2. Plants absorb two essential things from soil: water and nutrients. The availability of water in soil is related to physical properties of the soil such as percolation and retention rates. The availability of nutrients is a function of the chemical characteristics of the soil, in which water also plays a key role.

3. Soil is made up of decomposed organic matter (humus) and weathered minerals (sand, silt, and clay). Minerals are compounds with a definite chemical composition, crystal structure, and an orderly internal organization of atoms. In all, about 90 elements are found in the earth's crust, and these combine in different ways to produce about 2000 different minerals. These minerals are generally found in mixtures we call rocks.

4. Approximately 98% of the earth's crust by weight is made up of only 8 elements: oxygen, silicon, aluminum, iron, calcium, sodium, potassium, and magnesium. In fact, 75% of the weight of the earth's crust consists of just 2 elements: oxygen and silicon. These elements comprise the majority of the material found in the earth's minerals and in the mineral fraction of soil.

5. Plants absorb essential minerals in the form of ions dissolved in the water fraction of soil. Most of these ions originate from the mineral fraction of the soil itself, but they can also arise from the decomposition of organic matter. The mineral component of soil slowly releases ions in a process called weathering.

6. Weathering refers in general to natural processes that cause physical and chemical changes in rocks and minerals. Some weathering processes are physical; for example, water seeps into small cracks in a rock, freezes, and enlarges the crack; wind blowing sand gradually erodes the outside surface of a rock.

7. Some of the most important chemical weathering processes occur at exposed edges of mineral surfaces where they interact with water. Some minerals, such as halite (sodium chloride), are gradually dissolved by the water and washed away in a process called leaching. Insoluble minerals take up water molecules on their surfaces (an interaction called adsorption), where the water molecules can be a source of hydrogen ions. These small positively charged ions can very gradually replace other cations found in the mineral, such as Mg2 +, Ca 2 +, K+, and Na +. Gradually, the composition of the mineral is altered.

8. Once the ions are dissolved, they are in the proper chemical form to be absorbed by plant roots. Whether plant roots will have time to absorb the ions depends in large part on how well the soil holds them.

9. Bulk retention of dissolved ions depends in part upon how fast water percolates through soil. Soil also holds dissolved ions through electrostatic interactions at the surfaces of soil particles. Large particles such as sand have relatively little surface area and are therefore relatively unimportant for ion binding. Highly sandy soil has little capacity to retain water, so most of the dissolved ions entering sandy soil will be leached away. Percolation of water through the soil eventually rinses out ions that are not bound by soil particles.

10. Clay and humus particles have much greater surface area and are the centers of ion binding in soil. Their ion binding properties depend on their chemistry, and also the environment within the soil.

11. Humus is composed of humic acid, a highly complex material with many carboxylic acid groups. At relatively neutral pH values, the carboxylic acid groups are negatively charged and can bind cations. -COOH

12. . Most clays are composed of systems of silicates containing varying amounts of aluminum, magnesium, iron, calcium, potassium, and trace minerals. Silicate clays are negatively charged, and also bind positive ions.

13. The adsorption of cations by negatively charged binding sites is, like other chemical associations, governed by a chemical equilibrium. This means that a given cation spends some time associated with a binding site, and some time dissociated from it. If the soil water contains other dissolved cations, one of these can replace the original ion on the binding site. This process is called cation exchange.

14. Cation exchange is the means by which plants absorb essential positive ions such as potassium. Plant roots secrete hydrogen ions (H+) which eventually effect an exchange with adsorbed cations such as potassium. In this manner, plants slowly deplete soil of available nutrient cations, unless these are replaced by mineral weathering or by application of fertilizer. The cation­binding capacity of soil also enables soil to purify water of positively charged contaminants.

15. Although silicate clays and humus have an overall negative charge, particles of these materials do contain a few sites where anions can bind. However, the capacity of these soils to bind anions is far less than their capacity to bind cations. A few soils are positively charged. In these rare soils, the interactions with cations and anions are reversed: anions are adsorbed and cations are leached.

16. Plant Nutrition and Soil Minerals The elements required for plant growth can be grouped roughly according to the relative amount the plant needs: macronutrients, which are necessary at concentrations of at least 500 parts per million in plant tissue; and micronutrients, which are necessary only in extremely small amounts, usually less than 50 parts per million.

17. Like all living things, plants require carbon, which they obtain from carbon dioxide in the atmosphere. The macronutrients that plants get from soil are nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur.

