Chapter 14. Calcium And Soil PH


Calcium is necessary in cell membranes and in the growing points of plant roots and tops; a deficiency causes them to wither and die.

It also helps to neutralize toxic materials in a plant.

Calcium assists in the development of soil structure. It also acts as a filler to maintain balance among cation nutrients and to limit the influence of acid cations

Retention of calcium in the soil and its function as a balancing agent depends upon the cation exchange properties of the soil.

Soil pH has no direct effect on plants; it is important only in its influence on biological activity and the availability of phosphorus and trace elements.

Table 23. Fertilizers For Supplying Calcium lists calcium amendments.

Calcium In The Plant

Calcium is the Servant, opening and closing doors and keeping out unwelcome intruders. It exists at all interfaces. It is part of cell walls and controls movement into and out of the cell. It is in the growing tips of the roots and tops and is part of the sticky substance that surrounds the roots and binds them to the soil.

It also reacts with waste products, either precipitating them or chelating with them, rendering them harmless to the plant.

Plant roots are inefficient at absorbing calcium from the soil, about 10 times less efficient than they are at absorbing potassium. Consequently the actual amount of calcium taken up by the plant is small, despite the large quantity that may be present in the soil.

A deficiency of calcium causes dieback of growing tips, in both roots and tops, and causes cell membranes to lose their impermeability and to disintegrate. Roots are short, thick and bulbous, also symptomatic of aluminum toxicity.

Calcium In The Soil

The low ability of the plant to take up calcium coincides with the large amount in most soils. Its presence is usually in the range of thousands of pounds/acre, at least an order of magnitude greater than that of other nutrients. Except under conditions of severe calcium deficiency, no relation exists between the amount of calcium in the soil and the amount in a plant. Furthermore, like potassium, soil organisms require little calcium.

Calcium has two major effects in the soil. One is as a bonding agent in the aggregation of soil particles, wherein it helps to bind organic and inorganic substances. It is important in the development of a good soil structure1.

Secondly, it acts as a nutrient filler, to maintain balance among nutrients and occupy space which otherwise would be taken up by acid elements.

The value of lime as a neutralizer is not in the calcium it contains, otherwise gypsum would be as good in raising the pH. The carbonate in lime is the neutralizing agent. Other carbonates would raise the pH as well, and some would do it faster, but they would throw the soil out of balance. Potassium carbonate would produce such a high level of potassium that plants might not survive the lack of other nutrients.

Carbonates2 are necessary to control the pH in an acid soil, but maintaining a nutrient balance requires calcium.

The role of calcium as a filler exists because the soil contains a reservoir of mobile cations. The flow of cations in and out of the reservoir is known as cation exchange. It has four effects:

  1. limits leaching losses
  2. establishes the availability of the major cations to plants
  3. influences the soil pH
  4. determines the quantity of lime needed to change the pH.

Soil PH And Cation Exchange

Cation Exchange

Cation exchange is due to the presence of either very fine clay or humus particles; these have a negative electric charge. They attract cations, which have a positive charge, and the result is a collection of cations floating around the particles. The particles are called micelles (short for microcells).

Cations are not chemically bound to the micelles but rather held loosely as a collection. They constantly drift back and forth between the micelles and the soil solution. Those associated with the micelles form the pool of exchangeable cations and those in solution the pool of soluble cations. In equilibrium, a balance is reached between exchangeable and soluble cations; this balance determines the soil pH.

In most soils, the exchangeable cations dwarf the soluble cations. Soluble cations taken up by plants or those lost by leaching are replaced by exchangeable cations. The application of lime produces cations that first enter the soil solution and then drift onto the micelles.

The principal exchangeable cations are calcium, magnesium, potassium, hydrogen, in many areas aluminum, and in acid soils ammonium. In a soil containing mostly calcium, the majority of the exchangeable cations are calcium ions.

Exchangeable calcium, magnesium, potassium and ammonium are directly available to plants. A plant root in the immediate vicinity of a micelle can take up one of these nutrients and substitute an equivalent amount of hydrogen ions.

The cation exchange reservoir is determined by the number of micelles. A measure of this number is the cation exchange capacity, or CEC3. The larger the exchange capacity, the larger is the number of exchangeable cations.

A large exchange capacity, however, does not assure a fertile soil but only that the soil contains a large number of exchangeable cations. The CEC does not indicate whether the cations are nutrients. The purpose of adding lime is to replace acid with alkalline ions. Plants are less sensitive to the calcium concentration than to the other major cation nutrients in the soil, and calcium plays the major role in this process.

