CHAPTER 8 AGRONOMIC IMPORTANCE OF HUMIC MATTER 8.1 IMPORTANCE IN SOILS It is well known that soil organic matter has a favorable effect on the physical, chemical, and biological characteristics of soils. With the increased knowledge in humic acid chemistry, this effect is now realized to be caused by the active components of the inorganic and humus fraction. 8.1.1 EFFECT ON SOIL PHYSICAL PROPERTIES The physical properties of soils are noted to change due to addition of soil organic matter yielding humic substances. These changes are usually interrelated, and the cl;lange in one physical property will often be followed by changes in other physical properties. Soils high in organic matter usually exhibit high water-holding capacities, display well-developed structures, have low bulk density values and are often fluffy or friable in consistencies. Although all the physical properties are no doubt very important for the well-being of the soil ecosystem, it is perhaps soil structure that 9 5 E B Z 4 5 8 - 8 2 0 s DO '% 6 MARCEL DEKKER, INC. 270 Madison Avenue, New York, New York 1001 6 TM Copyright n 2003 by Marcel Dekker, Inc. All Rights Reserved. Agronomic Importance of Humic Matter 255 is the most significant in soil formation and soil degradation as well as in plant growth and environmental quality. The formation of soil structure favorable for plant growth is assumed to be caused by the interaction between humic acid and clays and/or by complex reactions between humic acid and A1 and other metal ions. Since soil structure also infers a mutual arrangement of the three soil phases, solid, liquid, and gas, a change in soil structure will affect the balance between these three phases. The liquid and gas phases are especially vulnerable, since they are also subject to continuous exchanges with the environment. The effect of humic acid is to create and preserve a stable structure that can provide the proper amounts of pore spaces for the storage of optimum amounts of water and oxygen. The cementation effect of humic acid has long been considered a major factor in formation of soil structure (Baver, 1963), which is very important in especially sandy soils. These soils are usually very loose and friable and often single grained or structureless. The amounts of clay present are insignificant to cement the sand particles together. The dominant amounts of sand, which are chemically inert, are incapable of reacting with either cations or humic acids. Consequently, aggregation of sand particles can only be enhanced by cementation with humic acid. In contrast, another problem arises in soils rich in silt and clay. Here, crust formation occurs when the soils are low in organic matter contents. For farmers, the occurrence of surface crusts provides a very big problem. Seed germination, soil aeration, and infiltration of water will be inhibited. Enhancing aggregation by adding soil organic matter is often noted to prevent the formation of these surface crusts. The absence of soil organic matter in clayey soils causes in addition the development of structureless conditions, and massive structures are common which inhibit aeration, water penetration, and root growth. By creating granular structures due to the interaction of clay with humic acid the unfavorable physical conditions may be alleviated. Although its cementation effect plays an important role, the interaction of humic acid with metal ions and clays is believed to be of a more decisive factor in many soils for the creation of stable soil structures. One of the most striking examples in this respect is the unique physical condition in andosols. These soils are known for their MARCEL DEKKER, INC. 270 Madison Avenue, New York, New York 1001 6 TM Copyright n 2003 by Marcel Dekker, Inc. All Rights Reserved. 256 Chapter 8 black color due to high organic matter contents, low bulk densities and crumb to granular structures. The soils can become very wet due to their extremely high water-holding capacities. However, in such wet conditions, they still display low plasticity and stickiness and are friable in consistence (Tan, 1998; Shoji et al., 1993). The consensus is that the high organic matter content, allophane and A1 in andosols have played a major role in the development of the unique physical properties. Although several hypotheses can be presented, it is commonly assumed that the amorphous clay and humic acids are responsible for the exceptional physical properties. Exposed groups of A1 and Si on the surfaces of allophane and imogolite are capable of interacting with humic acids forming humo-Al-allophane or humo-Si- imogolite complexes or chelates. The structural A1 acts in essence as a connecting bridge, hence the interaction can be called Al-bridging. As discussed in Chapter 7, A1 ions in the soil solution can react in the same way acting as bridges between humic acids and negatively charged surfaces