18. Nitrogen presents a special case because it is not a component of any mineral, and higher plants cannot use the elemental nitrogen found in the atmosphere. Plants require nitrogen in the form of ammonium (NH 4 + ) or nitrate (N0 3 - ) ions. Atmospheric nitrogen is converted into these forms by soil microbes in reactions that form part of the global nitrogen cycle. Plants absorb and use some of the converted nitrogen, then eventually die and decompose, releasing the nitrogen back to the soil.

19. Alternatively, plants are ingested by animals, which excrete nitrogen-containing wastes into the soil and eventually die and decompose. Other soil microbes convert part of this organic nitrogen back into nitrogen gas, keeping the concentration of nitrogen in the atmosphere constant.

20. The amount of nitrogen introduced into the soil via natural processes is not sufficient to sustain the intensive agriculture upon which our economy depends. Therefore farmers add nitrogen to the soil in the form of nitrate salts. Nitrate is an anion and is not bound by soil to any significant extent, so excess nitrate leaches through the soil and can end up in surface water or groundwater. In addition, since nitrate is not retained by the soil, farmers must frequently reapply nitrate fertilizer.

21. Phosphorus In theory, there is enough phosphorus present in the earth's crust to supply the needs of plants. However, the distribution of phosphorus around the world is not uniform, and even when phosphorus is present, it is not necessarily available to plants. Most phosphorus is present as phosphate salts, which generally have quite low solubility. The low solubility means that the concentration of available phosphorus in soil water is low at any given time

22. Although phosphate is an anion (P0 4 3- ), it does not usually leach from the soil into groundwater, due to its low water solubility. Phosphate ions usually bind to the surface of clay particles through exchange with OH groups. From there they maintain a low concentration in soil water, governed by their solubility constants.

23. Phosphorus availability commonly limits plant growth. For this reason, the introduction of phosphates (for example, from detergents) into the environment can result in rapid plant growth. This stimulation of growth by phosphorus often results in algal blooms, which create problems for aquatic ecosystems.

24. Other Nutrients The other macronutrients are absorbed from soil as cations, except for sulfur, which is absorbed as sulfate (S0 4 2- ). All of these elements are introduced into the soil via mineral weathering. The cations Mg 2 +, Ca 2+, and K + are all bound by negatively charged clay and humus particles, where they can be exchanged into the soil water and absorbed by plants.

25. Soil pH Soil pH is measured by suspending a sample of water in soil, allowing it to equilibrate, and measuring the pH of the water. Therefore, soil pH is determined by the amount of hydrogen ions dissolved in soil water. Recall from the above discussion of weathering that mineral cations in soil are gradually replaced by H+ over time.

26. . This means that, over time, the concentration of H + ions in soil water will also increase (because they are in equilibrium with the bound H+), and the pH of the soil will decline. The rate of replacement of nutrient cations by H+ is dependent on the moisture in the soil and the buffering capacity of the soil.

27. . Soils in the humid tropics weather the most quickly. Many tropical soils are ultimately weathered; that is, their nutrient cations have been depleted and replaced by H+, so that the only nutrient cycling in the ecosystem occurs in the living organisms.

28. Soil pH is important for several different reasons, one being that it affects the solubility of soil nutrients. For example, phosphorus (in the form of phosphates) is most soluble between pH 6 and 8. Soil pH also affects the ability of certain microorganisms to fix nitrogen. For example, azotobacteria, a group of free-living nitrogen fixers, can survive at pH values below 6 but can no longer fix nitrogen.

29. Aluminum Toxicity As the cations Mg2+, Ca2+, and K+ are removed from soil by weathering and absorption by plants and are replaced by H+, the soil pH drops, making the aluminum ion (Al 3+ ) become more soluble. Another consequence of soil pH is its effect on the solubility of the potentially toxic aluminum ion.

30. As soil pH drops, aluminum ions begin to dissolve in the soil water and occupy cation binding sites in greater and greater concentration. This has two effects. One effect is on soil pH. Aluminum ions increase the acidity of a solution, because they interact with water molecules to liberate H +. The other effect is plant toxicity. Many plants show toxic effects when the concentration of Al 3 + in soil water exceeds one part per million.

31. Soil Chemistry and Acid Deposition Acid rain and other forms of acid deposition can, in essence, put chemical weathering processes on fast forward. If a soil lacks buffering capacity (provided by carbonate material ), acid rain introduces relatively high concentrations of H +, which speeds replacement and leaching of nutrient cations and increases aluminum solubility.

