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Chapter 7 Soil chemistry.


After studying this chapter, the student should be able to

* Discuss the basic principles of soil fertility

* Explain the effect of soil pH on plant growth

* Describe the materials used to raise or lower soil pH

* Discuss soil salinity


Soil chemistry is a complex subject. It is not necessary for the turf manager to understand every chemical reaction that occurs in the soil. However, the knowledge of some basic soil chemistry is very helpful. Soil fertility and soil reaction (pH) have a major effect on plant growth.

Soil Fertility

Nutrients in the Soil

An actively growing turfgrass plant is 75 to 85 percent water. Water is absorbed from the soil by plant roots. The remaining 15 to 25 percent of the plant's weight is dry matter. This dry portion is composed of seventeen essential elements (Figure 7-1). They are known as essential elements because a plant cannot successfully complete its life cycle if any one of these nutrients is lacking.

A major portion of the plant dry matter content consists of three elements--carbon, hydrogen, and oxygen. Plants obtain carbon and oxygen from the air. Carbon dioxide (CO2), a gas, enters the leaves through the stomata. Water (H2O) taken in by the roots supplies hydrogen and oxygen. These three elements are so common in a plant's environment that abundant quantities are always available for plant use.

The other fourteen essential elements are acquired by plants from three sources. As minerals in the soil weather and break down, nutrients are released. Fertilizer applications supply some of the essential elements. When plant tissue decomposes, nutrients are returned to the soil.

The major portion of nutrients found in the soil are not available for plant use. They are part of the structure of complex, insoluble compounds. Plant roots primarily absorb only nutrients that are in simple, soluble forms dissolved in water. The soil solution consists of soil water containing dissolved nutrients.

Insoluble forms of nutrients that are "tied up" or tightly bound in minerals and organic matter are not permanently unavailable. Essential elements found in minerals are slowly released by weathering processes. As the minerals are broken down, nutrients become available for plant use. Microorganisms decompose organic matter and convert complex molecules into simpler, soluble forms. The nutrients released can be absorbed by plant roots.

These simple, available forms are called ions. Some are positively charged and are known as cations. Others are negatively charged and are called anions (Figure 7-2). For example, calcium is taken in by plants when it is in the cation form [Ca.sup.++]. Nitrogen is primarily used in the anion form N[O.sub.3.sup.-].

The reason why clay and humus particles have a large surface area was explained in Chapter 5. These surfaces are primarily negatively charged. Negatively charged nutrients--anions--are not held on these surfaces to any great extent because like charges repel each other. Cations, however, are absorbed to these surfaces because unlike charges attract each other.

This is one of the reasons grass often experiences nitrogen deficiencies if nitrogen fertilizers are not applied regularly. Most of the nitrogen in the soil is locked up in insoluble compounds in organic matter. When the available form, nitrates (N[O.sub.3.sup.-]), is released it enters the soil solution. Much of the nitrate that is not absorbed by roots can be leached away (washed down through the soil by rain or irrigation water) because the anion is not held by the soil.

Larger amounts of cations are stored in the soil. Only a small portion of the nutrients in available cation forms are found in the soil solution. Many are loosely bound to the surfaces of clay and humus particles (Figure 7-3). Despite the fact that they are held on these negatively charged exchange sites, they can still easily become dissolved in the soil solution. These stored cations are protected from leaching until they enter the soil solution. The ability of a soil to absorb exchangeable cations is referred to as cation exchange capacity (CEC). Cation exchange capacity is measured in milliequivalents (mEq) per 100 grams of soil. Sand may have a CEC as low as 1 mEq/100 g, and organic matter may have a CEC as high as 500 mEq/100 g.


Nutrient Absorption

Turfgrass plants have extensive fibrous root systems that are ideal for locating nutrients. The roots primarily absorb nutrients from the soil solution. However, roots in direct contact with soil particles can also take in cations held on the surface exchange sites.

To understand nutrient uptake, it is necessary to understand how water moves into roots. Water enters roots by a process known as osmosis. Water tends to move from regions of high water concentration to regions of low water concentration.

Plants "attract" water by accumulating dissolved substances (solutes) in root cells. The greater the solute concentration, the lower the water concentration. The dissolved materials in root cells cause water to flow into the roots.