of the clay mineral (Figure 7.6). Not only will the organic substances be protected from rapid decomposition by such an interaction, but these chelates constitute the nucleus for formation of granular or crumb structures. In turn, crumb structures provide for an abundant amount ofmacro- and micropore spaces, which together with those present in the amorphous clays account for the high total porosity exhibited by andosols. The accumulation of organic matter (humic acids) known for its high adsorption capacity for water, together with the increased amounts of pore spaces, therefore increases the water-holding capacity of the andosols. A similar soil physical development can also be noticed in molli- sols. However, the difference is that mollisols are neutral soils contain- ing high amounts of Ca and crystalline clays, assisting in the accumu- lation of organic matter and the creation of excellent physical conditions. The clay fraction of these soils is characterized by smectite with kaolinite as an admixture. Smectite, the dominant clay mineral, does not have surfaces containing exposed A1 hydroxyl groups. The planar surfaces are composed of siloxane surfaces and are mostly negatively charged due to isomorphous substitution. Consequently, smectite can only be attracted to humic acid with Ca ions acting as bridges (Figure 7.6). On the other hand, the kaolinite minerals present MARCEL DEKKER, INC. 270 Madison Avenue, New York, New York 1001 6 TM Copyright n 2003 by Marcel Dekker, Inc. All Rights Reserved. Agronomic Importance of Humic Matter 257 contain on one side surfaces composed of A1 octahedrons. However, these surfaces are usually negatively charged due to dissociation of the exposed hydroxyl groups. Hence, the interaction of kaolinite with humic acid is made possible only through the Ca-bridging mechanism. The humo-Ca-clay mineral chelates are considered the reasons for the preservation of soil organic matter (humic acids) and for the development of granular structures and other favorable physical characteristics in mollisols. Soils, containing smectite in their clay fraction, are often experiencing a common problem, manifested in formation of large cracks when dry. This is known to be caused by the high swell and shrink capacity of the expanding 2:l type of clay minerals, such as smectite. The large cracks formed modify the soil's behavior with respect to aeration and water penetration, and may cause damage to plant roots. The effect of humic acid as discussed above by interacting with the expanding clay is noted in mollisols to reduce the extent of shrinking and swelling. 8.1.2 EFFECT ON SOIL CHEMICAL PROPERTIES Humic matter can affect the soil chemical properties in various ways, since it can generate a variety ofchemical reactions. As indicated before, the chemical behavior of humic matter is in general controlled by two functional groups: the carboxyl and phenolic-OH groups. The carboxyl groups start to dissociate their protons at pH 3.0 (Posner, 1964), and the humic molecule becomes negatively charged (Figure 7.1). At pH < 3 .O, the charge is very small, or even zero. At pH 9.0, the phenolic-OH groups also dissociate their protons, and the humic molecule attains a high negative charge. The issue of these pH values in the dissociation of functional groups of humic substances in natural soils has been discussed in Chapter 7. Since the development of the negative charge is pH dependent, this charge is called pH-dependent charge or variable charge (Tan, 1998). A number of reactions can take place because of the presence of these charges. At low pH values, the humic molecule is capable of attracting cations, and such electrostatic attraction leads to cation exchange reactions. This kind of reaction will no doubt affect the cation MARCEL DEKKER, INC. 270 Madison Avenue, New York, New York 1001 6 TM Copyright n 2003 by Marcel Dekker, Inc. All Rights Reserved. 258 Chapter 8 exchange capacity (CEC) in soils. The CEC of humic matter can be estimated from its total acidity values, which are usually very high. Humic acid shows CEC values, in terms of total acidity values, ranging from 500 to 1200 cmoYkg, whereas fulvic acid exhibits a somewhat higher range of 600 to1500 cmoVkg (Tan, 1998; 2000; Schnitzer and Khan, 1972). At high pH values, when the phenolic -OH groups are also dissociated, complex reactions and chelation become of importance (Figure 7.2). Complex reactions are considered to be a weaker bonding mechanism than chelation, due to formation of a coordinate bond with a single donor group. On the other hand, chelation is viewed to be a stronger bonding process because of formation of a chelate ring structure. Both adsorption and complex reactions can also take place by a water or metal bridging process. This was the vehicle for interac- tion reactions between humic matter and clay as discussed earlier. It is assumed that the interactions