Chapter 11: Soil pH and salinity.
After completing this chapter, you should be able to:
* describe soil pH and its development
* describe how pH affects plant growth
* tell how to lime or acidify soil
* describe saline and sodic soils
* describe methods to treat saline and sodic soils
TERMS TO KNOW
calcium carbonate equivalent
sodium adsorption ratio
total neutralizing power
"Sweet" and "sour" are old terms used to describe soil quality, which remind us of a farmer raising a handful of soil to the lips to taste sweetness or sourness. "Sweet" and "sour" are simple terms for soil reaction, or soil pH. What is soil reaction and how does it affect crop growth?
Soil reaction describes the acidity or alkalinity of a soil. Soil users are concerned about soil reaction because it strongly affects plant growth. Reaction is measured by the pH scale, as shown in figure 11-1, which gives sample pH values for common substances. The scale runs from a pH of 0 to a pH of 14.0. Readings between 0 and 7.0 are said to be acid. A pH of 1.0 is extremely acid and a pH of 6.0 is slightly acid. Examples of acid materials include vinegar, tomato juice, and lemon juice. These acid foods have a sour taste.
[FIGURE 11-1 OMITTED]
Readings between 7 and 14 are alkaline or basic. The larger the number, the stronger the base. Soap is slightly basic, while household ammonia, with a pH of 11, is strongly basic. Bases, or alkaline substances, taste bitter.
The midpoint of the scale, pH 7.0, is the neutral point, which is neither acid nor base. Pure water, which has a neutral pH, can be a model in our discussion of pH. A very few water molecules break up to form a cation and an anion as shown in reaction (a):
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The cation in the reaction is the hydrogen ion ([H.sup.+]). It makes a solution acid. The anion is the hydroxyl ion ([OH.sup.-]). It makes a solution basic. In pure water, the number of hydrogen ions equals the number of hydroxyl ions to maintain a balance. Thus, pure water is neither acid nor base. However, substances dissolved in water may change the balance, causing one ion to outnumber the other.
For instance, if pure water is exposed to air, carbon dioxide from the atmosphere dissolves in the water to form carbonic acid. Carbonic acid quickly breaks down to liberate hydrogen ions (b):
(b) C[O.sub.2] + [H.sub.2]O [right arrow] [H.sub.2]C[O.sub.3] [right arrow] [HC[O.sub.3.sup.-] + [H.sup.+]
Now there is an excess of hydrogen ions, so this dilute solution is acidic. Water in equilibrium with air has a pH of about 5.6. In fact, rainfall itself is slightly acid even without air pollution that creates lower pH "acid rain."
The pH scale indicates how acidic or basic a solution is by giving the concentration of hydrogen ions. The pH scale is a special scale for expressing hydrogen ion concentration as one over the log of the hydrogen ion concentration (1/log [[H.sup.+]]). The smaller the number on the pH scale, the stronger the acidity of a substance. Each pH point multiplies acidity by a factor of 10. A pH of 5.0 is 10 times more acid than pH 6.0 and 100 times more acid than pH 7.0.
The balance between hydrogen and hydroxyl ions dictates pH. Soil with far more hydrogen ions than hydroxyl ions is very acid. With only a few more hydrogen ions, it is slightly acid. On the basic or alkaline side of the scale, the reverse is true.
Development of Soil pH
Soil does not reach the extreme pH limits shown in figure 11-1--the most acid soil has about a pH 3.5 and the most basic soil is pH 10.5. These are extreme values. Growers more commonly find that soil ranges between pH values of 5.0 and 8.0.
Soil pH results from the interaction of soil minerals, ions in solution, and cation exchange. Different reactions govern at different pH ranges. In the simplest terms, high pH is caused by the reaction of water and the bases calcium, magnesium, and sodium to form hydroxyl ions. Low pH is caused by the percolation of mildly acidic water, which results in the replacement of exchangeable bases by hydrogen ions. To understand the full range of soil pH, it is easiest to start with alkaline soil.
Very basic soils (pH greater than 8.0) are more than 100 percent base-saturated. That is, all exchange sites are filled with bases, and the soil contains particles of mineral carbonates (C[O.sub.3]) like calcium carbonate (CaC[O.sub.3]). The pH of very alkaline soils results from the reactions of carbonates with water to form hydroxyl ions, according to reactions (c) and (d):
(c) CaC[O.sub.3] + 2[H.sub.2]O [right arrow] [Ca.sup.+2] + [H.sub.2]C[O.sub.3] + 2O[H.sup.-]
(d) [Na.sub.2]C[O.sub.3] + 2[H.sub.2]O [right arrow] 2[Na.sup.+] + [H.sub.2]C[O.sub.3] + 2O[H.sup.-]
This reaction with water is called hydrolysis ("hydro" meaning water). The hydrolysis of calcium carbonate in reaction (c) results in a pH range of about 8.0-8.5. Soils in this range, which are 100 percent base-saturated and contain enough free calcium carbonate, are called calcareous soils. Calcareous soils can be tested with dilute hydrochloric acid--they will fizz from carbon dioxide given off by the reaction between the mineral and the acid (figure 11-2). They result from the weathering of calcareous parent materials like limestone. If the sodium saturation exceeds 15 percent, then the hydrolysis of sodium produces lye (sodium hydroxide, NaOH), which can raise the pH to 10.0 as in reaction (d). Such high-sodium soils are termed sodic soils.
In humid climates, further weathering and leaching removes excess basic minerals. When these minerals reach a low level, the soil ceases to be calcareous. However, the remaining minerals ensure that the soil is 100 percent base-saturated. This occurs at a pH of about 8.0, though it varies for different soils. At this point, pH begins to be controlled by the hydrolysis of exchangeable bases (rather than free carbonates), as in reaction (e):
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]
[FIGURE 11-2 OMITTED]
This reaction maintains a pH level between about 7.0 and 8.0, depending on soil type. Note that reaction (e) replaces calcium with hydrogen. As this reaction proceeds, and rainfall leaches out the calcium, base saturation goes below 100 percent. The same is true of soils of noncalcareous parent materials. Exchangeable hydrogen, if adsorbed, does not acidify the soil. When it does enter the soil solution by cation exchange, however, it makes the soil more acid, reaction (f).
