Soil 205- General Soils

Study questions Quiz 5

1. What are all of the important sources and sinks in the K cycle?

Sources: mineral weathering; plant residue/manures; fertilizers

Sinks: biomass removal (crop or timber harvest); leaching; erosion; fixation

2. Explain three “problems” with K nutrition.

-The majority of K is found in mineral (insoluble) form and is not plant available.

-Plants take up a lot of K (luxury consumption).

-K can be leached from sandy soils with low CEC

3. Explain two factors that influence micronutrient availability in soils.

pH- In general, the lower the pH, the greater the availability of micronutrients. Using Fe as an example, as you increase the pH, Fe will go from a relatively soluble form Fe⁺³, to an insoluble (not readily plant available) form Fe(OH)_3.

Mo behaves differently because at higher pH values, it reacts with colloidal surfaces much like P.

-Redox potential- In general, the reduced form of the micronutrient will be more soluble and available. For example Fe⁺² which forms in low O₂ conditions is more soluble than Fe⁺³, the oxidized form of iron.

4. What determines K availability in soils? What are the different mineral forms of K and how are they related?

Availability

-The rate of mineral weathering: K released through mineral weathering is the major source of K into the soil system.

-The return of K through litter fall and decomposition of biomass is important in determining K availability. Decomposition is recycling of K (the plant takes K out of the soil, incorporates it into its biomass, and then K (because of its high solubility) is returned to the soil through throughfall (leaching of the plant canopy) and the decomposition of plant material. When biomass is harvested a lot of K is taken away from the soil and deficiencies can occur.

-CEC: K deficiencies can occur in sandy soils with low CEC due to leaching

Mineral forms:

K in primary minerals à non-exchangeable K à exchangeable K

K is found within the mineral structures of K-feldspars and micas. As minerals weather, the structures loose rigidity and the K becomes somewhat more available (non-exch. K). Finally, as the K is released from the structure of the mineral, it can be held on the exchange sites of colloids and is considered exch. K (readily available). These three forms of K are distinguished by their availability.

5. What are three specific concerns over micronutrient management?

-Increased yields have resulted in greater removal of micronutrients from the soil; deficiencies are becoming more common

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High-analysis fertilizers (low impurities); at the same time that more micronutrients are being removed from the soil, lower amounts of micronutrients are being added to the soil

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Very narrow margin between deficiency and toxicity

6. In your own words (chemical reactions are not required) explain why most micronutrients are more available at low pH. Why does Mo behave differently?

As the pH is increased, Fe will go from a relatively soluble form (Fe⁺³), to an insoluble (not readily plant available) form (Fe(OH)₃). Mo behaves differently because at higher pH values, it reacts (is fixed) with colloidal surfaces much like P.

7. In what type of conditions are micronutrients most often found to limit plant growth?

-highly leached, acid, sandy soils

-organic soils

-soils of very high pH

-soils that have been very intensively cropped and fertilized only with macronutrients

8. What is a chelate and how does it increase micronutrient availability?

A chelate is a chemical compound where a metallic ion is firmly

combined with an organic molecule by the means of multiple chemical bonds. It increases the micronutrient availability by protecting it from reacting with inorganic colloids.

9. Explain the shape of the following graph showing plant growth vs. micronutrient concentration. Plant growth initially increases as the micronutrient concentration increases (the deficiency is corrected). Eventually a point will be reached where the micronutrient is sufficient and the plant growth will level off (this is the point that should be targeted when applying micronutrients to soil). Eventually, if further micronutrient is added, it will become toxic to the plant and the growth will drop off. A major concern over micronutrient management is that the range between deficiency and toxicity is very narrow. For example, the application of as little as 3-4kg/ha of Boron to an initially deficient soil can cause B toxicity.

		Concentration of available nutrient in soil

10. Explain the four major goals of nutrient management.

1) cost-effective production of high quality plants: How this goal is met will depend on what is being produced. In traditional agriculture we would be interested in relatively short term increases in biomass which requires intensive management. For forestry we are going to be interested in either the production of biomass for harvest or the use of forests for wildlife and/or recreation. Since both of these uses require a longer time frame, trees may be harvested approximately every 10 years instead of every year like traditional ag., the management tends to be somewhat less intensive. In the production of ornamentals, we may want to manage the soil for plant quality (flower production), instead of biomass production.

2) efficient use and conservation of nutrient resources: Two keys to achieving this goal are to reuse/recycle nutrients and to balance our nutrient inputs and outputs. Examples of the reuse of nutrients include- application of manure to ag. Fields; irrigation with secondary treated sewage water; and leaving crop residue to decompose in the field.

