Soil Science Final Review

3 Important things about Silicate clays

*Dominant in most soils (except Andisols, Oxisols & Histosols) *Crystalline structure consists of sheets of oxygen, silicon, and aluminum atoms *(Predominantly negatively charged but vary in their intensity of charge, stickiness, plasticity, and swelling behavior

What are exchangeable cations?

*Generally are available for uptake by both higher plants and microorganisms. *By cation exchange, hydrogen ions from root hairs and microorganisms replace nutrient cations from the exchange complex. *The nutrient cations are forced into the soil solution, where they can be assimilated by the absorptive surfaces of roots and soil organisms, or they may be removed by drainage water.

Vermiculite

*The most common are 2:1-type minerals in which the octahedral sheet is aluminum dominated, but some magnesium-dominated *The tetrahedral sheets of most vermiculites have considerable substitution of aluminum in the silicon positions, giving rise to a cation exchange capacity (CEC) that usually exceeds that of all other silicate clays, including smectites

Approximately what percentage of the P in surface soils is typically found in organic forms?

25-75%

In what range of soil pH is phosphorus generally most available to plants?

5.5-7.0

Which of the following added to a soil would be likely to result in the greatest level of soil organic matter after several years of decomposition? A. 5 Mg of carbon in plant roots B. 5 Mg of carbon in plant tops C. the manure produced by feeding cattle 5 Mg carbon in plant residues D. the compost produced by composting 5 Mg of plant leaf material.

5mg of Carbon on plant roots.

Green plant residues such as grass clippings or a green manure crop generally have a water content of about ____% by weight.

75

Ch 8- The Colloidal Fraction What is a colloid?

A tiny particle that carries a swarm of positively and negatively charged ions (cations and anions) that is attracted to electrostatic charges on its surface. The ions are held tightly enough by the soil colloids to greatly reduce their loss in drainage waters, but loosely enough to allow plant roots access to the nutrients among them.

Disease suppressing soil often results from _____.

Adding organic materials to the soil that stimulate diverse microorganisms

Which of the following is (are) major sources of the carbon dioxide now being added to the atmosphere in excess of the amount taken out by plants? A. loss of soil organic matter B. burning of fossil fuels C. destruction of natural forests D. all of the above E. none of the above

All of the above (loss of soil organic matter, burning of fossil fuels, destruction of natural forests)

Which of the following is a greenhouse gas contributed to the atmosphere by soils? A. nitrous oxides B. carbon dioxide C. methane D. all of the above E. none of the above

All of the above (nitrous oxides, carbon dioxide, methane)

The sulfur dioxide in the atmosphere ______. A. can be an important source of sulfur nutrition for plants B. is a major contributor to acid rain C. comes from fires such as fossil fuel burning, volcanoes and forest fires D. all of the above E. none of the above

All the above

After drying to eliminate the water in plant litter, which two elements account for about 80% of the dry weight?

C&O

Explain the Cation exchange capacity as it relates to soil?

CEC is defined simply as the sum total of the exchangeable cationic charges that a soil can absorb.

Ch: 11 Organisms & Soil Ecology Soil detritivores feed principally on ___.

Dead Tissues

All but a few nematodes are pests, attacking either economic or wild plants. T/F

False

Humus has a much higher cation adsorption capacity at pH 5.5 than at pH 8.0. A. True B. False

False

Hydrogen ions dominate the exchange complex of very acid soils (pH<5). T/F

False

Of the silicate clays, smectite has the least tendency to expand upon wetting and shrink upon drying. A. True B. False

False

On the average, agricultural soils in the US are currently being depleted of their nitrogen supply. T/F

False

Research has shown that most herbicides and insecticides used in agriculture drastically reduce the numbers of living meso- and microorganisms in soils for several years after the application of these chemicals. T/F

False

The cation capacity of soil humus is two to three times greater than that of an equal volume of high CEC silicate clays. A. True B. False

False

The cation exchange capacities of alkaline soils are commonly lower than those of comparable soils in humid regions. T/F

False

The total number of microorganisms in a hectare of soil is far greater than the number of macroorganisms, but the total biomass of macroorganisms in 1 hectare is greater than the total biomass of microorganisms. T/F

False

While 2:1 type clays have significant internal surfaces, their external surfaces are generally much more extensive. A. True B. False

False

A homeowner in New Mexico attempted to grow azaleas in a well drained soil (pH 7.5) but the plants were stunted and the leaves were yellow. What is the most likely cause of this constraint?

