In the Beginning - Ion Exchange - Soil Mineralogy and Ion Fixation

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In the Beginning

The emergence of the discipline we call "Soil Chemistry" began with the early observations by experimenters concerning the ability of soils to modify solutions. In 1819, the Italian chemist, Gazzeri, observed that liquid manure, once passed over clay particles became discolored without losing its soluble substances. In similar work, Huxtable noticed in 1848 that soils alsoserved to deodorize liquid manure.

The study of soils as a chemical entity formerly began with J. Thomas Way. Way became familiar with the work of H.S. Thompson, who reported in 1845 that when he leached a soil column with ammonium sulfate, to his surprise, calcium sulfate ran out the other end. Experimenting with different soils, pipe clays, and some "home-made" alumino-silicates, Way demonstrated that soils could retain cations such as NH₄⁺, K⁺, and Na⁺, in exchange for equivalent amounts of Ca²⁺ ions. With time, Way managed to refine (and in some cases correct) his initial conclusions about soils behaving as cation exchangers, thus earning him the title, the "Father of Soil Chemistry.

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Ion Exchange

With Way's discoveries came an explosion of research involving ion exchange reactions in soils. F. Stohmann and W. Henneberg were the first to develop the adsorption isotherm, a tool that still remains popular among soil chemists today. In 1859, Samuel Johnson utilized this new tool to resolve the early discrepancy surrounding the supposed inability of soil organic matter to adsorb NH₄⁺ ions. Johnson found that not only was organic matter capable of adsorbing NH₄⁺, but in significant more quantities than clay. Furthermore, Johnson found that adsorption of ions (common plant nutrient ions) was reversible in soils, coining the familiar term "exchange of bases". In 1888, van Bemmelen discovered cation exchange was not restricted solely to Ca²⁺ ions, but could be involve Na⁺ ions as well. Thus, such experiments laid the groundwork for the simple concept we commonly call today, cation exchange capacity.

 With the advent of the 20th century, scientific understanding of the physical chemical mechanisms mediating ion exchange reactions grew in sophistication of theory and application. Kinetics of exchange reactions in soils were studied separately by K.K. Gedroiz and D.J. Hissink. Prominent among the progress made in describing ion exchange reactions were attempts to interpret exchange equilibrium quantitatively by empirical analysis. One of the very first approaches, employed by Weigner in 1931, modified the Freundlich adsorption isotherm to create an exchange isotherm. In 1932, Albert Vanselow developed his ion exchange equation based on H. Kerr’s earlier equation (in 1928) incorporating principles of mass action to describe exchange. Vanselow’s improvement, the mole fraction, was particularly useful in obtaining exchange constants for heterovalent (e.g., Na-Ca) exchange reactions. In contrast, the Russian scientist E.N. Gapon in 1933 developed a relatively simple exchange equation, which expressed the soil exchange phase in terms of milliequivalents per 100 grams of soil. Because of its simplicity, the Gapon equation remains today as the most popular exchange equation, yet this fact has not deterred investigators from trying to improve on it. Notable efforts were made by Krishnamoothy, Davis, and Overstreet in 1948 (employing entropic considerations by statistical physics), Gaines and Thomas in 1953 (who replaced the Vanselow mole fraction with the quantity equivalent fraction), and Schofield in 1953 (who revised the concept of the exchange constant to the unitless exchange coefficient, which implied soil selectivity for ions).

 As it became clear that ion exchange phenomena could not solely explain much of the variability in plant responses to soil conditions, researchers took other avenues of investigation, many times developing new subdisciplines of soil chemistry along the way, or at least adding significantly new insights to the existing knowledge. Specifically, we highlight four additional topics of proven importance to soil chemists: soil mineralogy and ion fixation, soil acidity and alkalinity, soil organic matter, and surface charge and flocculation/dispersion behavior in soils.

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Soil Mineralogy and Ion Fixation

Once it became clear that exchange reactions occurred on soil surfaces, experimenters began attempts to determine the bulk composition, structure, and nature of those surfaces. In 1926, Sante Mattson developed the technique electrodialysis for studying adsorption/desorption of ions from soils. Mattson concluded from his ion adsorption experiments on hydrous oxides and gelatinous silicates that soils exhibited analogous behavior, and must therefore be comprised primarily of these materials. In 1927, Hendricks and his associates improved on Mattson’s work by demonstrating that clay particles were inherently crystalline in nature, giving unique and distinguishing x-ray diffraction patterns. This discovery ushered in a new era in soil science research, allowing one to relate specific physico-chemical properties of soils to its composite clay minerals. Linus Pauling is credited with being the first to resolve the crystal structure of a clay mineral: mica. Pauling’s work was soon followed by Ross and Kerr in 1931 (kalonite), and Hofman, Endel, and Wilm in 1934 (vermiculite). Mineralogy and structure led to the discovery of 2:1 expanding and non-expanding clays and processes such as ion "fixation".

