Natural Organic Soil

    Soils are formed through a very complex process involving the nature of the parent bedrock, climate, animals, vegetation, slope of the land-form, and length of time the soil has been in existence. Most of Michigan’s soils were developed from glacial sediments deposited during the Ice Age. As the huge ice sheets moved southward over Michigan, they picked up, eroded, and deposited rocks, sand, gravel, and silt. When the ice receded the material collected in and underneath the ice sheets was left behind. Since then the surface layers have been changed by the action of water, ice, wind, plants, animals, and people. Thus, there are a great variety of soils, and in Michigan, soil characteristics may differ dramatically from region to region, from field to field, or even within a single field.

    Soils are characterised by the differences in the various layers (horizons).  In natural settings, the mineral soil is overlain by the O (organic) soil horizon, which is made up of decaying leaves, twigs, etc.

    The topmost mineral horizon is called the A horizon and contains the partially decomposed vegetable matter called humus, which helps to hold moisture and provide food for plants to grow. In forest soils, the dark A horizon is underlain by a light-coloured E horizon, from which various amounts of clay, Fe and Al have been removed by infiltrating water.  The B horizon (below the E and A) is the zone that accumulates these compounds.  It is often brown in colour.

    The soils of Michigan vary greatly. Sandy soils are dominant in the western and northern portions of the Lower Peninsula; clay and loam, in the southern Lower Peninsula. The size of particles, or texture, varies in different kinds of soil. Soils with a loam texture have a combination of soil particle sizes; there are sandy loam, silty loam, loamy sand, and clay loam.

    Loam soils are best for plant growth because sand, silt, and clay together provide desirable characteristics. First, the different-sized particles leave spaces in the soil for air and water to flow and roots to penetrate. The roots feed on the minerals in the suspended water. Deep sands do not hold moisture well and are often infertile. Clay hold moisture better than sands and may be more fertile, but they tend to swell when they get wet, which may limit the movement of water and roots. Clay crack when they dry and the clods become very hard and difficult to manage (for humans as well as plants!). A desirable soil is a loam with enough sand to drain well yet with enough clay and silt to hold to hold moisture. Silt-sized grains also contain nutrients and help make a soil workable.

    Some soils contain a large amount of partially decayed vegetation (humus) and are called peat or muck. They are often used for growing vegetables because of their high fertility. Thus, the distribution of various kinds of soils in Michigan relates closely to the various types of crops grown and how productive the agriculture is.

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07-07-2017