This concept also explains fertilizer "burn" and the injury caused by high levels of salts in the soil. Water continues to enter the roots as long as the concentration of water in the soil solution exceeds the concentration of water in root cells. If too much fertilizer or salt is applied, the water outside the plant may have a greater concentration of dissolved substances than the water inside the plant. The direction of water movement reverses, and water leaves the root cells. This lack of water may kill the plant or the salts in the root cells may cause injury when water content is decreased.

If the turf manager applies too much of some fertilizers by mistake, a fertilizer "burn" may be the result (Figure 7-4). He or she should immediately water the area. A heavy irrigation will wash the fertilizer off the leaves and will help to leach the fertilizer salts below the root zone.

Under normal soil conditions the concentration of dissolved substances is higher in root cells than in the soil solution. Because nutrients are more abundant in root cells, a natural tendency for them is to move out into the soil solution. Membranes surrounding the cells prevent nutrients from leaving the roots.


The actual absorption of nutrients into the roots can be a complex process. Energy is often expended by the plant to move nutrients across membranes into the roots.

Several factors influence the rate of nutrient uptake. Sufficient quantities of nutrients in available forms must be present in the soil. A well-developed root system enables the plant to obtain essential elements. Adequate soil moisture is necessary because ions move through the soil solution to the roots.

Soil temperature affects nutrient absorption. Plants use energy to carry ions in through cell membranes. This energy is produced by root respiration. Very low or high soil temperatures inhibit respiration.

Roots need oxygen ([O.sub.2]) for respiration to occur. In waterlogged or compacted soils low [O.sub.2] levels can limit nutrient uptake.

Soil Reaction

Soil reaction is the degree of acidity or alkalinity of a soil. The pH scale is used to measure this acidity or alkalinity. A pH of 7.0 is considered to be neutral--neither acidic nor alkaline. Values lower than 7.0 indicate acid soils, and values higher than 7.0 indicate alkaline (basic) soils (Figure 7-5).

Soil pH is actually a measure of the concentration of hydrogen ions ([H.sup.+]) in the soil solution. An increase in the number of hydrogen ions in the soil solution decreases the pH--the soil becomes more acidic. The [H.sup.+] concentration increases or decreases ten times for each unit change in pH. A pH of 5.0, for example, is ten times as acidic as a pH of 6.0.

Soil pH varies greatly throughout the United States--it may be as low as 3.5 and as high as 9.5. Most turfgrass soils are in the 4.5 to 8.0 range.

The major factor influencing soil pH is precipitation. Calcium and magnesium are minerals that neutralize acidity and raise pH. The greater the rainfall, the larger the amount of calcium and magnesium that is leached from soils.

Arid, dry regions of the western United States usually have soil pH in the alkaline range. Less rainfall results in less leaching of neutralizing minerals. The effect of precipitation on soil pH is illustrated in Figure 7-6. Regions experiencing minimal rainfall tend to have alkaline soil conditions. In the more humid regions acid soils occur.


Other factors influence soil pH. Soils formed from limestone contain large amounts of calcium, and the pH does not drop even if rainfall is abundant. In arid regions sodium (Na) contributes to soil alkalinity (alkali soil). Soils with high levels of organic matter may have low pH values because acids are released from organic materials. Plant roots give off acid-forming substances. Nitrogen fertilizers may have an acidifying effect.

Most turfgrasses grow best in soils having a pH of 5.5 to 7.0, depending on the species. Other species, such as centipedegrass and carpetgrass, prefer more acidic conditions. Some species, such as crested wheatgrass and blue gramagrass, can tolerate greater alkalinity. Optimum pH ranges for the primary turfgrass species are listed in Figure 7-7.


The major reason that the preferred pH range is generally slightly acid is that all of the essential elements are in available chemical forms in this range. Significant amounts of each nutrient are available for plant use when the soil pH is around 6.5 (Figure 7-8).

Nitrogen, for example, is normally found in the soil as part of the structure of organic matter. Essential elements such as nitrogen are released from the organic material and become available for plant use when the material is decomposed by microorganisms. Extreme acidity or alkalinity inhibits microorganisms and results in decreased nitrogen availability.

Plants have difficulty obtaining phosphorus at high or low soil pH values. Below a pH of 6.5, it combines with iron, aluminum, or manganese and forms insoluble compounds. As the pH increases above 7.0, phosphorus reacts with calcium or magnesium and is, again, "fixed" in unavailable compounds.


