rapid rates of SOM decomposition in the world occur in irrigated soils of hot regions (Brady and Weil, 1999). Conversely, decreases in temperature and precipitation cause Increasing SOM decomposition decomposition rates to slow. This results P r e c i p t a o n in greater SOM accumulation and a less rapid release of nutrients. Generally, SOM decomposes above 77oF (25oC) and Temperature accumulates below 77oF (Brady and Weil, 1999). Figure 4. Effects of temperature and precipitation Cropping practices, such as tillage and on SOM decomposition. fertilization, have had long-term effects on SOM levels over the last 75 years. greater in fine-textured soils, resulting in a Cultivated land generally contains lower larger return of residues to the soil. levels of SOM than do comparable lands Poorly drained soils typically under natural . soils of accumulate higher levels of SOM the Northern Great Plains originally had at than well-drained soils due to poor least 4% SOM, whereas present day SOM aeration causing a decline in soil content in most Montana and Wyoming concentrations. Many soil microorganisms agricultural generally ranges from involved in decomposition are aerobic 1 to 4% (Jacobsen, unpub. data). Unlike (oxygen requiring) and will not function natural conditions where the majority of well under oxygen-limiting conditions. plant material is returned to the soil, only This anaerobic (absence of oxygen) effect is plant material remaining after harvest evident in wetland areas in which the ‘soil’ makes it back to the soil in cultivated is often completely composed of organic areas. Furthermore, tillage aerates the material. soil and breaks up organic residues, The C:N ratios of various organic thus stimulating microbial activity and materials, such as manure, municipal increasing SOM decomposition. Residue sludge, biosolids, and straw, will affect burning lowers SOM levels by reducing microorganism activity and subsequent the amount of residue available for SOM decomposition rates (Nutrient formation. Fertilizer applications can Management Module 3). Organic materials result in an increase in SOM levels due to with relatively high C:N ratios (greater greater yields creating a larger return of than 30:1) generally experience slower crop residues to the soil (Albrecht et al., rates of decomposition than materials 1997). However, tillage practices typically with lower C:N ratios. To obtain a desired associated with fertilizer applications may balance between SOM decomposition and decrease this effect (Jones et al., 2002). accumulation, different organic materials can be mixed (see Table 4 in NM Module Ch e l a t i o n 3 for C:N ratios of various organics), As introduced in Nutrient Management or N fertilizer can be added to enhance Module 7, many organic substances can decomposition of high C:N materials such serve as chelates for micronutrient metals. as straw. Chelates (meaning ‘claw’) are soluble Climate impacts decomposition organic compounds that bind metals such and accumulation by affecting growth

8 Module 8 • Soil pH and Organic Matter as copper (Cu), iron (Fe), (Mn), and zinc (Zn), and increase their solubility Plant Formation of and availability to plants (Clemens et al., Organic Chelates 1990; Havlin et al., 1999). The dynamics Chelate Fe 2+ Humus of are illustrated in Figure 2+ 2+ 5. A primary role of chelates is to keep Fe Fe metal cations in solution so they can Fe 2+ diffuse through the soil to the root. This Soil SOIL SOLUTION is accomplished by the chelate forming 2+ a ‘ring’ around the metal cation that Fe protects the metal from reacting with other inorganic compounds (Brady and Weil, 1999). Upon reaching the plant root, the metal cation either ‘unhooks’ itself Figure 5. Cycling of chelated iron (Fe2+) in soils. from the chelate and diffuses into the root membrane or the entire metal-chelate complex is absorbed into the root, and then breaks apart, releasing the metal. # Both cases result in the metal being taken Q&A 3 up by the root and the chelate returning to the soil solution to bind other metals. Chelation may be particularly I have alkaline soils and low micro- important for regions in which alkaline nutrient availability. Will commercial soils predominate. As previously noted, metal availability is often inhibited under chelating agents benefit crop production? alkaline soil conditions, causing plant Commercial chelating agents can improve metal micronutrient deficiencies to occur. Iron, availability to crops. However, certain factors should for instance, becomes nearly insoluble as be considered before using them. The stability of soil pH nears 8 and chelation can greatly chelates (how well they complex metals) will depend increase availability (up to 100 fold) upon specific micronutrient forms, soil pH, and the (Havlin et al., 1999). Chelation can be presence of bicarbonates and other metals ions in the increased through the use of commercial soil and is related to the ‘stability constant.’ The stability chelating agents, synthetic organic constant is a value corresponding to how well chelate- compounds such as EDTA (see Q&A #3), or metal complexes will form with given metal and chelate by maintaining and increasing SOM levels concentrations. Stability constants for EDTA complexes (described below). typically range from about 14 (Mn2+) to 25 (Fe3+) with higher values corresponding to a greater tendency for Ca r b o n Se q u e s t r a t i o n and Co ns e r v a t i o n metals to stay chelated (Clemens et al., 1990). Stability Carbon (C) cycling is the transfer of constants should be on all chelate product labels. both organic and inorganic C between Other factors include product effectiveness and cost. the pools of the atmosphere (carbon For instance, a chelated metal complex may be X times dioxide and ), terrestrial and more expensive than a non-chelated metal, but not aquatic organisms (living plants, , X times more effective. Also, a chelating agent that is microorganisms), and the soil. Research effective with one metal at a given pH and in a particular within the last few decades indicates C soil may not be useful with different metals or at different concentrations in the atmosphere have pH or soil conditions (Clemens et al., 1990). So while increased with inputs linked to industrial chelating agents may improve micronutrient availability, emissions (i.e., extraction and chelating stability, soil properties, and economics need to of fossil fuels) and land use changes (e.g. be considered. cutting and burning large areas of forest)