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]
The actual pH of soil with a base saturation below 100 percent depends on the balance between hydroxyl ion production by base hydrolysis, reaction (e) and hydrogen ion production by hydrogen exchange, reaction
(f). This balance controls pH in slightly acid to slightly alkaline soils.
When pH declines to about 6.0, aluminum begins to leave the structure of silicate clays. Aluminum ions react in several steps with water to form hydrogen ions and aluminum hydroxide compounds. The reactions are summarized in reactions (g) and (h):
(g) micelle-[Al.sup.+3] [right arrow] [Al.sup.+3] + micelle-
(h) [Al.sup.+3] + 2[H.sub.2]O [right arrow] Al [(OH).sub.2.sup.+1] + 2[H.sup.+]
Aluminum hydrolysis can lower soil pH to about 4.0. This is the most acidic the majority of upland soils become. Figure 11-3 summarizes the pH ranges and associated conditions.
Causes of Acidity. Relatively young soils--those not exposed to long periods of weathering and leaching--share the pH of their parent materials. Acidic parent materials include granite, sandstone, and shale. These materials are common in New England, the Great Lakes, and the Appalachian states. The soils of many states, including many in the Great Plains, developed from calcareous parent materials like limestone. When young, these soils tend to be neutral to alkaline.
The pH of most soils is controlled by the percolation (or lack of percolation) of acidic water. This percolating water leaches away bases and replaces them on the exchange sites with hydrogen and aluminum ions, reaction (i):
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]
Figure 11-4 portrays this reaction graphically. This type of percolation occurs in humid climates, where precipitation exceeds evapotranspiration. In a humid climate, net movement of water over the course of a year is downward, leaching out bases. In semiarid or arid zones, net water movement is upward, since water is being pulled out of the root zone by evaporation or transpiration. With little or no percolation, soils of dry regions tend not to become acidic. They may even become quite alkaline from calcium or sodium being carried upward into the root zone by capillary movement. Figure 11-5 shows that leaching has the greatest effect on the soils of the eastern half of the United States and the Pacific Northwest.
A number of processes produce the hydrogen ions that make soil more acidic. Some processes occur naturally, and others result from human activities. A major natural process that contributes to soil acidity is the respiration of plant roots and other soil organisms. During respiration, organisms give up carbon dioxide which reacts with water to produce carbonic acid, reaction (j). Carbonic acid, in turn, breaks down to release hydrogen ions:
(j) C[O.sub.2] + [H.sub.2]O [right arrow] [H.sub.2]C[O.sub.3] [right arrow] HC[O.sub.3.sup.-] + [H.sup.+]
Thus, plant growth and organic-matter decay both produce hydrogen ions. This is the same reaction that acidifies rainfall, which is a second cause of soil acidity. Crop plants acidify soils in two additional ways. First, when roots take up cation nutrients like potassium, they "give back" an equivalent number of hydrogen ions. Second, growers take calcium and magnesium with each crop harvested. For example, every ton of alfalfa hay is a loss from the soil of thirty pounds of calcium and eight pounds of magnesium. This removal of magnesium and calcium during harvest speeds up the acidification of soil.
[FIGURE 11-4 OMITTED]
[FIGURE 11-5 OMITTED]
Nitrification also contributes hydrogen ions to the soil. When nitrifying bacteria oxidize ammonium ions (N[H.sub.4]), hydrogen ions result:
(k) 2N[H.sub.4.sup.+] 3[O.sub.2] [right arrow] 2N[O.sub.3.sup.-] + 8[H.sup.+]
This reaction is most important because of the common use of ammonium fertilizers, otherwise called acid-forming fertilizers.
Effects of pH on Plants
Each crop grows best in a specific pH range. The pH ranges for a selection of crops are shown in figure 11-6. Most plants growing on mineral soils do well at a pH range of 6.0-7.0. For organic soils, most crops prefer a pH of 5.5 to 6.0. An exception is a group of acid-loving plants that includes mostly woody plants like blueberry and azaleas and many evergreens. Alfalfa is one of a few crops that prefer a slightly basic soil.
[FIGURE 11-6 OMITTED]
Except at pH extremes, the actual number of hydrogen or hydroxyl ions does not seem to be the main factor in plant growth. Rather, several soil conditions related to pH are more important to plants. These include (1) the effect of pH on nutrient availability, (2) the buildup of toxic levels of aluminum or other metals, and (3) effects on soil microbes. Which factor has the greatest effect on limiting crop growth varies from soil to soil and from crop to crop.
Effect of pH on Nutrient Availability. Many soil elements change form as a result of reactions in the soil. These reactions, controlled by pH, alter the solubility, and therefore the availability, of nutrients. A good example is phosphorus, which gets tied up with aluminum and iron at low pH, and with calcium at high pH. Therefore, phosphorus is most available to plants between a pH of 6.0 and 7.0.
[FIGURE 11-7 OMITTED]
Figure 11-7 shows the availability of nutrients at different pH levels. Note that the major nutrients and molybdenum are most available in near-neutral or higher pH soil. The other trace elements are more available in acid soil. Note that pH in the range of 6.0-7.0 is a good average level for all nutrients. This range is also the best pH range for most crops.
Soils that cannot supply enough of a nutrient may actually contain the element, but the nutrient is tied up because of acid or alkaline soil. Figure 10-2 showed an oak leaf deficient in iron; the deficiency resulted from growing on an alkaline soil.
pH and Element Toxicity. At low pH, particularly below 5.5, aluminum and manganese can reach toxic levels in the soil. Aluminum actually leaves the structure of clay minerals, reaches high concentrations in the soil solution, and occupies most of the cation exchange sites. Aluminum toxicity severely inhibits root growth, especially in acidic subsoils, and restricts the uptake of calcium and magnesium. Aluminum toxicity also increases water stress during dry periods because of poor root growth.