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Soil Acidity and Alkalinity

The discovery of soil acidity originated from struggles of Eastern United States farmers in the early 1800’s, who were searching for ways to improve their poorly crop-productive soils. One notable figure was Edmund Ruffin, who being a similarly unsuccessful farmer in Virginia, felt inspired by the 1813 book Elements of Agricultural Chemistry, by Sir Humphrey Davy. In the book, Ruffin learned how to determine the calcium carbonate status of his soils. Realizing from these determinations that the soils on his farm were lacking calcium carbonate (which was thought to be a determining factor of productive soils, such as in Great Britain), Ruffin decided to lime his soils with "marling" (partially decomposed oyster shells). The overwhelming success of his experiment forever established Ruffin as the first in recorded history to promote the values of liming. Ruffin’s early demise during the American Civil War left his discoveries in obscurity until their reemergence by Emil Truog (in 1938), and particularly F. Vietch in 1902. But the driving mechanisms behind soil acidity remained elusive for years. From numerous studies on this subject, there emerged two camps: one claiming that soil acidity was the result of soil aluminum; the other claiming that soil acidity resulted from exchangeable H+. For example, Daikuhara (in 1914) and others showed that Al- and Fe-compounds were readily released from acid soils by neutral salts. On the other hand, Dutch scientist Van der Spek was the first to show (in 1922) that clay particles contained exchangeable H+ ions. Some researchers, such as Riehm in 1932, Paver and Marshall in 1934, and Mackerjee in 1942 attributed sources of soil acidity to both exchangeable H+ and Al³⁺. Finally, N.T. Coleman and M.E. Harward resolved this debate, publishing a definitive work in 1953 which showed H-clays (prepared by H-resin) behaved entirely different from Al-clays. In fact, H-clays behaved analogous to Na- or Li-clays, testifying to the significance of exchange-phase H+. While both clays may be categorically acidic, clays "prepared" in nature were decidedly Al-clays. The realization of Al-clays led to an important discovery by Rich and Obenshain in 1955, that acid soils possessed nonexchangeable, hydroxy-Al polymers bound inside the interlayer of vermiculites. These Al polymers, resulting in what is now referred to as hydroxy-interlayered vermculite or HIV, not only impacted mineral ion selectivity but also served to inhibit the ion-fixing properties characteristic to vermiculite. In retrospect, such investigations represent the finest in soil chemistry research, linking the form and function of the soil mineral to ion selectivity.

 While soil acidity involved the attention of soil chemists in the Eastern U.S., researchers in the Western U.S.(especially in California) searched for answers for a seemingly distinct problem: deleterious soil conditions due to high alkalinity. Once again, this turned out to be a problem related to the exchange-phase ionic composition of the soil surface. Eugene Hilgard was the first to undertake the study of alkalinity in soils in 1906, but significant progress in this area only began with the simultaneous, yet separate work of Kelly, Hissink, Gedroiz, and de' Sigmond, who all applied a cation exchange approach to the problem. These investigations established the effect of a Na-dominated exchange phase on soil physical condition, and root uptake of ions.

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Soil Organic Matter

 After the 1920's, soil chemists began to make great strides in their work, in part due to contributions by giants like Linus Pauling. With Pauling, research in to structural chemistry in soils began to catch on. Progress in understanding organic chemistry and consequently soil organic matter chemistry began to accelerate. The work of Gillam, and Broadbent and Bradford in the 1940's started to shed light on the acidity and complexation potential of organic matter. In the 1960's major contributions on the chemical make up and function of soil organic matter were made by Stevenson, Schnitzer, and Skinner. Also, during and after the 1930s major contributions were made with respect to the structure and reactivity of water with mineral surfaces by a number of chemists including Linus Pauling, and soil chemists, e.g., Mortland.

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Soil Colloid Surface Charge and Dispersion

 While much was being done during the late 1800s and early 1900s on exchange reactions, acidity, mineralogy, sodicity, and organic matter chemistry, another group of scientists were making major breakthroughs in basic science relevant to soil chemistry, which lead to major contributions in understanding soil as a surface. For example, Reuss in 1807 discovered that by passing an electrical current through a porous diaphragm, the water moved through the capillaries to the cathode and the flow of water stopped immediately when the electric current was switched off. Reuss was puzzled with the phenomenon but had no explanation. In 1879, Helmholtz reproduced the phenomenon and provided the following explanation (which is essentially the same as accepted today by soil chemists, with some improvement): Water in a capillary is comprised of free-water and bound-water adjacent to the capillary walls, forming what is called the "Double Layer". Helmholtz postulated that the double layer consisted of two water layers. One layer located directly at the surface (which is very thin in comparison to the total double layer thickness) carrying negative charges, contains water that is rigidly attached to the capillary wall. The second, much thicker layer of water molecules (hence the term "double") carries positive charges that move toward the negative pole under an applied potential. In doing so the charges carry with them water molecules through the thicker part of the double layer. 