Fe deficiency

Gypsum is the most widely used chemical for the reclamation of sodic soils. What characteristic likely accounts for this popularity?

Gypsum is abundant and low priced.

Phosphorus availability in acid soils is constrained by the presence of significant quantities of reactive ______.

Iron

Poor plant growth in a well-drained irrigated soil in an arid region (pH=8.0) is most likely due to ______.

Iron Deficiency

What makes isomorphous substitution so important?

Isomorphous substitution is of vital importance because it is the primary source of both negative and positive charges of silicate clays

If you want to reclaim a saline soil, which of the following practices would you most likely use? A. Leach the soil with low salt containing water. B. Leach the soil with water having a high SAR ratio. C. Add calcium carbonate to reduce the exchangeable Na+ level D. Leach the soil with water high in bicarbonates. E. Leach the soil with water high in calcium and magnesium.

Leach the soil with water high in calcium and magnesium.

Chapter 8 assessment questions Which of the following is not a characteristic of soil colloids? A. low water holding capacity B. very small size C. high external surfaces D. electronegative charges E. ability to exchange ions with the soil solution

Low water holding capacity

Ch 13-14: Nutrients (Note: Questions #1-5 are from Chapter 13) The great majority of nitrogen (95 to 98%) in soils can be found in the form of _____.

Organic Compounds

Ch 12: Soil Organic Matter Which of the following pools contain the largest amount of the world's carbon? A. soil B. atmosphere C. animals D. vegetation

Soil

Explain the surface area of clays

Some silicate clays posses extensive internal surface area between the layers of their platelike crystal units. aka Very Large To grasp the relative magnitude of the internal surface area, remember that these clays are structured much like a book. If you were to paint the external surfaces of a book (the covers and edges), a single brush of paint would do. However, to cover the internal surfaces (both sides of each page in the book) you would need a very large amount of paint.

In desert areas, the soil under scattered clumps of natural vegetation is usually depleted of nutrients compared to the unvegetated soil between clumps. T/F

T

What are exchangeable anions?

They are an important mechanism for holding negatively charged constituents, especially in acid subsoils. Adsorption of exchangeable anions is not as extensive as that for exchangeable cations.

A significant portion of the global production of methane comes from the activity of termites. T/F

True

Iron and aluminum hydrous oxides are characteristic of soils that are highly weathered chemically. A. True B. False

True

Leguminous forest trees, such as Locust, enrich the soil in nitrogen, to the benefit of non-nitrogen fixing plants growing nearby. T/F

True

Nitrogen fertilization is a significant cause of acidity in many cultivated soils. T/F

True

Nitrogen taken up by plants growing in a compost-treated soil is indistinguishable from the nitrogen taken up by plants growing in a fertilizer-treated soil. T/F

True

Soil colloids are too small to be seen with an ordinary light microscope. A. True B. False

True

The amount of both P and K fertilizer applied to US farmland has increased dramatically since 1980 as American farmers attempt to maximize profits and meet record demands for food. T/F

True

The cation exchange capacity of a soil is determined primarily by the amount and kind of colloids in the soil and by the pH. A. True B. False

True

The soils of the world contain more carbon than all the world's vegetation and the entire atmosphere, combined. A. True B. False

True

If you were looking for a soil with high cation exchange capacity and were not concerned with the soil's physical properties, which of the following would most likely fit your needs? A. Mollisol B. Oxisol C. Aridisol D. Vertisol E. Ultisol

Vertisol

Among the soils of which order would you most likely find the highest buffering capacities?