with metals are going to take place first at the sites that form the strongest bonds, e.g., coordinate bonding and chelating sites. As these stronger bonding sites become saturated, attraction to the weaker sites, e.g., electrostatic bonding and water bridging sites, becomes increasingly greater (Stevenson, 1994). However, under certain conditions the assumption above is very diffi- cult to justify. At low pH values, the only sites available are the sites for electrostatic attraction and the sites for water bridging. Neither the complexing nor the chelation sites are ready for reactions. The complexing and chelation capacity of humic matter is con- sidered today of utmost importance in many environmental quality issues. Depending on several factors, e.g., pH, saturation of sites, and electrolyte concentration, humic matter can form both soluble and insoluble complexes with metals, hence providing for a dual function in the soil ecosystem. In natural conditions most of the chelates may be in insoluble forms due in part to the participation of clays in the reaction process. The fulvic acid fraction is assumed to form the more soluble metal chelates because this humic substance is soluble in water to begin with, is lower in molecular weight and has higher contents of functional groups. The fulvo-metal chelates remaining soluble may serve then as carriers of trace metal elements to be transported to plant roots. On the other hand, humic acid tends to produce more insoluble metal chelates, and the humo-metal chelate is considered to MARCEL DEKKER, INC. 270 Madison Avenue, New York, New York 1001 6 TM Copyright n 2003 by Marcel Dekker, Inc. All Rights Reserved. Agronomic Importance of Humic Matter 259 serve as a sink for toxic metals. Large amounts of free A1 in acid soils are made chemically inactive by chelation with humic acid, preventing A1 toxicity in crops and plant growth (Tan and Binger, 1986). Hence, humic acid can act as a buffer in alleviating adverse effects of heavy metals and toxic substances such as pesticides and other xenobiotics. However, depending on many factors, such as the type of metallic ion, cationic valences, cation saturation, and degree of dissociation of the humic molecule, humic acid is also capable of forming soluble metal chelates. This is considered of extreme importance by many chemists and environmentalists in the mobilization and concentration of radionuclides in the environment (Gaffney et al., 1996). The chelates, carrying the toxic compounds, can migrate long distances and may pollute the ground water or reappear at other locations to be precipitated. Nonpolar hydrophobic compounds, e.g., DDT and PCB, can be made soluble in this way, hence preventing their accumulation in soils and sediments. However, the creation of insoluble and soluble chelates by humic acids seems to generate controversial problems for the environment. In the form of humo-chelates, these xenobiotics are indeed prevented from adsorption by soil clays, but the interaction with humic acids decreases their rate of decomposition, photolysis, volatilization, and biological uptake. The latter is expected to lengthen their lifetimes or to increase their mean residence time. This is expected to also affect their transport distances in our natural ecosystem. 8.1.3 EFFECT ON THE SOIL REDOX SYSTEM The reduction and oxidation reactions in soils, called redox reactions, are chemical processes involving electron transfer. They affect formation and accumulation of humic matter. As explained earlier, more humic matter will be formed in reduced than in oxidized environments as exemplified by formation of peats. However, it is also noted that hurnic matter is capable of inducing reduction and oxidation reactions, hence affecting the redox system in the environment. Humic substances are in fact important components of the soil redox systems, MARCEL DEKKER, INC. 270 Madison Avenue, New York, New York 1001 6 TM Copyright n 2003 by Marcel Dekker, Inc. All Rights Reserved. 260 Chapter 8 capable of transferring electrons (Flaig, 1988; 1972). They are considered by Ziechmann (1994) electron donor -acceptor complexes, with the aromatic structures, containing OH and COOH groups, functioning as electron donors, and quinonoid structures as electron acceptors. However, most of the data presented so far are based on the capacity of humic substances as electron donors with the transition metals at the higher oxidation states serving as the electron acceptors. A reaction of such an electron transfer is illustrated below in Figure 8.1, by which a divalent ion (M2+) is reduced into a monovalent ion by accepting an electron from the humic acid molecule. In reduction-oxidation chemistry, compounds, capable of donating electrons, are sometimes referred to as electron-rich substances embodied by substances in the reduced state, whereas their counterparts, compounds capable of