Manganese problems are less common but can be equally toxic under certain conditions. In the greenhouse, iron toxicity may occur on certain crops like geraniums if the pH of the potting mix drops too low. Figure 11-7 shows that these three elements become highly soluble below pH 5.5.
Aluminum toxicity occurs primarily in mineral soils of temperate climates. Aluminum problems seldom appear in organic soils because these soils have little aluminum. In fact, plants tolerate acid organic soils better than acid mineral soils mainly because of the low aluminum level.
pH and Soil Organisms. Soil organisms grow best in near-neutral soil. In general, acid soil inhibits the growth of most organisms, especially bacteria and earthworms. Thus, acid soil slows many important activities carried on by soil microbes, including nitrogen fixation, nitrification, and organic-matter decay. Rhizobia bacteria, for instance, thrive at near-neutral pH and are sensitive to aluminum.
The simplest way to ensure proper pH is to choose a crop that matches the present soil pH. Indeed, matching the crop and pH may be the only answer in some cases--growers may find it impractical to lower the pH of calcareous soils or to raise the pH of acid peat soils. Tropical soils may be very difficult to adjust profitably. In many of these situations, it is best to raise crops or select landscape plants that tolerate the existing soil pH. Refer to figure 11-6 and appendix 6 for examples. Breeders are also creating crop varieties tolerant of poor pH conditions, such as high-pH tolerant soybeans.
The other approach is to change the soil pH to match crop needs. Many field crops grow best in slightly acid soil. However, leaching of exchangeable bases, acid fertilizers, and other factors may slowly make soil more acid than is best for good growth. Liming is practiced by growers to counteract soil acidity.
Benefits of Liming. Liming acid soils has long been an important, but sometimes neglected, agricultural practice. Liming improves crop response to fertilizers by improving nutrient uptake, especially phosphorus, reducing aluminum toxicity, and promoting the activities of such desirable organisms as the Rhizobia bacteria that fix nitrogen for legumes.
Because calcium is itself a plant nutrient, lime is also a fertilizer, especially for high-calcium crops like alfalfa. Certain limes also supply magnesium, which is important to many acid sandy soils.
Liming Materials. We apply the term agricultural lime to ground limestone or other products made from limestone. These materials contain calcium. When lime is mixed into soil, it neutralizes excess acidity. Common liming materials include calcitic limestone, dolomitic limestone, burned lime, and hydrated lime.
Calcitic limestone is nearly pure calcite or calcium carbonate (CaC[O.sub.3]). It forms on the sea floor as deposits of calcium drop out of solution in seawater. Limestone deposits are widespread in the United States. The deposits are mined and ground into agricultural lime (figure 11-8).
Dolomitic limestone is a mixture of calcium carbonate and magnesium carbonate (CaC[O.sub.3] and MgC[O.sub.3]). Liming with dolomitic lime helps the calcium/ magnesium balance in soil. Dolomite is especially helpful in sandy soils, because they often lack sufficient magnesium. Magnesium has the same effect on soil pH as calcium.
Burned lime, or quicklime, is made by heating limestone. Heating drives off carbon dioxide resulting in the lighter calcium oxide:
(l) CaC[O.sub.3] [right arrow] CaO + C[O.sub.2] (gas)
Because calcium oxide is lighter (has a lower molecular weight), a smaller weight of it has the same effect as a larger weight of ground limestone. Burned lime also reacts more quickly in the soil. However, the material costs more and is hard to handle. Burned lime is caustic and may cake during storage. Burned lime can be used where fast action is needed but is not usually recommended.
Hydrated lime, or slaked lime, is produced by adding water to burned lime, forming hydrated lime, or calcium hydroxide:
(m) CaO + [H.sub.2]O [right arrow] Ca[(OH).sub.2] Like burned lime, hydrated lime is unpleasant and hard to handle, but fast acting. Hydrated lime is used more often than burned lime. Because of processing steps, it is more expensive than regular ground lime, but it may be used where speed of reaction is needed.
[FIGURE 11-8 OMITTED]
Growers may find other locally useful materials:
* Marl is a soft, chalky freshwater deposit in swamps that receive alkaline runoff water from nearby land. Although marl is difficult to harvest and spread, it may be useful where locally mined.
* Ground seashells, a by-product of shellfish industries, may be used in areas where those industries thrive.
* Lime-rich by-products of several industries may also be locally available.
* Wood ashes can be used by gardeners who burn wood.
It should be noted that gypsum (CaS[O.sub.4]) does not change soil pH, so cannot be used as an agricultural lime.
How Lime Works. Lime neutralizes soil in two ways. First, calcium replaces hydrogen and aluminum ions on exchange sites by mass action. In doing so, liming raises the percent base saturation. Second, lime converts hydrogen ions to water. Let's look at a couple of examples to see how this works.
The simplest reaction is that of hydrated lime (figure 11-9). As hydrated lime dissolves, it releases calcium and hydroxyl ions. Calcium replaces hydrogen and aluminum on the exchange sites, releasing those cations to the soil solution. Aluminum ions undergo complete hydrolysis to form insoluble aluminum hydroxide, with the release of more hydrogen ions. All the hydrogen ions react with the hydroxyl ions from the lime, forming water.
Calcite and dolomite act in a similar fashion, with a couple of additional steps. Hydrogen ions resulting from the other steps react with the carbonate to form carbonic acid, which quickly decomposes to carbon dioxide and water. Figure 11-10 shows this process.
[FIGURE 11-9 OMITTED]
[FIGURE 11-10 OMITTED]
Other liming materials undergo similar reactions. The important thing to remember is that calcium (or magnesium) replaces hydrogen and aluminum on cation exchange sites and hydrogen ions are changed to water. The speed of this overall process varies according to the type of material. Hydrated lime dissolves quickly in the soil and reacts quickly. Ground limestone, on the other hand, dissolves more slowly and takes more steps to neutralize acid.
