5 SIL CHEMISTRY 5.1 Introduction A knowledge of the chemical composition of a soil is less useful than a knowledge of its component minerals and organic materials. These dictate the reactions that occur in the soil, and the availability of nutrients, the elements of which, though maybe present from the analysis, are not necessarily free for plant uptake. CLASS EXERCISE 5.1 List processes occurring in soils that cause changes in the levels of ions. Processes causing ion decrease Processes causing ion increase 5.2 Clay minerals Minerals are naturally occurring inorganic compounds, with defined chemical and physical properties. Their parent materials form in the crystallisation of molten rock material: these are known as primary minerals, and include olivine, quartz, feldspar and hornblende. Primary minerals are not stable when exposed to water, wind and extremes of temperature. They weather, which means they break down physically and chemically. Some of the elements that are released during weathering reform and crystallise in a different structure: these are the secondary minerals, and include vermiculite, montmorillonite and kaolinite. Secondary minerals tend to be much smaller in particle size than primary minerals, and are most commonly found in the clay fraction of soils. All but the youngest and unweathered of soils will contain mainly secondary minerals. The major elements in the earth s crust are shown in Figure 5.1. Ca Si Al Fe K Mg Na thers FIGURE 5.1 Elemental composition of the earth s crust
The dominant element, oxygen is negatively charged, while the other major elements are positively charged: thus oxygen bonds with one or more of the cations, producing a chemistry of oxides. Silicon oxides (silicates) and aluminium oxides (aluminates), generally in combination as aluminosilicates, dominate the minerals. Small concentrations of other elements account for the differences in minerals. In silicate, silicon binds to four oxygens in a tetrahedron while aluminate has six oxygens (often as H) surrounding the central aluminium ion in an octahedron. However, in each case, it is not a matter of individual Si 4 or Al(H) 6 units. A number of the oxygens are shared between the silicate or aluminate units, giving rise to a two-, or some cases three-, dimensional structure. The most common structure in clay minerals is the formation of sheets, which are flat layers of silicate tetrahedra or aluminate octahedra. In clay minerals, these sheets stack on top of each other, and are held together by hydrogen bonding or electrostatic attraction. The common structures are classified by the proportion of each type of sheet as shown in Figure 5.2. 1:1 2:1 2:2 FIGURE 5.2 Sheet arrangements in clay minerals (silicate sheet in grey) Real clay crystals are not pure silicates or aluminates: some Si or Al atoms are substituted during the crystallisation process, creating spare charges which give the overall crystal a charge which must be balanced by loose cations or anions. For example, a silicon atom (4+ charge) is substituted by an aluminium ion (3+). This creates a 1- charge, which is then counterbalanced by a cation, such as potassium, as shown in Figure 5.3. Si Si replaced by Al in crystal Al - X + has only 1 bond, so has -ve charge; requires balancing positive charge from free cation FIGURE 5.3 Generation of cation-exchange sites in clay minerals These cations generally are held on the surface of the clay, and are not strongly held. They can be exchanged for other cations in an equilibrium process. The extent to which this process can occur is known as the cation exchange capacity (CEC). This will be discussed more later in this chapter. The majority of clay minerals have exchangeable cations, and the soil ph has no effect on the exchange capacity of the mineral matter. As minerals weather, they lose silicon (as soluble silicic acid), leading to increasing proportions of aluminate in weathered clays, such as kaolinite. Aluminium hydroxide species are amphiprotic, meaning that they can accept or lose protons. As a consequence, soils dominated by oxides of aluminium (and other metals) can have positive sites, allowing anion exchange, as shown in Equation 5.1. Al-H + H + <=> Al-H 2 + + X - Eqn 5.1 30