accepting electrons, are called electron-poor substances, which are the materials in the oxidized state (Tan, 1998; Sposito, 1989). Electron-rich substances are usually char- Figure 8.1 A schematic representation of a redox reaction of humic acid showing electron transfer to a metal M2+. MARCEL DEKKER, INC. 270 Madison Avenue, New York, New York 1001 6 TM Copyright n 2003 by Marcel Dekker, Inc. All Rights Reserved. Agronomic Importance of Hurnic Matter 261 acterized by negative pe values, whereas the electron-poor compounds commonly exhibit positive pe values. This parameter pe is often used for measuring the capacity of substances to donate or accept electrons (Tan, 1998). It is derived from a generalized redox equation as follows: Oxidation + e- * Reduction (8.1) for which the electrochemical potential, measured against a standard hydrogen electrode, is defined as: RT Oxidation EH = EO + - In nF Reduction in which EO= standard electrochemical potential, R = gas constant, T= absolute temperature ("Kelvin), n=valence, and F=Faraday constant. Since the reaction is a reduction-oxidation reaction, the electro- chemical potential, E,, is called a redox potential. In the above equations, the term oxidation is referring to substances in the oxidized states and reduction to materials in the reduced state. The oxidized materials carry higher valencies than their reduced species, and the electrons have the responsibility for balancing the equation. In the case above, the oxidized substance is one valence higher than the reduced element, which carries one lower valence due to reaction with the electron. If electron (e-) activity increases, the reaction shifts to the right, meaning reduction takes place. When, on the other hand, electron activity decreases (no e- available), the reaction shifts to the left, or in other words oxidation occurs. In analogy to the concept of pH, electron activity can be represented by pe as illustrated below (Tan, 1998): (8.3) pH = - log (H') MARCEL DEKKER, INC. 270 Madison Avenue, New York, New York 1001 6 TM Copyright n 2003 by Marcel Dekker, Inc. All Rights Reserved. 262 Chapter 8 pe = - log (e-) (8.4) It should be realized that the analogy also includes the fact that neither electrons nor hydrogen ions can exist as free particles in natural conditions or in the soil solution. Both can exist only in association with the solvent or a solute species. The parameter pe is closely related to the redox potential EH according to the following equation (Tan, 1998): From the above, it follows that pe can also represent the redox potential, since conversion of EH into pe (or vice versa) can be accomplished very easily by using equation (8.5). However, it should be realized that pe was not defined as a redox potential, but is by definition the activity of electrons (see equation 8.4). The redox potential is the electrochemical potential of the reduction-oxidation reaction and has been defined as reflected in equation (8.2). Electron transfer in redox reactions is sometimes accompanied by a proton transfer as illustrated by the reaction below: MnO, + 2e- + 4H+ * Mn2+ + 2H,O (8.6) This leads some scientists to believe that a redox reaction can accordingly be generalized as follows (Bartlett, 1999): Oxidation + e- + H' * Reduction The equilibrium constant k is derived by Bartlett (1999) as follows: MARCEL DEKKER, INC. 270 Madison Avenue, New York, New York 1001 6 TM Copyright n 2003 by Marcel Dekker, Inc. All Rights Reserved. Agronomic Importance of Humic Matter log k = log red -log ox - log e- -log H+ log k = pe +pH (8.8) in which log k = the log of the equilibrium constant k. Considering log k equal to pe+pH by ignoring the most important reaction parameters, log red and log ox, is very confusing and misplaced. Even more mind- boggling is the contention of the author above to consider pe + pH as the redox parameter or redox potential. This is stretching the basics of electrochemical potentials a little bit too far. According to equation (8.8), pe+pH equals the logarithm of the equilibrium constant, but an equilibrium constant is not the electrochemical potential of a redox reaction. The electrochemical potential of a redox reaction is formulated and defined differently, as can be noticed in equation (8.2) The electrons are added in redox equations for the purpose of balancing the equations by reducing the charges of the oxidized substance and should not be applied for reaction with the H+ ion. The H+ ions, added in redox reactions involving oxides, must be accounted for and are often used for convenience to balance the equation by converting them into H20, as noted in reaction (8.6). Additional examples are presented below to illustrate more clearly the issue of incompatibility of equation (8.7) for application with ionic components: is incorrect (8.9) is correct (8.10) is correct (8.11) In soil chemistry, standard half-cell reactions involving electron transfer between ionic components are written in accordance with MARCEL DEKKER, INC. 270 Madison Avenue, New York, New York 1001 6 TM Copyright n 2003 by Marcel Dekker, Inc. All Rights Reserved. [...]