Vertisols

Humus is an important soil colloid. It differs from 2:1 type minerals in all but one of the following characteristics (select the one). A. structural framework of the particles B. CEC dependence on soil pH C. influence of isomorphous substitution D. capable of adsorbing cations E. influence on soil aggregate stability

capable of adsorbing cations

Which of the following plants would be most apt to tolerate high soil salinity? A. cotton B. apples C. alfalfa D. tomato E. corn

cotton

Chapter 9 Assessment Questions The exchange complexes of strongly acid (pH 4.5) mineral soils are saturated mainly with _____.

exchangeable Al3+ ions

A farmer started irrigating a soil some 10 years ago but in recent years yields have declined and the soil physical condition has deteriorated. Which of the following most likely accounts for this situation? A. high content of Ca and Mg in the irrigation water B. failure to use conservation tillage to reduce salt accumulation at the soil surface C. high SAR of the irrigation water D. excessive drainage of the soil E. high gypsum content in the soil

high SAR of the irrigation water

Which of the following characteristics of irrigation water is most apt to stimulate the formation of a sodic soil? A. high selenium content B. high calcium and magnesium content C. high content of chlorides D. low pH E. high SAR values

high SAR values

Ch 10: Soils of Dry Regions Normal alkaline soils are not characterized by ______.

high levels of exchangeable sodium

The total phosphorus lost from various watersheds is most closely correlated with ____ .

high soil erosion

Some irrigation systems have stimulated the formation of saline and saline-sodic soils. Which of the following contributes to this process? A. the low sodium adsorption ratio in the irrigation water B. inadequate drainage systems to remove soluble salts C. low pH of the irrigation water D. high Ca and Mg contents in the irrigation water E. high contents of sulfates and chlorides in the irrigation water

inadequate drainage systems to remove soluble salts

Which of the following human actions is most apt to result in a long term increase in soil pH?

irrigation with high sodium salt-containing waters

Biomass consists of ______. a. Identifiable biomolecules b. living organisms and cells c. dead tissues and wastes d. polymerized, non-identifiable molecules e. all of the above

living organisms and cells

An acid soil is known to contain toxic quantities of a certain plant nutrient. Which of these essential elements is it most likely to be?

manganese

Which of the following groups of organisms are responsible for greatest amount of organic matter decomposition in soils?

microflora

Compared to the bulk soil in which the earthworms live, earthworm casts contain ____.

more of most plant-available nutrients

Under which of the following conditions would you favor a finely ground dolomitic limestone as your choice of a liming material?

need to provide magnesium in addition to calcium

The drainage of certain wetlands has resulted in extreme soil acidity. This is due primarily to which of the following processes that take place when the soil is drained?

oxidation of sulfer-bearing minerals that produces sulfuric acid

Char is made up of _____. A. living organisms and cells woven through wood ash B. dead plant & animal tissues C. identifiable biomolecules D. pyrolized plant materials E. all of the above

pyrolized plant materials

The addition of large quantities of plant residues to the soil generally ____ the incidence and severity of soil-borne plant diseases. A. increases B. reduces C. has no effect on

reduces

Smectite

shrinking and swelling clay; unlike Kaolinite, have a layer of exposed O at the top and boom planes that create weak links which allow water to move in The flake-like crystals of smectites have a high amount of mostly negative charge resulting from isomorphous substitution.

In most soils the overall population (numbers) of microorganisms changes with time, generally following changes in the _____.

supply of decomposable organic matter

The swelling/shrinking tendency of some silicate clay minerals is due primarily to _____.

the movement of water molecules in and out of the interlayers of the crystal

Mycorrhizae improve the nutrition of many plants by ______.

transporting phosphorus through the soil to the plant root

Kaolinite

used for 1000s of years to make pottery, roofing tiles, and bricks by far the most common 1:1 silicate clay in soils . As implied by the term 1:1 silicate clay, each kaolinite layer consists of one silicon tetrahedral sheet and one aluminum octahedral sheet.