Module 8 • Soil pH and Organic Matter 9 (USDA, 1998). This increase is causing strategies which sequester C include the C balance between pools to shift converting marginal lands to native and may also be affecting global climate systems (i.e., wildlife habitat), practicing change. In response to these concerns, the no-till or conservation-till farming, United States Department of Agriculture reducing the frequency of summer fallow (USDA) along with other national and in , and incorporating, rather international organizations (see Appendix than disposing of organic amendments for additional information) have begun such as manure (Lal et al., 1998; promoting management practices to USDA, 1998). Figure 6 demonstrates a conserve and sequester (store) C. The goal hypothetical decrease in SOM with time of C sequestration is to reduce atmospheric and the effects various management C concentrations by taking carbon dioxide practices will have on future SOM levels.

(CO2) out of the atmosphere and storing it in ‘sinks’ or storage compartments (USDA, SOM Te s t i n g 1998). Soil organic matter tests are useful An important sink within soil is SOM, in establishing SOM’s influence on soil in which C (organic) levels are over twice properties and determining fertilizer

as large as the atmosphere CO2 pool and or organic matter application needs. In 4.5 times larger than the C pool in land sampling for SOM, the top 6-inch soil plants (Delgado and Follett, 2002). Soil C sample should be collected and organic sequestration is accomplished through soil material on the surface (i.e.,duff or visible conservation practices that not only reduce plant parts) should be excluded, as it is , but also increase the SOM not part of SOM and can result in invalid content of soils. Possible conservation readings. Soil testing laboratories will return results as a SOM percentage for 6 the total soil sample. In interpreting SOM tests, it is important to understand what is being tested for and what testing 5 method was performed. Most SOM values are derived from organic C because the D 4 direct determination of SOM has high variability and questionable accuracy C (Nelson and Summers, 1982). Organic C O M ( % ) 3 represents approximately 50% of SOM, so B a conversion factor of 2 is often used to estimate SOM concentrations (e.g. SOM 2 A = 2 x organic C). Two common methods for testing SOM are Walkley-Black acid 1 digestion method and weight loss on 0� 20� 40� 60� 80� 100� 120 ignition method. It is important to note � � YEARS that both of these methods test for total SOM and do not distinguish between Figure 6. Hypothetical situation of SOM changes with time. different SOM forms. For example, two At 50 years, changes in soil and crop management system soils may have similar quantitative SOM can either decrease (A), continue (B), or increase (C, D) contents, yet SOM influenced soil fertility SOM. B represents no change in cropping system, while A and properties may differ considerably represents a change that would accelerate SOM loss (i.e., between the two soils due to differences in more intense tillage). C and D represent the adoption of SOM forms. one or more SOM conservation practices. Combinations of conservation practices may yield the highest SOM gains (D). (From Havlin et al., 1999)

