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6.1.3: Soil Formation and Geography

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  • How do soils form in different places?

    Soil Formation Factors

    Soils around the world have different properties that affect their ability to supply nutrients and water to support food production, and these differences result from different factors that vary from place to place. For example the age of a soil -- the time over which rainfall, plants, and microbes have been able to alter rocks in the earth's crust via weathering-- varies greatly, from just a few years where soil has been recently deposited by glaciers or rivers, to millions of years in the Amazon or Congo River Basins. A soil's age plus the type of rock it is made from gives it different properties as a key resource for food systems. Knowing some basics of soil formation helps us to understand the soil resources that farmers use when they engage in food production. Below are some of the most important factors that contribute to creating a soil:

    1. Climate: climate has a big influence on soils over the long term because water from rain and warm temperatures will promote weathering, which is the dissolution of rock particles and liberating of nutrients that proceed in soils with the help of plant roots and microbes. Weathering requires rainfall and is initially a positive process that replenishes these solubilized nutrients in soils year after year and helps plants to access nutrients. However over the long run (thousands to millions of years) and in rainy climates, rainwater passing through a soil (leaching) leaves acid-producing elements in the soil like aluminum and hydrogen ions, and carries away more of the nutrients that foster a neutral pH (e.g. calcium, magnesium, potassium; see the next page on soil properties for a discussion on soil pH). Old soils in rainy areas, therefore, tend to be more acidic, while dry-region soils tend to be neutral or alkaline in pH. Acid soils can make it difficult for many crops to grow. Meanwhile, dry climate soils retain nutrients gained in weathering of rock -- a good thing -- but may lack plant cover because of dry conditions. A lack of plant cover leaves the soil unprotected from damage by soil erosion and means that dry climate soils often lack dead plant material (residues) to enrich the soil with organic matter. Both dry and wet climate soils have advantages as well as challenges that must be addressed by human knowledge in managing them well so that they are protected as valuable resources.
    2. Parent material: soils form through gradual modification of an original raw material like rock, ash, or river sediments. The nature of this raw material is very important. Granite rock (magma that hardened under the earth) versus shale (old, compressed seabed sediments) produce very different soils. An important example of parent material influencing soils with consequences for human food production are soils made from limestone or calcium and magnesium carbonates. These rocks strongly resist the process of acidification by rainfall and leaching described above. Limestone soils maintain their neutral level of acidity (or pH) even after thousands of years of weathering, and thus can better maintain their productivity. An example of this parent material influence is the Great Valley in Pennsylvania, USA, where the Amish reside. These Pennsylvania soils are considered some of the most productive soils in the U.S. even after hundreds of years of farming. Pockets of other limestone soils the world over are similarly productive over the long term. In summary, as part of learning about a food production systems of a region, it can be helpful to consider the types of rock that occur in that region, which you may want to consider for your capstone regions.
    3. Soil age: the time that a soil has been exposed to weathering processes from climate, and the time over which vegetation has been able to contribute dead organic material, are important influences on a soil. Very young soils are often shallow and have little organic matter. In a rainy climate, young (e.g. 1000 years) to medium aged (e.g. 100,000 years) soils may be inherently very fertile because rainfall and weathering have not yet removed their nutrients. Old soils are usually deep and may be fertile or infertile depending on the parent material and long-term climatic conditions. Soils in previously glaciated regions such as the northern U.S. and Europe are usually thought of as young because glaciers recently (~10,000 years ago) left fresh sediments made from ground up rock materials.
    4. Soil slopes, relief, and soil depth: Steep slopes in mountains and hilly regions causes soils to be eroded quickly by rainfall unless soils are covered by throughout the year by crops or forest. These hilly and mountain regions may also have young soils, and the combination of young soils and erosion can make for soils that are quite thin. Meanwhile, flat valley areas are where the eroded soil is likely to accumulate, so soils will be deep. Along with the water holding capacity and the nutrient content of a soil, soil depth determines how much soil "space" or soil volume a crop's roots can explore for nutrients and water. Soil depth is an important and often overlooked determinant of crop productivity of soils. Moreover, these large-scale "mountain versus valley" differences can be mirrored within a single field, with small differences in topography creating differences in drainage, depth, and other soil properties that dramatically affect soil productivity within ten to twenty meters distance.

    A Summary of Soil Formation: The Global Soils Map

    These four factors along with the vegetation, microbes, and animals at a site, create different types of soils the world over. A basic global mapping of these soil types is given below in Fig. 5.1.2 We've attached some soil taxonomic names (for soil orders, categories used by soil taxonomists) to these basic soil types for those who are familiar with some of the terminology of soil classification. We should emphasize that understanding these orders is not essential to your understanding of food production and food systems, as long as you understand how the basic processes of soil formation described above, and the properties of soils described on the next page, contribute to the overall productivity of a soil. You should think about how the soil formation processes affect crop production in your capstone regions of your final project, and you should be able to find resources on how soils were formed in any place in the United States and around the world.

    global soil map.png

    Figure 5.1.2.: Simplified global soil map classified into broad categories. Credit: Steven Vanek based on USDA world soils map.

    Formation and Management Affect a Soil's Productivity

    Another important point is that soil formation processes described above largely determine only the initial state of a soil as this passes into human management as part of a coupled human-natural food system. Human management can have equally large effects as soil formation on productivity, either upgrading productivity or destroying it. The best management protects the soil from erosion, replenishes its nutrients and organic matter, and in some ways continues the process of soil formation in a positive way. We'll describe these best practices as part of a systems approach to soil management in module 7. Inadequate human management can be said to "mine" the soil, only subtracting and never re-adding nutrients, and allowing rainfall and wind to carry away layers of topsoil.

    The next page adds to this description of soil formation by focusing in on the basic properties that affect food production on soils, like acidity and pH which is discussed above.

    Knowledge Check (fill in the blanks)

    Answer the following questions on basic properties of soil and factors of soil formation.

    1) Fill in the missing words

    The materials that make up the solid part of soils are minerals and matter (i.e. not air and not water).

    2) Drag the words into the correct boxes


    holding water for plants

    Large pores in soil are important for ______ and medium size pores are important for ______.

    3) Fill in the missing words

    The amount of carbon held in soils globally is (larger or smaller) than the amount of carbon in all the earth's forests.

    4) Fill in the missing words

    Soils with what type of rock tend to resist the natural process of acidification that happens to soils in many climates?

    5) Fill in the missing words

    In global comparative terms and considering the global soil map above, a soil in the Northeast United States would tend to be . (young or old).

    6) multiple choice

    Which of the following is likely to have a low level of organic matter and why? A dry climate soil or a wet climate soil?

    • dry climate soil, because plant growth is limited and there is less dead plant material going into the soil

    • wet climate soil, because plant growth is rapid and there is more dead plant material going into the soil