5.3 Ion exchange in soils Ion exchange occurs when the loosely held cations or anions on the mineral surfaces are replaced by ions of the same charge (sign and magnitude) in solution. Cation exchange is by far the most common, and is necessary for soil fertility. As soils weather, they lose cation exchange capacity and lose fertility. CATIN EXCHANGE As described above, clay minerals have negative charge due to substitution of aluminium or silicon in the crystal lattice. Humus the stable organic matter of soil also contributes negative charge, due to the presence of dissociated organic acids, as shown in Equation 5.2. The negative charge is balanced by the presence of adsorbed positive ions (cations), which are held on the surface of the clay mineral or humus particle. humus-ch humus-c - + H + Eqn 5.2 CLASS EXERCISE 5.2 What effect would soil ph have on the amount of cation sites from humus? The process of cation exchange occurs when a cation in solution replaces an adsorbed cation on the soil particle, as shown in Equation 5.3, in which initially a sodium ion is held to the soil, but is replaced by a potassium ion in solution. soil-na + K + (aq) soil-k + Na + (aq) Eqn 5.3 You should also be aware that it is charges that are balanced, not number of charged species. CLASS EXERCISE 5.3 Write an equation for the exchange of adsorbed sodium with solution calcium. The exchange reaction is an equilibrium one, which means that it is reversible and dependent on the levels of each of the species, particularly the solution species. For example, if a soil solution becomes depleted in calcium, then some calcium will desorb from an exchange site into solution. This is known as buffering, and means that in all but the most leached and infertile of soils, there will be a balance between adsorbed and dissolved ions. CLASS EXERCISE 5.4 What do you think would happen to a soil which is treated with lime (calcium hydroxide), in addition to a ph change? 31
The cation exchange capacity (CEC) is defined as the moles of exchangeable positive charge per unit mass 100 g of dry soil (this can be mmole/100g or cmole/kg, giving the same value). Calcium and magnesium ions contribute twice as much to the CEC as an equivalent number of sodium and potassium ions because of their 2+ charges. Table 5.1 gives typical CEC values for some soil texture classes. The size and charge of the ion affects the strength of attraction to the soil particle. Smaller ions and those with 2+ or 3+ charges are more strongly adsorbed. Those that are more strongly adsorbed are less likely to be exchanged. TABLE 5.1 Typical CEC values Soil Class CEC Sand 2-4 Sandy loam 2-12 Loam 7-16 Silt loam 9-26 Clay, clay loam 4-60 CLASS EXERCISE 5.5 Comment on the trend in CEC in Table 5.1. Significance of CEC During periods of high rainfall, the relatively pure water passing down through the soil will tend to cause the adsorbed ions to be removed from the exchange sites, to be replaced by H + which is more common in rainwater than normal soil solution. Uptake of nutrient ions from plant roots occurs from solution only. As cations are absorbed into the roots, they are replaced in the soil solution by H + ions. nce enough are removed from soil solution to cause a disturbance to the exchange equilibrium, some of that ion will desorb from the soil particles and be replaced by another ion, possibly the H + released by the plant. However, if the nutrient is a weakly adsorbed one, such as potassium, there may not be enough adsorbed to replenish the soil, presenting a fertility problem. f the three most important cations for plant growth (K, Ca, Mg), potassium is the one most likely to be in short supply. Test Method Information - See Chapter 8 ANIN EXCHANGE The important soil anions, nitrate and phosphate, behave very different at exchange sites. Nitrate (and chloride) are only weakly held at positive sites on the clays, and are more likely to be found in soil solution. Phosphate (and sulfate to a lesser extent), on the other hand, are very strongly bound to the exchange sites, to the point where phosphate becomes covalently and irreversibly bound. 32
5.4 Soil ph The ph of a soil is one of its most important properties, because it affects so many other soil properties, such as ion exchange and nutrient availability. In this section, we will look at how soils develop different ph levels. The soil ph comes about from a balance between acidic and alkaline species in the soil. The soil ph reflects mainly the levels of dissolved H + and H -, but also the adsorbed H + on cation exchange sites. Soil ph values under normal circumstances range from 4-9. Sources of soil acidity rain - polluted or fresh will be slightly acidic due to dissolved gases microbial and root respiration this produces C 2, which is slightly acidic in solution oxidation of organic matter this produces organic acids known as humic acids, together with nitric and sulfuric acids Sources of soil alkalinity carbonate minerals calcium and magnesium carbonate are common materials in minerals; they are slightly soluble in water, and produce H - as they dissolve (these cations and sodium and potassium are known as bases because of their association with alkaline soils) mineral weathering many primary minerals as they weather release hydroxide salts of the basic cations Trends in soil ph As soils age by weathering and leaching, they tend to become more acidic. The primary minerals that