How Much Lime to Apply. Four factors tell the grower how much lime is required: the present pH, the desired pH, the cation exchange capacity of the soil, and the liming material to be used.
By testing the present pH, and by knowing the correct pH for a given crop, a grower or soil testing laboratory can determine how much a pH should change. For example, if alfalfa grows well at a pH of 6.5, and the present pH is 5.0, then the pH must be raised one and a half points. The pH of the soil solution can be measured with methods described in chapter 13. However, pH by itself does not tell how much lime to apply, because it measures only the hydrogen ions in solution, not the potential acidity (hydrogen and aluminum) adsorbed on the colloids. Hydrogen ions in solution can be termed active acidity, while adsorbed hydrogen and aluminum is reserve acidity. While some soil scientists consider these terms out of date, they are useful concepts for this discussion.
Effect of Cation Exchange Capacity on Liming. Picture the soil (figure 11-11) storing hydrogen and aluminum in a large bin (reserve acidity adsorbed on soil colloids) attached to a small one (active acidity in solution). pH measures only the active acidity in the small bin. If we add enough lime to neutralize those hydrogen ions, they are quickly replaced from the large bin. Thus, the soil resists a pH change, a process called buffering. Enough lime must be added to draw down both bins before the soil will become less acid.
The size of the large bin depends on the cation exchange capacity (CEC)--the larger the CEC, the more hydrogen a soil can hold, and the more lime it needs. A soil with a CEC of 20 needs twice as much lime as a soil at the same pH with a CEC of 10.
The buffering capacity of a soil depends on the amount of clay in the soil, the type of clay, and the amount of humus. The amount of clay can be estimated by knowing the textural class of a soil. Figure 11-12 suggests how much lime to apply to soils of various textures. The type of clay modifies the effects of texture, being highest for vermiculite and lowest for sesquioxides. The buffering capacity is also increased by the amount of organic matter in the soil.
[FIGURE 11-11 OMITTED]
The lime requirement of an acid soil depends on both pH and buffering capacity. The total lime requirement can be measured directly by a buffer test, for example, measuring the reaction of soil to a pH 7.5 buffer solution. The test does not measure actual pH; it is a guide to the amount of lime needed to correct pH.
The pH of a soil sample is first measured to see if liming is needed. Let's say a soil sample has a pH of 5.5. The laboratory technician adds pH 7.5 buffer solution to the soil sample and measures the pH of the new mixture as 6.2. This means the acidity of the soil lowered the pH of the buffer solution from 7.5 to 6.2. The pH of 6.2 is called the "buffer index." Looking at a table of buffer indexes (figure 11-13), the technician reads how much lime is needed to raise soil pH the correct number of points. In this example, the table suggests applying four tons of lime per acre of mineral soil to bring the pH to 6.5-7.0.
Neutralizing Power of Lime. Buffer tests suggest the lime needs of a soil based on an "average" calcite limestone. However, different lime products have different capacities to neutralize acidity. This capacity is called the total neutralizing power or calcium carbonate equivalent. Neutralizing power is based on comparing an agricultural lime with pure calcium carbonate, or calcite. Two factors affect the comparison: the chemical nature of the lime and the purity of the lime.
Calcium carbonate has a molecular weight of about 100 atomic mass units. Other chemicals weigh more or less, and so have a greater or lesser neutralizing power. For instance, the molecular weight of pure hydrated lime (calcium hydroxide) is 74, less than calcite. Thus, it would take a smaller weight of hydrated lime to obtain the same amount of calcium--hydrated lime has a greater neutralizing power. The calcium carbonate equivalent of hydrated lime is simply 100/74; or 135. This means 100 units of hydrated lime has the same neutralizing power of 135 units of calcite.
The second influence on neutralizing power is purity. For example, calcitic limestone is mostly calcite. However, it also contains other materials, like silt, that have no effect on acidity. A ground limestone that is 90 percent pure is only 90 percent as active as pure calcite. Since calcite has a neutralizing power of 100, the power of the limestone would be 90.
Figure 11-14 gives sample neutralizing values of several agricultural limes. If the lime requirement from a buffer test was based on 85 percent pure limestone, and a grower plans to use a different form of lime, a conversion is needed. The following problem shows how much burned lime with a neutralizing power of 150 is needed to replace three tons per acre of the calcitic lime:
rate burned lime = rate calcitic lime x
neutralization value calcitic lime/ neutralization value burned lime
rate burned lime = 3 tons/acre x 85/151 = 1.7 tons/acre
Most states regulate the purity of agricultural lime to protect the customer. The laws set the chemical guarantees for the neutralizing power of lime products used and/or produced in the state. For instance, the average requirement for ground limestone is a calcium carbonate equivalent of about 85.
Lime Fineness. The fineness of the grind affects how rapidly lime acts. The finer the grind, the more rapidly it can neutralize acidity. Lime producers measure the grind by passing lime through screens of a given number of squares per square inch. Figure 11-15 shows the relative efficiency of different grinds.
While finely ground lime acts most rapidly, it is also used up rapidly. A fine powder is also costly and hard to spread evenly. Most labs suggest a medium grind that contains enough "fines" (grains small enough to be dusty) for fast action. All of such a grind would pass an 8-mesh screen, and 25 percent to 50 percent would pass a 100-mesh screen.
Most states regulate the grind of agricultural lime as well as the purity. The laws specify the physical guarantee of agricultural limes. Laws differ from state to state. A sample law, for instance, might specify that all of a lime must pass through a 16-mesh screen and that 35 percent must pass through a 100-mesh screen.
Lime Application. Best results are obtained from liming when there is close contact between the grains of lime and the soil. To achieve this, lime should be spread evenly over the field and then mixed well into the soil. Lime-spreading trucks do a good job of spreading the material. The trucks have a V-shaped bed with a spinning disc mounted on the rear. Lime drops out of the truck bed at a controlled rate onto the spinning disc that flings the lime out in a fan-shaped pattern (figure 11-16).