Soil PH

Soil pH is an imperfect and limited concept. A discussion of why this is so may help in interpreting soil test results.

The term pH is chemical in origin. Its intended purpose is to indicate the hydrogen ion concentration, converted to logarithmic units, in a water solution. The conversion is such that a neutral solution has pH 7; a lower pH means that the solution is acid and a higher pH that it is alkaline. The pH can be estimated with the help of paper strips coated with chemicals whose color depends upon the acidity of the water solution. A more accurate measurement is with a pH probe; it measures the hydrogen ion concentration in solution.

The consequence is that pH refers to the water in a soil rather than the soil itself. The concept of pH has no meaning for a solid.

People who first tried measuring the pH of soil simply plunged a probe into a soil sample; but the results were erratic, because the probe did not make intimate contact with the soil water. More reliable results were obtained with a paste, adding just enough water to the soil to saturate it, just before it becomes shiny with excess water.

This procedure, however, is laborious. Eventually the test was standardized by combining soil and water in a predetermined proportion, commonly but not always equal quantities by weight.

The problem is the distinction between the hydrogen ion concentration actually in solution in the soil and and the concentration in a laboratory test. The relation between the two depends upon the amount of water used and its purity. The more water, the greater the dilution, the lower the concentration of soluble hydrogen ions and the higher the pH.

Salts in the water will replace some of the exchangeable acidity. Salty water will result in a higher soluble hydrogen ion concentration, or a lower pH. Moreover, during a dry season salts will accumulate in the soil, and when the rains come, these salts are leached out.

If the pH is tested during the dry part of a season, the soil salts will dissolve in the water used to measure the pH. But they will not during the wet season, having already leached out. The pH measured during a drought will be lower than the pH measured after a rainstorm.

So a pH meter does not measure soil pH nor even the pH of the soil water, but rather the pH of the water mixed with the soil by a technician or by a machine. Secondly, this distinction produces results which depend upon the amount of water used and its saltiness.

Different testing laboratories may give different results. Some laboratories use salted water in order to swamp out the effect of varying salt content in the soil; this produces more uniform results, but the pH is lower, by an amount which may vary between a tenth and more than a whole pH unit, depending upon the extent to which the soil has a high salt content on the one hand or is leached out on the other; the average drop in pH is about half a unit.

Unfortunately, most charts and tables showing the best pH for growing specific crops are based on the pH measured in pure water, although states that measure pH in salt solution have accumulated their own data.

Furthermore, the pH is not uniform throughout the soil; it is lower in the area around plant roots, owing to a higher biological activity.

However, it is the best that can be done at a reasonable cost, and soil pH is still the most important of the simple tests. Owing partly to the variations in pH that can occur, people who test their soils regularly should take samples at the same time of year and during typical weather conditions.

Importance Of Soil PH

Plants are not sensitive to soil acidity; rather they are sensitive to the effect of the acidity on the availability and form of plant nutrients. An acid soil inhibits the conversion of nitrogen from the ammonium to the nitrate form, and plants have evolved accordingly.

Most plants prefer nitrogen in the nitrate form, but grasses and grains do best with a mixture, and blueberries require nitrogen in the ammonium form. In an acid soil, levels of aluminum and available manganese can rise to the point where they become toxic to plants, while molybdenum may be deficient. In an alkaline soil, the availability of most trace elements may be too low for some crops. The availability of soil phosphorus drops in an acid or alkaline soil.

The soil pH is also a significant factor in determining the quantity and diversity of soil organisms; a neutral pH encourages a greater and more diverse population than an acid soil.

Exceptions do occur, because some plants have unique trace element requirements. Blueberries evolved in acid soils containing large amounts of iron and ammonium-nitrogen. Grasses and other monocotyledons have a requirement and a tolerance for moderately high amounts of manganese and zinc; these are most available in acid soils.

Exceptions also occur with soils. Some soils have a high aluminum content, and a higher pH may be warranted for the sake of improving the status of available phosphorus. Sandy, highly weathered and poorly buffered soils are typical of many areas of the Atlantic coast and the southeast. These soils require more acidity for satisfactory availability of some trace elements, notably manganese; the pH of those soils should not rise above 6.0 - 6.2.