Micronutrients (also called trace elements) such as iron and manganese become more available at pH values lower than 6.5; however, this is not usually a problem because plants need only small amounts of these elements. Manganese and aluminum may become so soluble in strongly acidic soils that they have a toxic effect on plants.

When soil is maintained at the ideal pH, plants can obtain nutrients they need and growth is increased. If the soil pH is too acidic or alkaline, plants may experience nutrient deficiencies. In such cases, the soil may contain abundant quantities of these essential elements, but they are locked up in complex, insoluble compounds and are unavailable to plants.

Neutralizing Soil Acidity

Lime materials containing calcium or magnesium are applied when the soil pH is too low. Calcium and magnesium neutralize acidity and raise the soil pH. Adding lime to a soil is also beneficial because calcium and magnesium are plant nutrients and they help to improve soil structure.

The most common liming material is calcium carbonate (CaCO3). It contains 40 percent calcium and is called ground or agricultural limestone. Calcium carbonate is the most widely used type of lime because it is very inexpensive.

This material is produced by grinding limestone rock. The rate at which calcium carbonate corrects acidity depends on how finely the material is ground. The finer the lime, the quicker it works. Ground limestone that passes through a 100-mesh sieve (100 holes per linear inch) raises pH much more rapidly than material that can only pass through a 20-mesh sieve (20 holes per linear inch) (Figure 7-9).

Lime used for turf should be ground finely enough to allow all of the particles to pass through a 20-mesh sieve, and at least 50 percent should pass through a 100-mesh sieve. This information is stated on the bag (Figure 7-10).

In areas where the soil is low in magnesium, dolomitic limestone should be used. It is ground limestone that contains both calcium and magnesium. Dolomitic lime is also very slow working and should be finely ground.


The amounts of ground limestone required to raise soil pH are listed in Table 7-1. Dolomitic limestone is applied at approximately the same rates.

Table 7-1 indicates that clay soils need more lime than sandy soils need. This is because finer textured soils have more particle surface area and exchange sites. They can hold more hydrogen ions than can coarser, sandier soils. Finer textured soils or soils with significant amounts of organic matter have reserve acidity and are said to be buffered because they resist pH change. Eventually the stored [H.sup.+] ions will become dissolved in the soil solution and contribute to soil acidity. More calcium or magnesium must be added to a clayey soil because it contains more [H.sup.+] ions.

Soil testing laboratories report "buffer pH" as well as soil pH. The soil pH number indicates the pH of the soil solution, but does not tell anything about the soil's buffering capacity. To determine the lime requirement, it is necessary to know the soil texture and percent of organic matter and refer to a chart such as the one in Table 7-1.

The buffer pH method determines the lime requirement by adding a buffering solution (pH 7.5 or 8.0) to the sample. The decrease in soil pH that occurs after the buffering solution is added indicates the buffering capacity ([H.sup.+] ions in reserve) and determines how much lime is needed. This is the method that labs use to make lime recommendations (Table 7-2).

Ground or dolomitic lime will not burn grass. However, excessive application rates can raise the soil pH too much. If the soil becomes too alkaline, nutrients will remain in unavailable forms. Overliming is most likely to cause problems in sandy soils. Lime raises the pH of a sandy soil faster than it changes the pH of a loam or clay soil.

Lime is applied with a fertilizer spreader. It is often sold as a finely ground powder. The small particles must be spread evenly. Lime moves straight down in the soil--it does not move laterally (horizontally). Soil beneath "missed" spots will not be neutralized.

The ground and dolomitic limestones move down through the soil very slowly. This is not a problem if the correct amount of lime is spread before the grass is planted. The lime can be rototilled or plowed into the soil. It should be mixed uniformly 6 to 8 inches (15 to 20 centimeters) deep. The pH of the root zone can often be raised to 6.0 or higher in a few months.

When lime is applied to an established lawn, it cannot be worked into the soil. Lime may move down through the soil only 0.5 to 1 inch (1.3 to 2.5 centimeters) a year. Surface applications can take two years or longer to increase the pH of the root zone to the proper level. The turf manager must not allow the pH to drop too low before adding lime.

In humid areas soil pH tends to become acidic because of the leaching of calcium and magnesium. Plants also remove these two neutralizing elements from the soil. The nutrients are not returned to the soil if the clippings are removed when the grass is cut. Many nitrogen fertilizers have an acidifying effect. Organic acids released from soil organic matter contribute to acidification.