Lime may also be applied in pelletized form, which is more easily applied than ground lime with common application equipment. Pelletized lime is particularly useful when applying over existing vegetation and in small applications like landscapes.
While most lime is spread in a dry form, some is finely ground and mixed with water or a fertilizer solution and sprayed on the field. Fluid lime acts more quickly than regular lime and so is useful where fast action is needed. Fluid lime remains active for a shorter time, so it must be reapplied sooner. Fluid lime is becoming popular because it can be applied in a fertilizer solution, combining two operations and saving time. Fluid lime is also popular with growers with short-term use of land, such as a rented field.
After lime is spread, plowing and/or discing mixes the lime into the soil. In established pasture or other situations where plowing is not possible, lime is simply spread evenly on the soil surface. If it is not mixed into the soil, the lime slowly moves into the soil.
[FIGURE 11-15 OMITTED]
[FIGURE 11-16 OMITTED]
Growers may lime at any time that is convenient. To avoid compaction, however, it is best not to drive the trucks on wet soil. Lime should not be applied with certain forms of nitrogen fertilizer because it can cause nitrogen losses (see chapter 14). The most important consideration is reaction time, because it takes a few months for lime to break down in the soil. If faster action is needed, a grower can use fluid lime, hydrated lime, or more finely ground lime grades.
Crop growth can be inhibited by alkaline soils, common to the areas of the country that are shown in figure 11-5. Here, excess lime or sodium keeps soil pH high, as shown earlier in this chapter in reactions (c), (d), and (e). Overlimed soils may also become alkaline.
As shown in figure 11-7, zinc, manganese, iron, and other trace elements are tied up in basic soils. In addition, free molybdenum can reach toxic levels. Where soil is strongly alkaline, many crops will grow poorly. Even near-neutral soils present a problem to acid-loving plants like azalea and pin oak. Such plants appear to have a very high iron requirement. In the author's home state, new, hardy azaleas are becoming popular, but most local soils are not acidic enough for their needs. Here, landscapers may amend planting beds with acid peat moss prior to planting. However, the pH lowering effects are temporary (the peat decays), and peat amendments are not suitable for large areas.
For longer-lasting pH reduction, and for larger areas, sulfur is preferred. Once applied and mixed into the soil (as described earlier for lime), Thiobacillus bacteria alter sulfur to sulfuric acid:
(n) 2S + 3[O.sub.2] + 2[H.sub.2]O [right arrow] 2[H.sub.2]S[O.sub.4] + energy
The sulfuric acid releases hydrogen ions, and the soil becomes more acid.
Sulfur is available in granular and powdered forms and as a flowable liquid. The powdered form acts most rapidly, but is more difficult to handle. Granular sulfur, while slower acting, spreads much more easily with application equipment. Figure 11-17 suggests sulfur application rates.
A number of other chemicals also acidify the soil. These include iron sulfate, [Fe.sub.2][(S[O.sub.4]).sub.3], and aluminum sulfate, [Al.sub.2][(S[O.sub.4]).sub.3]. These materials are less powerful than sulfur and usually more expensive. They should also be used with caution, because toxic levels of aluminum or iron may build up from repeated use. In addition, when increasing pH is a problem, strongly acid-forming fertilizer may be selected. Gypsum, as a neutral salt, is unlikely to lower pH.
Calcareous soils may be very difficult to acidify because there is such a large reserve of lime that must be leached out. The bin example in figure 11-11 pictures this, except one would relabel "reserve acidity" with "reserve alkalinity." Here, free lime buffers soil from pH changes. Nevertheless, in the American Southwest, sulfur is often included in the preparation of flower beds.
Where pH reduction is impractical, fertilization and crop selection are required. Deficiencies may be temporarily corrected by fertilizing with the proper nutrients. Crops should also be selected for tolerance to high pH. In alkaline soils, for instance, alfalfa would outperform soybeans. Varieties of the same species will vary in tolerance, so one soybean variety may grow where another would not. Appendix 6 lists pH preferences of common landscape trees.
In humid regions of the United States, acidity is a common problem for growers because percolation leaches calcium, magnesium, and sodium from the soil. Growers in the more arid parts of the nation often have a different but related problem--an accumulation of soluble salts of these same bases. A salt is a chemical that results from the reaction of an acid with a base, such as the reaction of hydrochloric acid with sodium hydroxide to form common table salt:
(o) HC1 + NaOH [right arrow] [H.sub.2]O + NaCl acid base water salt
A soluble salt is defined as a salt that is as soluble or more soluble in water than gypsum (calcium sulfate, CaS[O.sub.4]). The soluble salts of greatest concern in the soil are sulfates (S[O.sub.4.sup.-2], bicarbonates (HC[O.sub.3.sup.-], and chlorides ([Cl.sup.-]) of the bases calcium, magnesium, and sodium. These salts may come from parent materials, irrigation with salty water, or even deicing salts. Prominent locations for this problem in the United States include the San Joaquin Valley of California, the lower Rio Grande Valley of Texas, and such western and southwest states as Arizona, New Mexico, Utah, and some of the northern Great Plains. Even farmland around estuaries of the East Coast may suffer some salinity problems. Salinity problems affect about 25 percent of the irrigated lands of the United States.
Growers of potted plants--greenhouses, nurseries, and interior landscapers--also experience soluble salt problems. Here high volumes of water, often containing dissolved salts, are needed to supply crop needs. Because fertilizers are salts, fertilization compounds the problem. Chapter 17 discusses this further.
Soil scientists define three types of problem soils based on the types of soluble salts: saline, sodic, and saline-sodic. Figure 11-18 summarizes these three. Saline Soils. Saline soils have high levels of soluble salts except sodium. Soil salinity can be easily measured by passing an electrical current through a solution extracted from a soil sample. The greater the salt content, the more electricity will pass. The amount of electrical flow is called electrical conductivity and is measured by the unit millimhos per centimeter (mmhos/cm). This unit of measure is presently being replaced by siemen per meter, which equals 10 mmhos/cm.