... whereas the remainder will be subject to ammonification and nitrification processes in continuation of the nitrogen cycle Fixation o f AProchemicals This is another indirect effect of humic matter on soil organisms With the agricultural revolution, increasing amounts of inorganic and organic compounds are introduced in the soils as wastes Most of them will affect the flora and fauna in the soil ecosystem,... complexes Since most of the humo-metal chelates remain soluble, they will move with the flow of soil water, hence provide the carrier mechanism for transport to the soil- root interface The metal chelation properties of humic matter find application today in the fertilizers industry in Europe, which will be discussed more in detail in the section on industrial importance in Chapter 9 8. 2.2 Effect on Plant... Clark, 1 989 ), though the opinion in clay mineralogy is that most enzymes are too big to penetrate the intermicellar spaces of clay minerals According to the purpose of this book, in the following discussion the focus will be on enzyme -humic acid interactions only For more details on clay-enzyme interactions, reference is made to Tan (19 98) Complex formation and interaction between enzymes and humic matter. .. that humic matter can affect the solubility of insoluble P-compounds in soils Its chelation capacity is a major force for decomposing rock phosphates and other insoluble forms of phosphates in the soil, e.g., AlPO, and FePO, The strong affinity of TM Copyright n 2003 by Marcel Dekker, Inc All Rights Reserved MARCEL DEKKER, INC 270 Madison Avenue, New York, New York 10016 Chapter 8 284 A and Fe ions in. .. prolifically in soils rich in humus By way of enzymatic decomposition and mineralization, the humic substances are eventually broken down into H,O and CO,, which completes the cycle Nitrogen Cvcle The nitrogen eycle is another indirect effect of humic matter on the biological properties of soils Very simply defined, it is the movement of nitrogen from the atmosphere through the plants into the soil, before... However, the larger (higher) soil flora and fauna will react differently with humic matter or clay They are generally too large and well insulated to be affected by the charge characteristics of the humic and mineral fractions In analogy to the preservation of organic matter in soils as a result of complex formation with clays, the interaction of microbial cells with humic matter or clays also ensures the. .. cycle, and an inner cycle which occurs in the soils In the inner cycle, the NO ,- is not denitrified, but consumed by plants and soil organisms by a process called immobilization The cycle is completed in the soil and it is here that humic matter plays an active role in affecting the nitrogen cycle As a result of decomposition of plant residues a variety of nitrogenous compounds are released, e.g., amino... with the remainder being lost as CO, in the TM Copyright n 2003 by Marcel Dekker, Inc All Rights Reserved MARCEL DEKKER, INC 270 Madison Avenue, New York, New York 10016 Agronomic Importance of Humic Matter 273 decomposition process, on the other hand N is being added in the humic structure These processes cause the C/N ratio of the humic substances to decline to the narrowest ratio at which C and N... (2) synthesis of humic acid-like compounds using pesticide residues, which brings about the inactivation of the pesticides Direct linkages occur for example when basic pesticides, such as s-triazine, react with the carbonyl groups of humic acids (Stevenson, 1994), and pesticides containing carbonyl groups react with amino groups of humic acids Cross-coupling of xenobiotics with humic acids is another... processes are enzymatic in nature, their complex and variable structure and most often the extreme difficulties and inability to directly determine enzymes in soils cause many soil researchers to shy away from studying the issue Enzymes are proteinaceous compounds and can be determined indirectly through their capacity to transform one compound into another By definition, enzymes are thermolabile catalysts . (2000) and Stevenson (1 986 ). As illustrated in Figure 8. 3, the nitrogen cycle involves an outer cycle representing the overall cycle, and an inner cycle which occurs in the soils. In the inner. plant roots. The effect of humic acid as discussed above by interacting with the expanding clay is noted in mollisols to reduce the extent of shrinking and swelling. 8. 1.2 EFFECT ON SOIL CHEMICAL. Importance of Humic Matter 273 decomposition process, on the other hand N is being added in the humic structure. These processes cause the C/N ratio of the humic substances to decline to the narrowest