On the other hand, these tendencies to favor a departure from a biologically optimum pH can be reduced or eliminated by the presence of organic matter, which can tie up aluminum and chelate many trace elements. Soil pH is not as important to plant growth when the organic content is satisfactory.

In the southwest and most of the west, soils are low in aluminum. In addition, organic soils (with an organic content of 50% or more) contain little aluminum. In these soils there is no chance of aluminum toxicity; raising the pH to avoid it is unecessary. It is most necessary in the weathered, high aluminum soils of New England and in the coastal Northwest.

Soil PH And Calcium

Lime serves to increase the pH, by modifying the balance between acidity and alkalinity. The predominant sources of acidity are the acid-forming cations, hydrogen and, where it is significant, aluminum 4. The sources of alkalinity are the base cations, principally calcium, magnesium and potassium. The equilibrium between exchangeable acid and base cations determines the amount of hydrogen ions and the pH of the soil solution.

Accordingly, soil acidity can be neutralized with anything that supplies alkalinity. Wood ashes are excellent for neutralizing soil acidity, but only in moderately quantities; otherwise they would add too much potassium. The only substances that can effectively neutralize a soil without impractically disturbing the cation balance are alkaline materials containing calcium alone or calcium plus magnesium. The choice between these two should depend upon the magnesium status of the soil.

The pH as a measure of the equilibrium between the cation reservoir and the soil solution is only a flag. It may state that the soil is too acid but not how excessive the acidity is. A soil in Delaware with a pH of 5 will require much less lime to neutralize it than a typical soil in California at pH 5. The amount of lime required is dependent on the CEC. A Delaware soil typically has a low CEC and fewer acid cations; it requires less lime to neutralize than a young California soil at the same pH but with a high CEC.

The advantage of a high CEC well balanced in nutrient cations is that it constitutes a reservoir against the natural acidifying tendency of the soil. A soil with a higher CEC is more strongly buffered5. This buffering capability counteracts the detrimental effect of acid rain on most soils6.

Calcium Fertilizers


Table 23. Fertilizers For Supplying Calcium is a list of calcium fertilizers. Poultry manure, wood ashes and seashells or lobster shells are the only common organic sources of enough calcium to be useful when spread in typical amounts.

Animal manures also contain a large quantity of carbonates and have a liming value. The actual carbonate content, however, is variable, and the liming value of manure is unpredictable. Much of the calcium in poultry manure comes from the lime in poultry feed, most of which passes through the animal. With overuse of poultry manure, there is a danger of driving the pH too high.

Legume hay contains calcium, but the calcium has no liming value because the hay has no carbonates.

Two popular waste materials for liming are clam shells and wood ashes. Clam shells are almost pure calcium carbonate, and wood ashes comprise a variety of carbonates and oxides. The lime equivalent of wood ashes shown in table 23. Fertilizers For Supplying Calcium takes into account the total carbonate and oxide content, not just calcium alone7. Clam shells have a low solubility and require several years to be effective. They should be ground as finely as possible for the quickest results. Wood ashes are effective almost immediately. But they are also caustic and require care in spreading.

Agricultural limestone is the most common method for adjusting the soil pH. Two kinds of limestone are available, one being primarily calcium carbonate, or calcitic limestone, and the other a mixture of calcium and magnesium carbonate, often called dolomitic or simply high magnesium limestone. Both have approximately the same liming capability.

At one time, other more soluble forms of lime were used. Burned lime, or quicklime, is calcitic limestone which has been heated in a furnace to drive out carbon dioxide, leaving calcium oxide. It is soluble and caustic. It is also difficult to spread effectively, because it tends to form flakes in the soil, which become insoluble owing to the formation of a crust of calcium carbonate on the surface.

Hydrated lime, or slaked lime, is burned lime to which water has been added. It is soluble and even more caustic than burned lime. It is also unstable and eventually changes to insoluble calcium carbonate upon exposure to air.

Rock phosphate and bone meal raise the pH to a modest extent8; this adds to their value in an acid soil. They act more slowly than limestone, however, because of their lower solubility.

Gypsum, sometimes called land plaster, has no liming capability. It improves the growth of clover in an acid soil, but not as well as lime. Its most likely value is in its sulfur. It is, however, useful in improving the soil structure of alkaline soils, by facilitating the removal of excessive amounts of sodium9.