The turf manager must test the soil to determine whether lime is needed. The soil should be tested before the grass is planted, and then tested again every two or three years. If test results show that the pH has dropped to 6.0 (or to a minimum optimum pH value), lime should be added. For the majority of turfgrasses lime should be applied at 6.0. Soil testing will be discussed in detail in Chapter 8.

Ground or dolomitic lime and fertilizer should not be applied to turfgrass at the same time. If they are put down together, nitrogen may be lost. It is converted into a gas and escapes into the air. Phosphorus is "fixed" by the calcium and magnesium. It is bound up in insoluble, unavailable compounds. When lime is spread, a week should be allowed to pass before fertilizer is applied, and vice versa. However, lime and fertilizer can be worked into a seedbed together if incorporation is not delayed.

Lime is very inexpensive and can be applied at any time during the year. In acid soils, applications are often necessary every two or three years. Turfgrass quality is greatly diminished if the soil pH becomes too acidic.

Very finely ground 200-mesh lime particles can raise soil pH one unit in several weeks. However, it is difficult to spread the dustlike particles evenly when they are in a dry, powdered form. The particles can be suspended in water and applied with a pesticide sprayer. When lime is mixed with water and sprayed on turf it is called slurry or fluid lime. Lime can also be purchased in a pelletized form (Figure 7-11). The pellets are easier to spread but more expensive than the powdered forms. After application the pellets dissolve into powder when they come into contact with moisture. Pelletized lime is very popular.


Correcting Soil Alkalinity

Moderately alkaline pH in the range of 7.5 to 8.5 may develop in soils with an excess of sodium, calcium, or magnesium. This usually occurs in arid or semiarid regions where rainfall is infrequent and leaching is minimal. Less leaching is also associated with poorly drained soils. Irrigation water may contain sodium, calcium, and magnesium and contribute to alkalinity.

At alkaline pH values, micronutrients such as iron, manganese, copper, zinc, and boron become unavailable to plants. Alkalinity is corrected by applying sulfur, which forms sulfuric acid (Table 7-3). Elemental sulfur is commonly used. Aluminum sulfate is also effective, but is toxic to grass if improperly applied. Fertilizers containing acidifying chemicals such as ammonium sulfate and iron sulfate help to lower pH. At some golf courses various forms of sulfur are injected into the irrigation water (Figure 7-12).

Turf managers have to be careful when applying sulfur to established turfgrass. Rates should not exceed 5 pounds per 1,000 [ft.sup.2] (2.3 kilograms per 93 [m.sup.2]) per application. Sulfur can injure turfgrass even at low rates if the plants are stressed. For example, it should not be applied to cool season grasses during periods of higher temperatures. Applications should be a month apart, and the sulfur should be immediately watered in.


Some soils have a high pH because they are formed from alkaline parent material such as limestone. They are called calcareous soils. Because of their buffering capacity they are very resistant to pH changes. If the soil pH cannot be lowered to the desired level, it is a good idea to apply larger amounts of the nutrients that are less available at alkaline pH levels. Some of these nutrients may be sprayed directly onto the foliage and be absorbed into the leaves.

Salted Soils

In semiarid or arid regions high levels of salts in the soil can be a serious problem. These salts are generally chlorides and sulfates of sodium, calcium, and magnesium. In high rainfall areas excess salts are leached from the root zone, but extensive leaching does not occur in regions where total annual precipitation is less than 20 inches (51 centimeters). Consequently, the salt content of the soil can build up to injurious levels in these drier areas. Salted soils are common in the southwestern United States.

High salt concentrations reduce the ability of plants to absorb water and nutrients. Salts may also enter the plant and cause toxicity. The symptoms of salt injury shown by turfgrass include wilting, a blue-green appearance to the leaves, a stunting of growth, and tip burn (tip of the leaf is yellow). Grasses that lack salt tolerance may die.

Salted soils are placed in one of three categories based on their soluble salt content and exchangeable sodium percentage. Saline soils contain levels of soluble salts high enough to reduce plant growth. Their pH is usually below 8.5 because only a small amount of exchangeable sodium is present. Sodic soils contain sufficient amounts of sodium to interfere with plant growth. Their pH is generally higher than 8.5 because of the high levels of sodium. Saline-sodic soils contain large enough quantities of both soluble salts and exchangeable sodium to cause plant injury.