Balancing inputs and outputs: When outputs are greater than inputs, the soil quality is being degraded since there is a net loss of nutrients from the system. When inputs are greater than outputs, the system is not economically efficient and extra N or P may be lost to the environment, decreasing water quality.

3) maintenance or enhancement of soil quality: the goal here is to go beyond just supplying enough nutrients to meet this year’s crop demand and to improve the ability of nutrient supplying and cycling ability of a soil. An example would be increasing organic matter levels by no-till farming.

4) protection of the environment beyond the soil: Achieved through the development and implementation of best management practices (BMPs).

11. What are three ways to protect environmental/water quality outside of the soil?

Specific example of BMPs to protect surface water quality include:

a. the development of buffer strips- dense vegetation planted near water bodies slow down water and allow sediment to drop out before reaching the water. The plants also take up extra nutrients.

b. the use of cover crops: cover crops increase the organic matter in the soil and therefore increase infiltration rates and decrease runoff. If runoff does occur, the presence of vegetation slows down the water and filters out sediment and nutrients.

c. selective timber harvest: streams that drain clearcut areas are often very high in nutrients. When the forest canopy is destroyed the soil temperature goes up and decomposition is high. The increase in decomposition releases nutrients and since there are no active roots left to take up these nutrients they are lost to surface water. Selective harvest keeps the litter layer intact and keeps live roots in the soil to take up any extra nutrients.

12. List and compare (in terms of nutrient supplying capacity) several organic and inorganic nutrient resources.

Typical inorganic fertilizers contain N, P, and K, in some combination, or alone. Inorganic N fertilizers range from 82% N for anhydrous ammonia to 11% for ammonium phosphate. Typical organic fertilizers such as manure or sewage sludge contain only about 2-4% N. Inorganic fertilizers tend to available right away, while nutrients from organic fertilizers are released over time through decomposition. Organic fertilizers can be considered slow-release fertilizers.

13. What are three things that can be used to better match the application of N fertilizers with plant uptake?

- split applications of fertilizer- apply ½ in fall, ½ in spring

- nitrification inhibitors- keep N in the NH₄⁺ form so it is not lost to leaching before plants need it

- slow release fertilizers- N is released slowly over time

14. What are the different application methods for liquid, solid, and gaseous fertilizers?

Solids-

Broadcast: fertilizer is spread across the top of the field and incorporated with tillage (except in no-till systems).

Banding: fertilizer is placed in soil near the seed during planting

Perforation: fertilizer is inserted in a hole that extends to the subsoil. The hole is within the influence of the tree’s canopy. This method is used exclusively for tree crops.

Liquids-

Applied through the irrigation system (either drip or sprinkler); sprayed on or injected into the soil; or sprayed directly on the plant leaves

Gasses-

Injected into the soil or dissolved in irrigation water

15. What is the Haber process?

The Haber process is the reaction through which ammonia is produced. The ammonia can either be applied to soils, or is combined to form other N fertilizers. It is carried out at extremely high temperatures and pressures and requires a significant input of fossil fuels.

16. List the possible application methods for each of the fertilizers in the following table.

Fertilizer	Method(s) of application
Ammonium nitrate	Broadcast, banding, perforation
Urea	Broadcast, banding, perforation
Anhydrous ammonia	Injected or dissolved in irrigation water
Aqueous ammonia	Sprayed on soil or plant, added to irrigation water

17. How do the two major different sampling schemes differ from each other?

Single rate- samples are taken either randomly or by using a grid system, the samples are combined and homogenized and one single subsample is sent for chemical analysis. Fertilizer is applied at one single rate (determined by the soil test) to the entire field.

Variable rate- samples are taken from geographically referenced locations across a field, usually following a grid pattern. Each sample is analyzed separately. This method allows a grower, using specialized computer equipment, to alter the rate of fertilizer application across the field. Although it is more labor intensive and requires expensive equipment, this method allows for much more efficient application and use of nutrient resources.

18. What is soil quality?

Soil quality is the capacity of a soil to function within and outside of its ecosystem boundaries to sustain biological productivity and diversity, maintain environmental quality, and promote plant and animal growth.

19. How does industrial and economic growth influence the importance of the various functions of soil?

In the industrialized countries functions such as protection of air and water quality and maintenance of wildlife habitat are more heavily weighted than in developing countries where poverty is more common. In these countries, food and fiber production is the most important function.