Gypsum may possibly be helpful in acid soils with a low cation exchange capacity, by supplying calcium without raising the pH. It can counter the toxic effects of aluminum in plants; it has been used effectively in growing cranberries and on some soils in the southeast. Why it works is not clear, although the fact that it does illustrates the point made earlier that plants are not sensitive to the pH but rather to its effect on the environment.

Calcium chloride is another source of calcium which does not raise the pH. It has been used as a foliar spray for fruit trees in an acid soil having an excess of potassium.

Lime Rates

As noted earlier, the cation exchange capacity depends upon the clay and organic content. In arid and semi-arid soils, the clay content is more important, but most of these soils need little or no lime. Where lime is most important, organic content is the predominant factor determining the exchange capacity. Some popular recommendations in articles and handbooks base the lime requirement only on whether the soil is sandy, silty or clayey. This may be satisfactory in some areas, but it is an oversimplification and least applicable to those soils most likely to need liming.

A reliable soil test offers a lime recommendation that takes into account the CEC of the soil. Most state laboratories have developed accurate correlations that reflect this dependence either directly or indirectly. Many private laboratories have done so also, but some universally apply results to all states which are valid only in a few.

A defect of a soil test kit is the lack of an adequate lime recommendation, and people who use one must develop their own correlations. To start, it is best to be conservative. If, for example, the pH goal is 6.8, then no lime should be necessary if the pH is above 6. If it is below 6, then one ton of lime per acre, or 5 lbs/100 sq ft is reasonable; it can be doubled if the organic content is known to be high. If the pH is below 5.0, two tons/acre, or 10 lbs/100 sq ft are more likely to be needed.

More than two tons/acre should not be added in one year, otherwise trace elements - particularly boron - could be tied up temporarily. Where magnesium is low, at least the first ton of lime per acre should contain magnesium.

Lime should be thoroughly tilled into the soil. If it is simply topdressed, it will take a long time to move down into the soil, at a rate of about one inch/year in New England10.

The quantity of gypsum needed to neutralize sodium in an alkaline soil depends upon the sodium content, but typical amounts are of the order of several tons/acre.

1 Chapter 2. Essentials of Soil Fertility has a discussion of soil structure.    [return to text]

2 or oxides, which are more expensive.    [return to text]

3 This definition is simplistic, but it gives a good image of the concept of the exchange capacity. More accurately, the cation exchange capacity is a measure of the number of positive electrical charges that can be attracted to the micelles. Some cations in the soil are associated with one positive charge, some with two charges, and some with three or more. In chemical terminology, these charges denote the ionic valence of the cation.    [return to text]

4 The reason for the influence of aluminum on soil acidity is discussed in [61], but for convenience the following is a brief summary of the argument.

Aluminum is a cation, like calcium, magnesium and potassium, but it differs from them in that it can combine with water in a process called hydrolysis. Water splits into hydrogen and hydroxide ions, and the aluminum combines with the hydroxide to become aluminum hydroxide. This leaves the hydrogen ions in solution.

Initially in a very acid soil, aluminum is without hydroxide ions, and hydrogen ions are numerous. If the soil is limed, some of the hydrogen ions are neutralized. This causes the aluminum to hydrolyze some water, releasing additional hydrogen ions into solution and tending to maintain the soil acidity. This continues until the aluminum in solution has hydrolyzed all the water it can. However, the calcium in the lime replaces exchangeable aluminum at the soil micelles, and this aluminum goes into solution and represents an additional source of hydrogen. So still more lime must be added until the exchangeable aluminum is consumed.

In the reverse process, the presence of increasing amounts of hydrogen ions causes some of the aluminum hydroxide to give up its hydroxide, which combines with the hydrogen to form water. With the increasing presence of hydrogen, increasing amounts of aluminum are freed and can become exchangeable, replacing exchangeable nutrient cations.    [return to text]

5 although it can be a problem in rejuvenating an abandoned soil, particularly where organic matter accumulates, as in the northeast.    [return to text]

6 see appendix C. Acid and Basic Fertilizers - acid rain .    [return to text]

7 see appendix C. Acid and Basic Fertilizers - wood ashes for details.    [return to text]

8 Appendix C. Acid and Basic Fertilizers - Bone Meal & Rock Phosphate derives quantitative estimates.    [return to text]

9 To learn how gypsum improves alkaline soils, see, for example, [21].    [return to text]

10 According to Winston Way, extension agronomist, University of Vermont.    [return to text]

© 2013 Robert Parnes

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