Another major contribution to soil chemistry was that of Walther Herman Nernst, a physical chemist. In 1889, Nernst (Nobel Laureate) was trying to understand the chemical reactions proceeding in a battery. Nernst showed that the electrical current produced by a chemical reaction could be used to calculate the free energy change in the chemical reaction involved in producing the current. This led to the development of the Nernst Equation, which allowed Nernst to relate the electrical potential of a surface to its chemical potential. In 1910, a Frenchman named Gouy and in 1913 an Englishman named Chapman, independently produced the Gouy-Chapman or diffuse double layer model, to describe ion accumulation at a surface. The Gouy-Chapman model treated a charged surface as a capacitor while allowing for random thermal motion (i.e., diffusion) to counteract electrostatic effects. The major weakness of this theory was the assumption that what we presently know as ions were considered to be infinitely small point changes. In addition, Helmoltz's description of the two water phases in the double layer was ignored. Specific knowledge about ions, e.g., shape, size, charge make up, etc., became available after 1919 when Niels Bohr (Nobel laureate) first proposed his description of the properties of atoms. In 1924, Otto Stern revised the diffuse double layer model to incorporate Helmoltz's biphasic double layer description. The Stern description allowed for ions to approach the surface within a certain minium distance forming a size-limited compact layer. Adding to the above, the description of the atomic bond in 1930 by Linus Pauling (Noble laureate) completed the picture of the so called triphasic properties of soil mineral surfaces: inner-sphere chemical complexes, outer-sphere physical complexes, and weak diffuse double layer physical complexes.

 In 1941, Derjaguin and Landau, and in 1948, Verwey and Overbeek, independently produced what is known today as DLVO theory. This theory was based only on the diffuse double layer (Gouy-Chapman) model, ignoring the improvements of the biphasic nature described by strict double layer. However, the DLVO theory contributed significantly to the soil chemist’s understanding of soil colloid dispersion, and how parameters such as cation type and valence, ionic strength, surface charge, and pH were quantitatively involved in predicting soil dispersion.

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The Present

 Looking back, one might describe today’s soil chemists as the third generation. Under this analogy, the pioneering soil chemists of the 19th century and beginning of the 20th century would represent the first generation. The second generation would be represented by those soil chemists working within the bulk of the 20th century, most having completed their careers by now. This middle generation greatly expanded our view of soils, such as describing the nature of variable charge soil surfaces, of soil solution and soil surface thermodynamics, and of solution single-ion activity coefficients, leading to computer simulations of soil-solution chemistry. Moreover, this generation refined our knowledge on how water molecules adhere to soil mineral surfaces, how the properties of water change when water associates with soil mineral surfaces, and how organic compounds react with soil mineral surfaces.

 The present third generation represents those soil chemists, whose careers have breached the end of the 20th century, spilling into the new millennium. At some point in the future, the third generation will probably be best remembered as the original computer-aided/spectroscopy-driven generation. By and large, the third generation is much better equipped and technologically advanced than any other past generation. However, this generation faces much stiffer competition than any of the other past generations from those in related scientific disciplines, as the boundaries separating soil chemistry from other fields blurs. In particular, soil chemists will find themselves competing against other scientific disciplines in response to society’s increasing demands to solve current environmental problems. These competing disciplines include pure physical chemists, environmental/chemical/agricultural engineers, and geochemists. The future challenges are great and only time will tell how the field of soil chemistry will continue to evolve.

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Future Challenges

 Soil chemistry has developed into an important and complex scientific discipline over the past two centuries. Beginning with J. Thomas Way’s simple base exchange work, studies on ion exchange, soil acidity/alkalinity, retention of ions by soils, clay mineralogy and chemistry of oxides and hydrous oxides have become the focal points of soil chemists’ research for decades. These subjects have evolved individually to be so extraordinarily complex and challenging that books and monographs have been and continue to be written on these topics. Up to the late 1960’s soil chemistry dealt mostly with understanding chemical reactions to improve crop productivity. The present research focus is now directed predominantly toward environmental soil research. For example, in the early 1970’s, soil chemists were among the first scientists to characterize the makeup of agricultural, industrial, and urban wastes. Soil chemists found that much of these wastes were comprised of both inorganic (such as soil material) and organic chemical substances.

 As soil chemists, our future depends on conducting cutting-edge research elucidating the chemical behavior of soil-related substances at the molecular and macroscopic levels. Such work will improve our ability to make accurate predictions regarding their transport and availability in the environment. This need drives soil chemists to creatively employ state-of-the-art spectroscopy as well as the vast array of new molecular-level technologies continuing to be developed. Our particular challenge as soil chemists is to couple these technologies with vision and computational skill, allowing us to make great strides in unraveling problems that would otherwise remain unresolved. As soil chemists, we should look beyond our own field of science, exploring more of the basic principles of chemistry, organic chemistry, and biochemistry, to improve our imagination and capacity to attain great advancements in our science.

 

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Note: It is not the intention of this review to cover all significant events in the evolution of soil chemistry as a science. Brief as it is, much has been omitted and much more could have been written. We hope that this outline will provide some insights on the voluminous and wondrous endeavors by the predecessors so that a strong foundation may be passed on to the successors. More to learn, more to discover… Knowledge is still insufficient!

 

This BRIEF SOIL CHEMISTRY HISTORY was inspired by G. W. Thomas' 1977 article entitled HISTORICAL DEVELOPMENTS IN SOIL CHEMISTRY: ION EXCHANGE, SOIL SCI. SOC. AM. J. 41:230-231.

 

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And now for a little more history...

 

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