32. Oxidation and Reduction in Soil In addition to nutrients and water, plants must have access to oxygen in the soil. Plant roots require oxygen for respiration, and plants will die if it is not available in sufficient quantities. In addition, the critical process of decomposition is largely carried out by microorganisms in reactions that require oxygen. Well-aerated soils contain ample oxygen for both microbial and plant root respiration.

33. When soils are flooded or saturated with water for any length of time, the available oxygen is consumed. Flooded soils with a good supply of decomposable organic matter can deplete their oxygen in a single day. When the soil environment becomes anaerobic, microbes that can live in the absence of oxygen multiply and metabolize, using ultimate electron acceptors other than oxygen.

34. ( Recall that aerobic respiration uses oxygen as a final electron acceptor and produces water.) Some of these reactions produce, among other things, methane and hydrogen sulfide. If you have ever disturbed pond muck and noticed a terrible smell, chances are you found hydrogen sulfide, a gas that has the smell of rotten eggs.

35. Another reaction that anaerobic organisms carry out is the reduction of the ferric ion (Fe 3+ ) to the ferrous ion (Fe 2+ ). This particular reaction dramatically alters the color of soil. The ferric ion imparts a bright red color to soil, while the ferrous ion is gray no longer fix nitrogen. Another consequence of soil pH is its effect on the solubility of the potentially toxic aluminum ion.

36. 2 Fe 2 0 3 ~ no oxygen 4FeO + O 2 ferric oxide red ferrous oxide gray

37. The reverse reaction, oxidation of ferrous oxide to ferric oxide, is exothermic and will occur spontaneously when oxygen is present. 4FeO + O 2 2Fe 2 0 3 ferrous oxide ferric oxide

38. This color change is helpful in assessing subsurface soil. Regions of subsurface soil that are frequently saturated with water are gray rather than brightly colored. Gray subsurface clay is a sign of oxygen deficiency.

40. Soil is a mixture of weathered rocks and minerals and decomposed organic matter. Formation: Weathered bedrock [Minerals] -> Plants die [+ organic] Top dark Rain percolates dissolving minerals Forms subsoil clay Lighter color

41. Plant roots require air and water. Just as a plant can die from lack of water, it can die in waterlogged soil from lack of air. Soil must retain water, allow for easy plant root penetration, and provide physical support for the plant.

42. Physical features of soil particle size and arrangement, nature of the soil layers, soil texture, and land slope determine how well a soil holds water, how freely water passes through it, how easily it permits root growth, and how readily oxygen permeates it.

43. Soil Horizons Formed from bedrock: very slowly, solid bedrock is weathered into looser material. Top area unconsolidated mineral material will support plants, which remove nutritious ions from the mineral material and replace them with hydrogen ions. This replacement, together with the gradual action of water, slowly changes the chemical composition of the mineral material. Plant growth, death, and decay results in the deposit of organic materials near the surface of the soil. Animals, bacteria, and fungi feed on these remains, contributing their own organic residues to the upper regions of the soil.

44. The surface region of the soil acquires a distinctly different composition and appearance (darker, because of the organic material) from the underlying material. This dark surface layer that is relatively rich in organic matter is called an A horizon. The A horizon is the topsoil layer. The soil may have a layer of relatively whole (undecomposed) organic matter on top of the A horizon. This is called an 0 horizon. In forests, it consists of leaf litter and other decomposing plant material.

45. Water percolating through the A horizon rinses some components into the underlying soil. The components that accumulate here are usually clay, some organic matter, and oxides of iron and aluminum. This layer is the B horizon, or subsoil. (There may be an additional light-colored horizon between A and B. this one is called E.) Beneath the B horizon is a layer that sits on the bedrock and contains broken-down rock. This is called the C horizon.

46. Soil profile =An analysis of soil that identifies the horizons, their thickness, and the individual properties of each layer

47. Soil texture is determined by the ratio of sand, silt, and clay in the sample. Sand, silt, and clay are all mineral components of soil, and are defined by their particle size. Particles with a diameter greater than 0.05 mm:are considered sand; between 0.002 mm anQ 0".05 mm, silt; and less than 0.002 mm, clay. (By definition, organic matter does not contribute to soil texture.) Soil scientists group soil into three broad classes based on texture: the sands, the clays, and the loams (a mixture of sand, silt, and clay).

48. Soils that separate easily into rounded peds are called granular. Granular soils have high permeability and therefore do not pack tightly. They are usually found near the soil surface where organic matter is abundant. Granular soils are particularly suitable for plant growth, because their structure permits air, water, and plant roots to easily penetrate the soil.