10 Module 8 • Soil pH and Organic Matter Summary References Soil pH is a measure of a soil Albrecht, S.L., H.L.M. Baune, P.E. Ras- Nelson, D. W., and L. E. Sommers. solution’s acidity and alkalinity mussen, and C.L. Douglas, Jr. 1997. 1982. , organic carbon, Light fraction soil organic matter in and organic matter. In A.L. Page et that affects nutrient solubility and long-term agroecosystems. Colum- al. (eds.), Methods of Soil Analysis, availability in the soil. Factors bia Basin Agricultural Research An- Part 2, 2nd ed., 9:539-579. influencing soil pH include organic nual Report. Spec. Rpt. 977: 38-42. + Nutrient Manager. 1996. Focus on pH matter decomposition, NH4 Belden, R.P.K. Unpublished data. and lime. Nutrient Manager Vol 3, fertilizers, of minerals University of Wyoming Soil Testing No. 2. College Park, MD: Maryland and parent material, climate, and land Laboratory. Laramie, WY: Depart- Cooperative Extension, University management practices. Availability of ment of Renewable Resources, Col- of Maryland. lege of Agriculture. nutrients for plant uptake will vary Prasad R. and J.F. Power. 1997. Soil depending on soil pH. The availability Brady, N.C. and R.R. Weil. 1999. The Fertility Management for Sustain- of cation nutrients is often hindered by Nature and Properties of Soils, 12th able Agriculture. Boca Raton, FL: decreased solubility in highly alkaline Edition. Upper Saddle River, NJ: CRC Press LLC. 356p. Prentice-Hall, Inc. 881p. soils and increased susceptibility to Rehm, G., R. Munter, C. Rosen, and M. leaching or erosion losses in very Clemens, D.F., B.M. Whitehurst, and Schmitt. 1992. Liming materials for acidic soils. For anion nutrients, G.B. Whitehurst. 1990. Chelates in Minnesota soils. agriculture. Fertilizer Research 25: availability is generally the opposite. 127-131. University of Minnesota Extension Ser- Soil pH levels near 7 are optimal for vice. FS-05957-GO. [Available on- overall nutrient availability, crop Delgado, J.A. and R.F. Follett. 2002. line] http://www.extension.umn.edu Carbon and nutrient cycles. J. of Soil distribution/cropsystems DC5957. tolerance, and soil microorganism and Water Cons. 57: 455-463. html. Accessed Feb. 17, 2003. activity. Soil pH can be modified by using chemical amendments; however Gavlak, R.G., D.A. Horneck, and R.O. Slaton, N.A., R.J. Norman, and J.T. Miller. 1994. Plant, Soil and Water Gilmour. 2001. Oxidation rates of these treatments may only be effective Reference Methods for the Western commercial elemental sulfur prod- for a relatively short amount of time Region. WREP 125. ucts to an alkaline silt from and are generally not economically Arkansas. Soil Sci. Soc. Am. J. 65: Grossl, P.R. and W.P. Inskeep. 1991. 239-243. viable. Precipitation of dicalcium phosphate Soil organic matter (SOM) is dihydrate in the presence of organic Sylvia, D.M., J.J. Fuhrmann, P.G. Har- an essential component of soil, acids. Soil Sci. Soc. Am. J. 55: 670-675. tel, and D.A. Zuberer. (eds.). 1998. Principles and Applications of Soil contributing to soil biological, Havlin, J.L., J.D. Beaton, S.L. Tisdale, chemical, and physical properties. Microbiology. Upper Saddle River, W.L. Nelson. 1999. Soil Fertility and NJ: Prentice Hall. 550p. SOM exists in three pools in the soil, Fertilizers, 6th Edition. Upper Saddle with each pool affecting the amount River, N.J: Prentice-Hall, Inc. 499 p. USDA Global Change Fact Sheet. 1998. and rate of SOM decomposition and Sequestration: Fre- Haby, V.A. 1993. Soil pH and plant quently Asked Questions. [Available nutrient mineralization. In addition nutrient availability. FertiGram Vol. online] http://www.usda.gov/oce/ to nutrient storage, SOM aids X, No. 2. gcpo/sequeste.htm. Accessed Jan. nutrient availability by increasing Jacobsen, J. S. Unpublished data. A 13, 2003. (see Web Resources) the soil’s CEC, providing chelates, summary of soil pH and organic Walworth, J.L. 1998. Potatoes-field and increasing the solubility of matter levels for Montana counties. crop fertilizer recommendations for certain nutrients in the soil solution. Bozeman, MT: Department of Land Alaska. Crop Production and Soil Resources and Environmental Sci- Furthermore, the humus fraction Management Series. FGV-00246A. ences, College of Agriculture. [Available online] http://www.uaf. of SOM improves edu/coop-ext/. Accessed Feb. 17, by increasing soil water-holding Jones, C.A., J. Jacobsen, and S. Lorbeer. 2002. Metal concentrations in three 2003. capacity, infiltration, and aeration. Montana soils following 20 years of Wander, M.M., S.J. Traina, B.R. Stin- By incorporating SOM conservation fertilization and cropping. Commun. ner, and S.E. Peters. 1994. Organic into management plans, farmers and Soil Sci. Plant Anal. 33 (9&10): and conventional management producers sequester atmospheric C 1401-1414. effects on biologically active soil or- and benefit from an overall increase in Lal, R., J.M. Kimble, R.F. Follett, and ganic matter pools. Soil Sci. Soc. Am. soil quality and possibly lower input C.V. Cole. The potential of U.S. J. 58: 1130-1139. costs. cropland to sequester carbon and mitigate the greenhouse effect. Boca Raton, FL: CRC Press LLC. 128 p. Module 8 • Soil pH and Organic Matter 11 APPENDIX

