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4.3.4: Chapter Summary

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    Essential elements are indispensable elements for plant growth. Plants can absorb mineral nutrients and water through their root system, and carbon dioxide from the atmosphere. Mineral nutrients are divided into macronutrients and micronutrients. The macronutrients plants require are nitrogen, potassium, calcium, magnesium, phosphorus, sulfur, and silicon. Important micronutrients include chlorine, iron, boron, manganese, sodium, zinc, copper, nickel, and molybdenum.

    Soil consists of organic and inorganic material as well as water and air. The organic material of soil is made of humus, which improves soil structure and provides nutrients. Soil inorganic material consists of rock slowly broken down into smaller particles that vary in size, such as sand, silt, and loam. Soils form slowly as a result of biological, physical, and chemical processes. Soil is not homogenous because its formation results in the production of layers called a soil profile. Most soils have four distinct horizons, or layers: O, A, B, and C. Their composition is influenced by the climate, presence of living organisms, topography, parent material, and time.

    The chemical elements that organisms need continuously cycle through ecosystems. Cycles of matter are called biogeochemical cycles, or nutrient cycles, because they include both biotic and abiotic components and processes.

    In the carbon cycle, carbon enters the soil through weathering of rocks and leaches into bodies of water. Aquatic organisms produce calcium carbonate, which ultimately forms ocean sediments. Through the process of uplifting, ocean sediments return to land. On shorter time scales, photosynthesis removes carbon dioxide from the atmosphere and converts it to organic carbon. Through aerobic cellular respiration by plants, animals, decomposers, and other organisms, organic carbon breaks down, releasing carbon dioxide back into the atmosphere.

    Nitrogen gas in the atmosphere cannot be used by plants. Through nitrogen fixation, bacteria convert nitrogen into ammonium, which can then be converted to nitrites and nitrates through nitrification. Both free-living and symbiotic bacteria, some of which form root nodules, fix nitrogen. Plants can asssimilate ammonium and nitrates, making organic nitrogen available to consumers. Decomposition of organic matter releases ammonium back into the soil through the process of ammonification. Denitrifying bacteria release nitrogen gas from unused nitrates back into the atmosphere.

    The phosphorus cycle is a simpler process than the nitrogen cycle. Phosphates enter the soil and water through weathering of rocks, where they can be assimilated. Assimilation and decomposition allow phosphates to cycle in terrestrial and marine ecosystems. Some phosphates are leached from the soil ultimately into the ocean, where the sediment, making ocean sediments a reservoir for phosphorus. Uplifting can move phosphorus-containing rocks to land.

    After completing this chapter, you should be able to...
    • List the essential elements required by plants and summarize their functions.
    • Describe how plants obtain nutrients, including the mechanism of cation exchange.
    • Distinguish between macronutrients and micronutrients.
    • Identify and describe each component of soil.
    • Distinguish among sand, silt, and clay and explain how particle size influences water holding capacity and soil texture.
    • Describe each horizon in a typical soil profile.
    • Explain how soils are formed, describing each of the five major factors that affects soil formation and composition.
    • Explain what determines soil pH and how soil pH affects nutrient absorption by plants.
    • Explain the processes of carbon, nitrogen, and phosphorus cycling.
    • Explain the process by which rhizobia infect legumes and form root nodules.


    Curated and authored by Melissa Ha using the following sources:

    This page titled 4.3.4: Chapter Summary is shared under a CC BY-NC 4.0 license and was authored, remixed, and/or curated by Melissa Ha, Maria Morrow, & Kammy Algiers (ASCCC Open Educational Resources Initiative) .

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