The soluble salts in saline soils are readily leachable, so frequent, deep irrigation will flush them beneath the root zone if the soil is permeable and well drained. Satisfactory turf can be grown on saline soils if excess salts can be leached by irrigation. Good drainage can be encouraged by mechanical aeration and the installation of drain lines. If salts cannot be removed from the root zone because the irrigation water has a high salt content, not enough irrigation water is available, or drainage is poor, the turf manager must use salt-tolerant grasses. Bermudagrass, St. Augustinegrass, creeping bentgrass, tall fescue, seashore paspalum, and alkaligrass (Puccinellia distans) exhibit good to excellent salt tolerance.

High levels of sodium are toxic to turf and create an unsatisfactory physical condition in the soil. Sodium causes the dispersion of clay particles, which means that aggregates or clumps of particles are not present. For this reason sodic soils are tight, structureless, and impermeable. Poor aeration and low water infiltration are common characteristics.

These problems may be corrected by adding gypsum (calcium sulfate) or sulfur to a sodic soil. Either amendment will cause the replacement of sodium on the exchange sites and promote the formation of clay particles into aggregates. After sodium has been replaced on exchange sites and has become dissolved in the soil solution, irrigation will help to leach it from the root zone. Gypsum is more widely used as a treatment for sodic soils than sulfur because it works faster. Sulfur, however, is very effective and will reduce the extreme alkalinity as well. Any practice such as core cultivation or drain line installation that encourages good drainage is beneficial.

All types of salted soils often experience nutrient unavailability, especially of iron and phosphorus, because of their high pH. Acidifying the soil with sulfur increases nutrient availability and improves drainage characteristics by dissolving some calcium carbonates that accumulate on the outside of sand particles and plug the soil.


1. Plants are composed of 75 to 85 percent --.

2. Which is not an essential element for plants?

a. Neon

b. Nitrogen

c. Iron

d. Boron

3. K is the chemical symbol for

a. Phosphorus

b. Kryptonite

c. Potassium

d. Manganese

4. Which of the following is a cation?

a. [K.sup.+]

b. [N.sub.2]

c. [Cl.sup.-]

d. N[O.sub.3.sup.-]

5. Why does the available form of nitrogen readily leach from the soil?

6. What is the soil solution?

7. Fertilizer burn is similar to drought injury. Why?

8. The preferred soil pH range for most turf-grasses is

a. 4.0-5.0

b. 5.0-6.0

c. 6.0-7.0

d. 7.0-8.0

9. A pH of 6.5 is

a. Alkaline

b. Slightly acid

c. Neutral

d. Extremely acid

10. Which plant nutrient also raises soil pH?

a. Nitrogen

b. Sulfur

c. Calcium

d. Potassium

11. Which lime material contains magnesium?

a. Hydrated lime

b. Ground lime

c. Dolomitic lime

12. How is very finely ground lime applied?

13. How many pounds of ground lime should be spread per 1,000 [ft.sup.2] to raise the pH of a clayey soil from 5.5 to 6.5?

14. To increase acidity and lower the pH the turf manager should apply

a. Iron

b. Phosphorus

c. Manganese

d. Sulfur

15. How can saline soil conditions be improved?

16. Give several reasons why a soil becomes more acidic.

17. Explain the difference between soil pH and buffer pH.
Table 7-1 Approximate Amounts of Calcium Carbonate Required
per 1,000 [Ft.sup.2] to Raise the Soil pH to 6.5 *

        POUNDS OF LIMESTONE/1,000 [FT.sup.2] (93 [M.sup.2])


6.0         20           35           50
5.5         45           75          100
5.0         65          110          150
4.5         80          150          200
4.0        100          175          230

* To convert rates to pounds per acre, multiply by 43.5.
Typically, it is best to limit each application to 50
pounds per 1,000 [ft.sup.2] when lime is applied to the
surface of an established turf. Larger amounts can result
in excessive alkalinity near the surface of the soil before
the lime eventually moves downward. Applications should
be spaced at least a few months apart.