A saline soil is defined as a soil with an electrical conductivity of four or more millimhos per centimeter. However, salinity levels as low as two mmhos/cm can injure very sensitive crops. Most salts are chlorides or sulfates. Less than half of the cations are sodium, and little sodium is adsorbed on soil colloids. Soil pH is 8.5 or less. A white crust may be seen on the soil surface, due to salts migrating to the surface by capillary rise.
The main effect of salinity is to make it more difficult for plants to absorb water from the soil. In nonsaline soils, only the attraction of water for soil particles (matric potential) contributes to total water potential. In saline soil, the water also is attracted to ions in solution (osmotic potential), so less water is available to plants (figure 11-19). Saline soils may also damage plants when chlorine or other ions reach toxic levels in the plant.
Soils can be classified for use based on salinity. Figure 11-20 shows the classification system. Figure 11-21 classifies common crops according to their salt tolerance. Appendix 6 lists salt tolerance of common landscape trees.
Sodic Soils. Sodic soils are low in the kinds of salts found in saline soils but high in sodium. The exchangeable sodium percentage (or sodium saturation) is 15 or more, and pH is in the range 8.5 to 10.0. Sodium is often measured by the sodium adsorption ratio (SAR). The SAR compares the concentration of sodium ions with the concentration of calcium and magnesium ions according to the formula:
SAR = [[Na.sup.+]]/[square root of [[Mg.sup.+2] + [[Ca.sup.+2]]]/2
[FIGURE 11-19 OMITTED]
Using this measurement, a sodic soil has an SAR greater than or equal to 13.
Sodic soil has a number of effects on plant growth. The importance of these effects varies according to soil and crop.
* Sodium reacts with water, reaction (d), to form lye. The resulting high pH, 8.5 or higher, limits growth of many crops.
* For many crops, the main effect of sodium is the destruction of soil structure (figure 11-22). When sodium ions saturate cation exchange sites, the colloids separate and disperse soil aggregates. Tiny soil particles lodge in the soil pores, sealing the soil surface and creating wet "slick spots." Tilth suffers and crusts hard enough to stop seed germination may form. Sodic soils may also show a poorly drained columnar subsoil structure. The effect of sodium is most extreme on fine-textured soils and least extreme on coarse soils.
* Crop plants may take up enough sodium to injure plant tissues. Crops vary in their tolerance to sodium. For the most sensitive crops, like citrus fruits, the nutritional effects of sodium are more important than its effects on structure. For sodium-tolerant crops, poor growth results mainly from soil conditions. Figure 11-23 lists the sodium tolerance of selected crops.
[FIGURE 11-12 OMITTED]
Saline-Sodic Soils. Saline-sodic soils contain high levels of both soluble salts and sodium. The electrical conductivity is greater than 4.0 millimhos per centimeter, the SAR is greater than 13, and pH is less than 8.5. The physical structure of these soils is normal. However, after periods of heavy rain or irrigation with low-salt water, soluble calcium and magnesium may leach out of the soil, leaving behind the sodium salts. The soil may then become sodic, with poor physical structure and drainage.
Reclaiming Salted Soils. The first step in reclamation of salted soils is to decide if the project is practical and will pay for itself. The basic step to reclaiming soil is to leach out salts, so there must be a source of acceptable water. Very fine-textured soils may not allow sufficient drainage. If a decision is made to reclaim the soil, the next step is to ensure good drainage to allow salted water to leave the soil profile.
Many salted soils have problems with drainage, including a high water table, hardpans, or fine soil texture. Subsoiling can help break up hardpans. Soil with a high water table must be drained to a depth of five or six feet so salty water can be removed from the root zone and carried off the field. After proper drainage has been installed, the next steps depend on the type of problem.
Saline soils are most easily reclaimed. Growers flood the soil surface so that percolation leaches salts out of the soil profile. High-quality water works best, but larger amounts of fairly saline water will also work. Treatment water should, however, be low in sodium. Ponding is one way to apply leaching water. In ponding, heavy equipment constructs low earthen dikes to divide the affected land into ponds, which are then flooded. The field may be ponded several times, allowing time for drainage between floodings.
The reclamation of saline soils has been improved by the use of organic mulches. Mulches reduce evaporation of water from the soil surface, increasing the net movement of water downward. In addition, organic matter keeps the soil loose and maintains structure to improve drainage.
Sodic soils cannot usually be reclaimed simply by leaching, because the sealed soil surface inhibits drainage. It is usually necessary to first remove the sodium. This is usually done by treating the soil with gypsum. Granular gypsum may be spread on the soil surface, or finely ground gypsum may be applied through an irrigation system. When gypsum enters the soil, it dissolves and calcium replaces sodium on the cation exchange sites. Sodium sulfate leaches out of the soil:
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]
Gypsum is the least expensive amendment, but other chemicals may be used as well. If soil contains some lime (CaC[O.sub.3]), finely ground sulfur will add calcium indirectly. The sulfur is converted to sulfuric acid by bacteria, reaction (n). The hydrogen ions from the sulfuric acid can replace sodium on the exchange sites. More importantly, the acid reacts with soil lime to make gypsum:
(q) CaC[O.sub.3] + [H.sub.2]S[O.sub.4] [right arrow] CaS[O.sub.4] + [H.sub.2]O + C[O.sub.2] (gas)
[FIGURE 11-24 OMITTED]
The conversion of sulfur to sulfuric acid takes some time, so sulfur treatment is relatively slow. Sulfuric acid can be added directly for faster action. This is more expensive and also more dangerous because sulfuric acid is highly caustic. One USDA research project achieved a similar effect on sodic soils by planting a sorghum-sudan grass hybrid. Its roots released large amounts of carbon dioxide, which reacted with soil water to form carbonic acid, reaction (j). The acid dissolved soil lime, freeing calcium which displaced sodium, as in reaction (p).