49. Clay and loamy soils often have blocky peds, which are angular and somewhat irregular in shape. Their irregularity ensures that soils composed of blocky peds contain pores that permit passage of air and water.

50. Soils with plate-shaped peds, which can resemble stacked sheets of ice, are tight are packed and difficult for air and water to penetrate. Platy soils usually have a high clay content and tend to be found in frequently flooded areas. These soils are called “clay pan”. On the other hand, sand itself is a structureless soil the primary particles do not aggregate but instead fall apart.

51. The degree to which soil resists pressure is referred to as its consistence. Farm and construction machinery and even a herd of cattle can put a great deal of pressure on the soil, so consistence is important when considering how land should be managed. The terms sticky, plastic, loose, friable, soft, firm, very firm, and hard are used to describe the consistence of the soil and how well the soil resists effects of wind, water, and machinery.

52. Bulk density (BD) expresses how much a soil weighs per unit volume. Soils contain soil particles and pore space, and the bulk density depends both on the amount of pore space in a particular soil and the density of the soil particles. Determining bulk density is simple: mass a sample of soil and measure its volume. Bulk density is expressed as mass/volume.

53. The aspect of bulk density that is important for understanding other properties of soil is porosity, the volume percentage of the total pore space. Soil contains both large and small pores (spaces between soil particles) that are occupied by both air and water.

54. For example, assume a 200-cc soil core weighs 260 g when oven dry and 360 g when saturated. The core can hold 100 g water, which is 100 cc water. Total pore space is 100 cc, and porosity is 100 cc/200 cc x 100%, or 50%. A 50% total pore space is rather typical for medium-textured soil.

55. By definition, sand has a larger particle size and coarser texture than loam, silt, or clay. Because there are fewer particles in a given volume of sand, there are fewer pore spaces than in finer- textured soil. Sand typically has a porosity of around 40%, though this varies with particle size. Fine-textured soils usually have a variety of particle sizes and shapes, which do not pack tightly. A clay­texturedA horizon with granular structure may have 60% porosity.

56. In our everyday experience with soil, we know that water usually runs through sandy soil faster than through soil with a high percentage of clay. The explanation for this fact is that most pores in sandy sail are large and permit air and water to pass easily through me soil. Clay has more total pore space than sand, but the pores are much smaller, and water cannot pass through them as quickly.

57. Bulk density can be used to calculate the particle density (PD), the density of the individual soil particles themselves, is known. The formula follows: Porosity = 100% - [BD/PD x 100] The BD/PD ratio gives the fraction of the soil volume occupied by solids. As bulk density decreases (PD remaining the same), the total pore space increases and the volume occupied by solids decreases.

58. Compaction Just as soil structure can be changed by disturbance, so can porosity. If soil is subjected to pressure, pores can collapse and-total pgre space decreases. Studies have shown that a,single pass of a motorcycle in the Mojave Desert increased the bulk density of loamy sand soils from 1.52 to 1.60 g/cc. Soil from the area around picnic rabIes and tent sites in Rocky-Mountain National Park was found to have a bulk density of 160 g/cc, as compared to 1.03 g/cc in less traveled areas of the park. Compaction reduces permeability of soil to water and air.

59. The way in which water moves through and is held by soil is of critical importance both to plant life and human activities. Soil particles have electrically charged sites on their surfaces. Most of these sites are negatively charged, but some are positive. Water molecules, though not charged, are strongly polar and form hydrogen bonds with one another. When water molecules encounter perfectly dry soil particles, the strong attraction of the electrically charged soil particles for the polar water molecules results in the spreading of a thin film of water molecules over the surface of the soil particles. [This is not available.]

60. This water is called adhesion water and is several molecular layers thick. The adhesion water molecules rarely move and are thought to exist in an almost crystalline array, similar to the structure of ice. Under normal conditions, adhesion water is always present on soil particles, even dust in the air. Adhesion water is not available to plants. To remove it requires oven heating.

61. Since water molecules are strongly attracted to one another by hydrogen bonding, the regular array of adhesion water molecules provides sites for additional water molecules to associate. Further away from the charged soil particle, these molecules are held only by hydrogen bonding. They have more energy than the adhesion water molecules and move about more. This water is called cohesion water. Cohesion water fills and coats soil particles about 10-15 molecules more thickly than adhesion water and fills micropores because surface tension (provided by hydrogen bonding) is sufficient to hold it there. Cohesion water constitutes the major source of water for plant growth.