release alkaline materials are replaced by neutral or slightly acidic secondary minerals, and leaching removes the carbonate minerals. Weathering occurs from the surface downwards so that the A and B horizons will tend to be more acidic than the C horizon. Cation exchange sites lose the basic cations and have increasing levels of adsorbed H + and Al 3+. Increased levels of aluminium are a problem, and the percentage of aluminium on the exchange sites is known as the percentage aluminium saturation. This can become as high as 90% in acidic soils. Soil buffering capacity Buffering capacity is the ability to reduce the effects of an added component. In this case, a soil will have some capacity to resist change in ph as a consequence of addition of extra acidic or alkaline materials. Significance of soil ph nutrient availability the ability of plants to take up nutrients is very much dependent on the soil ph (see Figure 5.4) effect on soil organisms soil organisms prefer different ph levels; fungi thrive in acidic soils, while bacteria prefer alkaline ones; worms cannot exist in highly acidic soils acid-sulfate soils - soils that are rich in inorganic sulfide minerals, such as pyrites, or other sources of sulfide (eg relatively anaerobic areas such as swamps) can lead to the formation of excessive levels of sulfuric acid through oxidation. The soil ph dives to very low levels, and causes solubilisation of toxic levels of aluminium, manganese and iron from soil minerals. plant preferences most plants prefer alkaline soils, but there are a few which need acidic soils and will die if placed in an alkaline environment; some of the more common of these are listed in Table 5.2 33
TABLE 5.2 Common plants requiring acid soils Plant group field crops fruit & vegetables flowers and shrubs trees Examples peanuts, rice, pineapple, blueberry, strawberry camellias, azaleas, orchids pines, cedars FIGURE 5.4 Nutrient availability at different ph levels (from www.fao.org) Soil ph management Because soils tend towards lower ph values as they age, the main need for ph management is to reverse that by making the soil more alkaline. Given that most plants prefer alkaline conditions, this provides another reason. The most common method of increasing soil ph is by liming. Agricultural lime is a mixture dominated by calcium carbonate, but also containing magnesium carbonate and calcium hydroxide. It normally comes from ground limestone, and being a base, will increase soil ph and add the nutrients calcium and magnesium to the soil. Dolomite lime has a higher proportion of magnesium carbonate. CLASS EXERCISE 5.9 What factors will affect the amount of liming required? 34
If the ph needs to be reduced, because a plant to be grown in it requires low ph, then iron, sulfur or peat can be added to increase acidity. Test Method Information - See Chapter 8 5.5 Soil redox potential The redox potential (E h ) of a soil is a measure of its ability to produce oxidation or reduction of chemical species in it. The most important soil property indicated by the soil E h is whether it is aerobic or anaerobic: aerobic soils give a positive value, and the lower the value the more anaerobic the conditions. It is, however, a value that is affected by soil ph. What You Need To Be Able To Do define important terminology describe the important aspects of clay mineral structure explain how cation and anion exchange sites occur in clays explain the importance of CEC outline the measurement of CEC and AEC list sources of soil acidity and alkalinity explain the importance of soil ph list sources of acidic and alkaline species in soils explain and calculate buffering capacity outline changes in soil ph over time discuss methods for managing soil ph list plants requiring acidic soils Terms And Definitions Match the term with the definition. A. primary mineral B. secondary mineral C. sheet D. cation exchange capacity E. ph buffering capacity F. liming 1. the amount of positive charge that can be adsorbed by a soil 2. minerals that form during crystallisation of molten rock 3. the addition of calcium carbonate-rich material to increase soil ph 4. the amount of resistance to changes in ph 5. a layer of silicate or aluminate in a mineral 6. minerals that form from the weathering of rocks Review Questions 1. What does ion exchange do to the level of ions in soil solution? 2. Why is anion exchange less important than cation exchange? 3. How do negatively charge sites occur in clay minerals? 4. In general, which has the higher CEC: a young or old soil? 5. In general, which has the higher CEC: an acid or alkaline soil? 6. In general, which has the higher CEC: a soil with low or high levels of clay? 7. Why are adsorbed cations on soil minerals important in terms of long-term soil fertility? 8. Why does heavy rain cause a decrease in soil ph? 35