After calcium replaces sodium on cation exchange sites, the soil slowly begins to aggregate. As the soil surface begins to improve, some growers plant salt-tolerant crops like barley. The plant roots and tops, if disced into the soil, help rebuild the soil structure.
A few additional words about gypsum may be added here. Gypsum is often sold to loosen clay soils in gardens and lawns. However, unless the soil is salt-affected, it is likely to have little effect. It may possibly improve soils damaged by road deicing salts.
Saline-sodic soils must be treated to remove sodium. If they are simply leached with low-salt water, calcium and magnesium salts are removed, but sodium remains in the soil, forming a sodic soil. Thus, gypsum treatments are useful. In the initial stages of reclamation, some growers leach these soils with fairly saline water. The calcium and magnesium salts in the water replace some of the sodium on the soil colloids, preventing destruction of soil structure.
Managing Salted Soils. Saline soils, especially irrigated land in arid climates, may be managed to reduce salt problems. One answer, of course, is to grow salt-tolerant crops. This step, however, does not really solve the problem but causes a shift over time to increasingly salt-tolerant crops. A number of practices can be used to help reduce salt problems, as follows:
* Prepare a field properly for irrigation. Proper leveling avoids low spots that collect salts. A grower may also install drainage during field preparation.
* If possible, use high-quality irrigation water. Figures 9-23 and 9-24 list the irrigation water classes.
* Keep the soil moist. Water dilutes soil salts, lowering the effect of osmotic potential. Salts tend to be most damaging in dry soil, when the salts are concentrated and both the salt and matric potential are high.
* Overirrigate enough to leach salts out of crop root zones.
* Return as much organic matter to the soil as is practical, including manures, crop residues, and green manures.
* Avoid overfertilization. Most fertilizers are salts and can compound salinity problems. Chapter 14 lists the salinity of common fertilizers.
* Maintain a good soil-testing program to monitor salinity and avoid overfertilization.
* Plant crops on ridge shoulders in furrow-irrigated fields. Salts tend to concentrate on the top of the ridge (figure 11-24).
* Use drip irrigation; it tends to reduce salt stress because it keeps the soil uniformly moist and moves salts out of the root zone of the crop plants and into the soil between plants and rows (figure 11-25).
[FIGURE 11-25 OMITTED]
Salted Water Disposal. One difficulty with methods for reclaiming and managing salted soils is that they do not eliminate soluble salts but move them to another place. Salty drainage water reappears in rivers downstream of affected farms, making that water even more salty. Individual growers do have some options to help reduce saline discharges from their fields. These suggestions stress reducing water use and retaining salt safely in the field:
* Improve water delivery systems to reduce seepage and evaporation from canals.
* Use techniques to improve irrigation efficiency, like surge irrigation and careful budgeting to reduce percolation and tailwater losses.
* Where feasible, adopt drip irrigation.
* Practice minimum leaching to carry salts below the root zone but not into the drainage system.
* Reuse salty water on salt-tolerant crops like barley or sugarbeets.
Soil pH depends on the balance of hydrogen and hydroxyl ions in the soil solution. Alkaline soil, with a pH between 7.0 and 10.0, results from the reaction of calcium and sodium with water to form hydroxyl ions. Acid soil, with a pH between 4.0 and 7.0, results from the leaching of these bases by mildly acidic water and from the release of hydrogen ions by aluminum hydrolysis.
Acid soils affect plant growth by lowering the availability of phosphorus and other nutrients, freeing toxic levels of aluminum, and inhibiting helpful soil organisms. Alkaline soils render several micronutrients unavailable and create many problems associated with salted soils. Most plants grow best between pH 6.0 and 7.0. A few, like potatoes, perform best in acid soil, while a very few, like alfalfa, do best in neutral or mildly alkaline soil.
Acid soils are treated with various forms of agricultural lime, mostly ground limestone. Lime replaces hydrogen and aluminum on the cation exchange sites with calcium and converts hydrogen ions to water. The amount of lime needed depends on the amount of pH change required, the buffering capacity of the soil, and the form of lime. Soils too alkaline for the crop being grown may be treated with sulfur.
Salted soils may be saline, sodic, or saline-sodic. Saline soils, which are high in soluble salts but low in sodium, reduce the water available to plants. Saline soils can be treated by flooding to leach out salts. Sodic soils are high in sodium and exhibit poor physical structure. They are treated with gypsum to displace the sodium. Saline-sodic soils contain both soluble salts and sodium. Care must be taken with these soils to avoid leaching the salts while leaving the sodium. After a salted soil is treated, it must be managed carefully to reduce salt problems.
1. It is said that in humid regions, soil is either acid or in the process of becoming acid. Explain.
2. Describe the effects of pH on plant growth.
3. A buffer test on a sample of mineral soil produces a buffer index of 6.4. How many pounds per acre of an 85 percent pure hydrated lime should be applied to the soil?
4. Explain why fine-textured or organic soils require more lime to neutralize acidity than coarse-textured soils.
5. Name soil conditions for which gypsum is an effective treatment and for which it is probably not. Explain your answer.
6. In excavation of some sites of ancient civilizations, archaeologists note evidence of a decline in agriculture. Early in the history of the site, wheat was grown; later, barley became the dominant crop; then agriculture ceased. Can you explain this?
7. Rising salinity is often a problem for plants grown in containers, as in greenhouses and interior landscapes. What would contribute to that rise? How could it be controlled?
8. How does common calcitic lime neutralize acid soil?
9. Describe four forms of agricultural lime.
10. A case study: assume you want to grow a river birch tree (Betula nigra) in your yard. Your soil is a moderately well-drained loam with a pH of 7.0 and a salinity of 2.0 mmhos/cm. Looking at appendix 6 and charts in this chapter, is this soil suitable for river birch as is? Explain your answer for each. If needed, how might you amend the soil, providing numbers?