20. What is the significance of the “Green Revolution”?

The “Green Revolution” is a term used to describe the time period (starting about 1950) when there was a major shift in production practices worldwide. In response to predictions of mass starvation following the population explosion (after WWII), scientists and farmers worked together to increase food production. Global grain production increased ~3x’s during this period and the price of food declined. The intensification of production practices averted starvation across the globe (except for Sub-Saharan Africa). The Green Revolution produced a new paradigm in agriculture: for the first time we were increasing production by increasing yield/ac instead of simply increasing the number of acres farmed. Although we avoided mass starvation, intensification of our agroecosystems had major impacts on soil quality.

21. How were we able to increase yields/ac during the Green Revolution?

-increased breeding (improved cultivars)

-more efficient irrigation practices

-use of inorganic fertilizers

22. What were the positive and negative impacts of the Green Revolution on soil quality?

Positive

-increased macronutrients

-increased plant production = increased plant residues

-reduced pressure on marginal lands

Negative

-micronutrient deficiencies

-excess nutrients (decreases air and water quality)

-salinization

-pesticides decrease biological diversity

23. Describe the three major factors that will determine future efforts to protect our soil resources.

1. population and economy drive demands for food and fiber- It is projected that 2.5 billion more people will be living on earth over the next 25 years. Approximately 90% of these new inhabitants will be living in developing countries. Increases in population will increase the demand on our current agricultural lands. As the economies grow in developing countries they will become more urban. With urbanization comes increased caloric consumption per capita and the increase of more animal products in diets. It takes more grain to feed animals to feed humans than it does if humans were to consume the grain themselves.

2. Extent of clearing of grasslands and forests- smaller gains are expected in the future from increases in fertilizer and irrigation technology. Therefore, as population grows, we will be forced to once again increase yields by adding more farmed acres. This will result in the cultivation of some of our more fragile (less resistant) ecosystems and further degradation of the soil resource. This problem will have global ramifications in that there is little arable land remaining in some developing countries where population growth is the largest. More pressure will be placed on areas such as America’s “breadbasket” to meet global demand.

3. Universal determination to stop/reverse environmental degradation- With industrialization comes the ability to focus on other issues than food and fiber production. The challenge we are now faced with is to increase food production by the same magnitude as during the Green Revolution, while protecting air and water quality.

24. What is the major objective of soil survey?

To identify the distribution of soil bodies across a landscape. The soil survey can be used for inventory purposes and for making land use decisions.

25. Why do we use soil mapping units? How are the five different soil mapping units defined?

Soil mappings are used as a tool to help us deal with or encompass the natural variability of soils across a landscape.

1. consociation- one principle soil type with allowable inclusions. To be an allowable inclusion, the soil must be less than 15% of the area.

2. and 3. complex and association- both of these soil mapping units contain two or more dissimilar soil types that occur in a characteristic pattern. The difference is that in a complex the soils can’t be separated at the 1:24,000 scale. In an association, the soils can be separated at the 1:24,000 scale, but do not make the requirements for consociation, so they are mapped together.

4. Undifferentiated Soil Group- soil mapping unit that contains two or more dissimilar soil types, that could be separated at the standard 1:24,000 scale, but it is not worth the time or money to do so. They are mapped together due to some common factor that limits landuse.

26. What are land capability classes? Which classes are suitable for agriculture?

Pasture? Which classes are best to keep as wildlands?

A LCC of I to IV indicates that the land could be used for production agriculture. A rating of III or IV doesn’t mean that a soil shouldn’t be used for agriculture, but that it will require much more intensive management than a soil with a LCC of I or II.

Land capability classes or the LCC is a grouping of soils based on their limitations for agriculture. They range from I to VIII.

I = prime agricultural land no limitations

II = prime ag. land with slight limitations

III = ag. land with moderate limitations

IV = ag. land with severe limitations

V = slight limitations

LCCs of V – VIII should be kept in permanent vegetation (grasses or forests). Classes V and VI can be used for pasture. Classes VII and VIII should be kept in native vegetation and can not support pasture.

VI = moderate limitations

VII = severe limitation

VIII = keep as wildlands

27. What are the possible subclasses to the land capability class?

e = erosion hazard

w = water hazard

s = soil limitation

c = climate limitation

28. Give an example of two uses of soil surveys.

Developers and perspective homeowners may use a soil survey to see if the land they are interested in buying will support their needs (housing, housing with basements, skyscrapers, etc)

Researchers use soil surveys to locate areas with the ideal conditions for an experiment.

Land use planners can use soil surveys to determine the best locations for playgrounds, schools, landfills, etc. in a county.