Bo o k s Pe r s o nn e l The Nature and Properties of Soils, Engel, Rick. Associate Profes- 12th Edition. N. Brady and R. sor. Montana State University, Acknowledgments Weil, 1999. Prentice-Hall, Inc. Bozeman. (406) 994-5295. engel@ We would like to extend Upper Saddle River, NJ. 881p. Ap- montana.edu our utmost appreciation to the proximately $125. following volunteer reviewers Jackson, Grant. Professor. Western who provided their time and Soil Fertility and Fertilizers, 6th Triangle Agricultural Research Edition. J.L. Havlin et al. 1999. Center, Conrad. (406) 278-7707. insight in making this a better document: Upper Saddle River, N.J: Prentice [email protected] Hall. 499 p. Approximately $100. Jones, Clain. Extension Soil Fertility Grant Jackson, Western Triangle Agri- Soil Fertility Management and Sus- Specialist. Montana State Univer- cultural Research Center, Conrad, tainable Agriculture. R. Prasad and sity, Bozeman. (406) 994-6076. Montana J.F. Power. Boca Raton, FL: CRC [email protected] Red Lovec, former Extension Agent, Press LLC. 356p. Around $70. Westcott, Mal. Professor. Western Sidney, Montana Agricultural Research Center, Valeria Okensdahl, NRCS, Spokane, Ex t e ns i o n Ma t e r i a l s Corvalis. (406) 961-3025. westcott@ Washington Nutrient Management Modules montana.edu (1-15) can be ordered (add $1 for Mike Waring, Bayer Crop Science, shipping) from: Wichman, Dave. Superintendent/ Great Falls, Montana Research Agronomist. Central MSU Extension Publications Agricultural Research Center, Moc- P.O. Box 172040 casin, (406) 423-5421 dwichman@ Bozeman, MT 59717-2040 montana.edu

All are online in PDF format in the category of ag and natural resourc- We b Re s o u r c e s es, at http://www.msuextension. http://www.omafra.gov.on.ca/english/ http://www.msuextension.org/ org/publications.asp crops/pub811/2limeph.htm#soil publications.asp A “Soil Acidity and Liming” Fact- Montana State University See Web Resources below for on- sheet, including how to correct Extension Publications ordering line ordering information. soil acidity and alkalinity. Source: information for printed materials. Canadian Ministry of Agriculture and Food, Ontario, Canada http://www.nrcs.usda.gov/feature/ outlook/Carbon.pdf Informational page about soil and USDA programs involved in conserving soil organic carbon. Source: United States Department of Agriculture

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