1. Put a few drops of a strong acid on a piece of limestone, and observe the fizz. Can you explain the bubbles? See figure 11-10. How does this relate to calcareous soil?
2. Use pH paper to test a number of household solutions, such as vinegar, lemon juice, tapwater, and ammonia.
3. Check a soil sample for pH by mixing one volume of soil with two volumes of distilled water. Let sit for twenty minutes, then filter with a coffee filter or other filtering device. Measure the pH of the liquid with pH paper or a pH meter.
4. At the beginning of the course, the instructor can mix finely ground lime, sulfur, and gypsum into separate soil samples in pots. Keep warm and moist for several weeks. Then check for pH in class, as described in (3) above.
5. This chapter claims that rainfall is acidic. Check out this map of the rainfall pH's for the United States. Why might it vary? <http://water.usgs.gov/nwc/NWC/pH/html/ph.html>
6. For more information on salted soils, try this Web site: <http://www.ext.colostate.edu/pubs/crops/00503.html>.
7. Here is a site on soil pH and modifying pH (note that it is a PDF file): <http://www.back-to-basics.net/efu/pdfs/ph.pdf>.
FIGURE 11-3 Reactions that determine pH ranges. pH Range Determining Reaction Saturation 8.5-10.0 [Na.sub.2]C[O.sub.3] 100 percent base saturation, hydrolysis sodium saturation more than 15 percent (sodic soil) 8.0-8.5 CaC[O.sub.3] hydrolysis 100 percent base saturation (calcareous soil) 6.0-7.0 Hydrogen exchange Base saturation below 100 percent, some hydrogen saturation 4.0-6.0 Aluminum hydrolysis Low base saturation, may be high Al saturation FIGURE 11-12 Amount of limestone needed to raise pH in a 7-inch soil layer in pounds per 1,000 square feet. These values apply to soils of northern and central states of average organic matter content. (Source: USDA, Kellogg, Soils: 1957 Handbook of Agriculture) Change 4.5 Change 5.5 Textural Class to 5.5 to 6.5 Sand, loamy sand 25 30 Sandy loam 45 55 Loam 60 85 Silt loam 80 105 Clay loam 100 120 Muck 200 225 FIGURE 11-13 Buffer index tables, such as this simplified one, can be used to determine the amount of lime needed to bring a 6 3/4 inch depth of soil to a pH between 6.5 and 7.0 with 90 percent pure calcitic lime. (Courtesy of A & L Agriculture Laboratories, Inc.) (Tons of Lime/Acre) Buffer pH Mineral Soil Organic Soil 7.0 0 0 6.8 1.0 0 6.6 2.0 0 6.4 3.0 1.0 6.2 4.0 2.5 6.0 5.5 4.0 5.8 6.5 5.0 5.6 8.0 6.0 FIGURE 11-14 Neutralizing values for major sources of lime. The first four values are for pure chemicals; the remaining values are averages for commonly available products. Form of Lime Percent Purity Neutralizing Value Pure Substances Calcium carbonate 100 100 Magnesium carbonate 100 119 Hydrated lime 100 135 Burned lime 100 178 Commonly Available Forms Calcitic limestone 85 85 Dolomitic limestone 85 88 Hydrated lime 85 115 Burned lime 85 151 Marl -- 50-70 Basic slag -- 60-90 Wood ashes -- 45-80 Ground seashells 85 85 FIGURE 11-17 Amount of sulfur needed to lower pH for an 8-inch soil layer. (Source: USDA, Kellogg Soil: 1957 Handbook of Agriculture) Ground Sulfur Pints/100 [ft.sup.2] Pounds/Acre To Lower pH by This Amount Sand Loam Sand Loam 0.5 2/3 2 360 1,100 1.0 1 1/3 4 725 2,200 1.5 2 5 1/2 1,100 3,000 2.0 2 1/2 8 1,350 4,400 2.5 3 10 1,650 5,400 FIGURE 11-18 Characteristics of salted soils. Salted Sodium Soil Conductivity Exchangeable Adsorption Class (mmhos/cm) Sodium (%) Ratio Saline >4.0 <15 <13 Sodic >4.0 >15 >13 Saline-sodic >4.0 >15 >13 Salted Soil Soil Soil Class pH Structure Saline <8.5 Normal Sodic >8.5 Poor Saline-sodic >8.5 Normal FIGURE 11-20 Crop responses to soil salinity. Salinity Class (mmhos/cm) Crop Response Nonsaline 0-2 Salinity effects unimportant Slightly saline 2-4 Yields of sensitive crops lowered Moderately saline 4-8 Yields of many crops lowered Strongly saline 8-16 Only tolerant crops yield well Very strongly saline More than 16 Only most tolerant crops yield well FIGURE 11-21 Tolerances of selected crops to soil salinity. Type of Crop Tolerant Medium Sensitive Field crops Barley Corn Beans Sugar beet Soybean Flax Cotton Sorghum Broadbean Wheat Forage Crops Bermuda grass Alfalfa Clovers Wheatgrass Orchard grass Tall fescue Perennial rye Vegetables Beets Spinach Lettuce Asparagus Tomato Bell pepper Broccoli Onion Cabbage Carrot Potato Beans Sweet corn Celery Fruits Date palm Grape All others Fig Olive FIGURE 11-23 Tolerance of some crops to exchangeable sodium. Damage to the most sensitive crops is from sodium toxicity. Damage to tolerant crops is due to poor soil conditions. (Source: USDA Agriculture Information Bulletin No. 216, 1960) Sensitive Sodium Percentage Moderately Tolerant Most (Exchangeable Tolerant (ESP = Tolerant [ESP] = 2-20) (ESP = 20-40) 40-60) (ESP Above 60) Deciduous fruit Clovers Wheat Crested wheatgrass Nuts Oats Cotton Tall wheatgrass Citrus fruit Tall fescue Alfalfa Rhodesgrass Avocado Rice Barley Bean Dallisgrass Tomato Beets