62. Cohesion water and adhesion water are retained in the soil by forces that exceed gravity. Water in macropores, however, is less strongly held and moves down and out of the soil unless an impermeable barrier stops it. Macropore water is called gravitational water because gravity attracts it more strongly than does soil. When soil is saturated as in the determination of total pore space, all its macropores are full. If the soil is allowed to drain, the macropores will lose their gravitational water, but the waters of cohesion and adhesion will be retained. In a field or pasture, once the gravitational water has drained, the area is said to be at field capacity.

63. When the soil is saturated, all pore space is filled with water. The water moves rapidly through larger pores and more slowly through smaller pores. The situation is analogous to water flow from a tank on the roof of a house down through a pipe to a faucet. The flow is a function of the height of the roof (gravitational potential difference) and the size of the pipe (conductivity). Water flow in pipes and pores is directly related to the fourth power of the radius. Thus water can flow 10.000 times faster through a pore with a radius 10 times greater than another one. If water draining down through a relatively permeable soil layer encounters a relatively impermeable layer, it will collect in the permeable layer. The rate of gravitational flow through soil is sometimes referred to as the infiltration rate.

64. Soils with a high rate of infiltration are easily leached, losing nutrient ions from the layers of soil where roots are the most abundant. Such soils can quickly become infertile without the addition of chemical fertilizers. The leachates (liquid that has passed through the soil) may contain fertilizer salts and pesticides that pollute the water table and rivers.

65. In addition, water can move through soil independent of gravity. Since there is a strong attraction between soil particles and water molecules, water can be drawn upward from the water table by capillary action when the soil becomes dry. For this to take place, soil particles must be packed closely enough to provide a continuous film of surface for water molecules to climb. On the other hand, as pore sizes become smaller, the rate of capillary movement will slow. Soil with a high rate of capillary action loses water more quickly through evaporation than does soil in which capillary action is slower.

66. Permeability and soil compaction Soil compaction primarily affects macropores, and thus can have significant effects on permeability and drainage. One study showed the permeability of soil on a logging road in Washington was only 8% of the permeability of undisturbed soil. By eliminating macropores, compaction reduces availability of both air and water to plants; by decreasing permeability, compaction also makes soils more vulnerable to water runoff and erosion.

67. Permeability and septic tanks Homes that are not connected to municipal sewage treatment systems rely on septic tanks for purifying waste from drains. A septic tank is a large, buried tank into which waste flows at one end. In its middle, a dividing wall reaches nearly to the top of the tank. Since most solid material in the waste stream sinks, the dividing wall traps most of it in the first half of the tank. A rich community of bacteria breaks it down. The mostly liquid waste that flows (slowly) over the dividing wall settles again in the second chamber.

68. Near the top of the second septic chamber is a one- way outlet to a system of underground, perforated pipes called field lines. The now mostly clear liquid from the septic tank trickles out of the perforations into the soil. Soil microbes break down remaining organic material and ions are absorbed on the surface of soil particles. As more liquid drains through the soil, it is purified.

69. If soil into which a septic were placed were highly impermeable, the liquid trickling from the field lines might not be absorbed into the soil and instead might accumulate and ooze out of the ground. Such a situation would obviously be highly undesirable. if not potentially a health hazard. Consequently- health departments ensure that any proposed septic tank site has adequate soil permeability. Sometimes this is done by considering the composition of the soil. Other times, health department workers go to the proposed site and conduct what is called a perc test. What do you suppose perc stands for in this context?

74. 5% Organic matter (living and dead plants and animals)

75. O horizon: layer of accumulated organic matter such as leaves, grass, twigs, etc A horizon (surface layer or topsoil): a surface mineral horizon enriched with organic matter. Generally, this is the soil's most productive horizon and is a zone of leaching, as well as the most climatically and biologically influenced layer. It is usually the darkest colored soil horizon due to accumulation of organic matter. E horizon: main feature is loss of silicate clay, iron, aluminum, humus, or some combination of these, leaving a concentration of sand and silt particles that is light colored.

76. R horizon (hard bedrock): bedrock underlying a soil. B horizon (subsoil): as water moves down through the topsoil, many soluble minerals and nutrients dissolve and leach into the subsoil. This horizon has an accumulation of leached clays and nutrients. The transition from topsoil to subsoil can usually be determined at a by a change of color and texture. C horizon (weathered parent material or saprolite): unconsolidated material from which the soil above it is developed.

79. Granular StructureBlocky Structure Platy Structure