Table 7-2 Pounds per Acre of CaC[O.sub.3] Needed
to Raise Soil pH to 6.5: An Example of One Laboratory's
Buffer pH Recommendations


   6.9           0
   6.8         500
   6.7       1,250
   6.6       2,000
   6.5       2,750
   6.4       3,500
   6.3       4,250
   6.2       5,000
   6.1       5,750
   6.0       6,500
   5.9       7,250
   5.8       8,000

Table 7-3 Approximate Amounts of Elemental Sulfur Required per
1,000 [Ft.sup.2] to Lower the Soil pH to 6.5 *

                                   POUNDS OF ELEMENTAL
LOWERING PH                SULFUR/1,000 [FT.sup.2](93 [M.sup.2])

IF PH               SANDY SOIL           LOAM              CLAY

8.5                    35-45             45-55             55-65
8.0                    25-35             35-40             40-50
7.5                    10-15             15-20             20-30

* To convert rates to pounds per acre, multiply by 43.5. The rate of
a single application should not exceed 5 pounds per 1,000 [ft.sup.2]
when sulfur is applied to the surface of an established turf. One
pound per 1,000 [ft.sup.2] is usually the maximum rate on putting

Figure 7-1 The seventeen essential elements.


Carbon                         Iron
Hydrogen                       Manganese
Oxygen                         Boron
Nitrogen                       Molybdenum
Phosphorus                     Copper
Potassium                      Zinc
Calcium                        Chlorine
Magnesium                      Nickel

Figure 7-2
Chemical symbols and available forms of the essential elements.


Nitrogen          N       N[O.sub.3.sup.-], N[H.sub.4.sup.+]
Phosphorus        P       HP[O.sub.4.sup.--], [H.sub.2]P[O.sub.4.sup.-]
Potassium         K       [K.sup.+]
Calcium           Ca      [Ca.sup.++]
Magnesium         Mg      [Mg.sup.++]
Sulfur            S       S[O.sub.4.sup.--]
Iron              Fe      [Fe.sup.++], [Fe.sup.+++]
Manganese         Mn      [Mn.sup.++]
Boron             B       [H.sub.2]B[O.sub.3]
                            and others
Copper            Cu      [Cu.sup.++]
Zinc              Zn      [Zn.sup.++]
Molybdenum        Mo      Mo[O.sub.4.sup.--]
Chlorine          Cl      [Cl.sup.-]
Nickel            Ni      [Ni.sup.++]

Figure 7-7
Optimum soil pH ranges for the major turfgrass species.
Species may tolerate wider ranges than those listed.

  TURFGRASS                    TURFGRASS
   SPECIES      pH RANGE       SPECIES             pH RANGE

Bahiagrass      6.5-7.5     Centipedegrass          4.5-5.5
Bentgrass                   Fescue
  Colonial      5.5-6.5       Fine                  5.5-6.8
  Creeping      5.5-6.5       Tall                  5.5-7.0
Bermudagrass                Grama, blue             6.5-8.5
  Common        5.7-7.0     Ryegrass
  Improved      5.7-7.0       Annual                6.0-7.0
Bluegrass                     Perennial             6.0-7.0
  Annual        5.5-6.5     St Augustinegrass       6.5-7.5
  Kentucky      6.0-7.0     Wheatgrass, crested     6.0-8.0
  Rough         6.0-7.0     Zoysiagrass
Buffalograss    6.0-7.5       Japanese lawngrass    5.5-7.5
Carpetgrass     5.0-6.0       Manilagrass           5.5-7.5

Figure 7-10
The fineness of the liming material is stated on the bag.


Calcium Oxide (CaO)                        30.0%
Elemental Calcium (Ca)                     21.4%
Magnesium Oxide (MgO)                      20.0%
Elemental Magnesium (Mg)                   12.0%


Total Calcium Carbonate                   104.0%
  (Ca[Co.sub.3]) Equivalent
Calcium Carbonate (Ca[Co.sub.3])           50.0%
  Equivalent from Magnesium Sources
Calcium Oxide (CaO) Equivalent             57.0%


% Through No. 20 U.S. Standard Sieve       98.0%
% Through No. 60 U.S. Standard Sieve       60.0%
% Through No. 100 U.S. Standard Sieve      40.0%
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Author:Emmons, Robert D.
Publication:Turfgrass Science and Management, 4th ed.
Date:Jan 1, 2008
Previous Article:Chapter 6 Soil modification.
Next Article:Chapter 8 Soil testing.

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