Soil Science Lecture Soil is the interface between the geosphere and the biosphere and thus plays a unique role in terrestrial ecosystems Ecosystem Services Provided by Soil Soil provides habitats for many organisms, both macroscopic and microscopic. These include bacteria, fungi, insects as well as vertebrates such as gophers. Soil also allows for the cycling of nutrients through terrestrial ecosystems. As soil organisms break down decaying organic matter, these nutrients are returned to the soil where plants can then take them up again. Soil provides a substrate for plant growth. Most land plants require something for their roots to grow into, so most plants would not be able to exist without soil. Soil filters water as rainwater passes through it on its way to rivers and groundwater. Formation of soil Weathering is the primary process that forms soil. Weathering breaks down the parent material, which is rock, into smaller particles. These small particles of mineral or rock combine with decomposing organic material to form soil. As a result, soil is neither completely biotic nor completely abiotic. There are three types of weathering that can contribute to soil formation. Physical weathering consists of water or wind separating piece of the rock. It can also occur when water in a crack in the rock freezes, wedging the rock apart. Chemical weathering occurs when the rock reacts with surrounding materials, such as when a weak acid dissolves part of a rock. Biological weathering is weathering caused by organisms such as tree roots wedging a rock apart or lichens secreting acids to dissolve a rock. Weathered material can be transported by wind or water or it can remain in situ Factors that influence Soil Formation There are a number of factors that can influence the quality of soil that forms in a particular location and how quickly that soil forms.
#1 Climate: Temperature and humidity will influence the speed of weathering. For example, in very cold climates, there will be little water movement, so physical and chemical weathering will proceed slowly. As a result you will have underdeveloped soils that do not contain a lot of minerals or rocks. They will mostly contain poorly decomposed organic material since biological activity is also slow at these temperatures. #2 Organisms: Burrowing animals can help to mix soil and distribute organic and inorganic components. Plants can secrete acids that can help to speed chemical weathering. #3 Topography: steeper slopes will lose soils due to erosion, so will not have very developed soils, whereas valleys will accumulate large amount of soils. Wind exposure or heavy rains on one side of a hill/ mountain could speed weathering as well. #4 Parent Material: The chemical composition of the parent material (ei. basalt, limestone, granite) determines how quickly it will weather and will determine the mineral content and many other properties of the soil. #5 Time: The more time has passed, the more developed soil will become. Old soils with productive ecosystems and moderate rainfall can become very deep and fertile (such as grasslands). However, a soil with heavy rainfall maybe less fertile due to the leaching of nutrients from the soil. Regions Differ in Soil Characteristics Many cultures throughout human history have classified soil to communicate its value for growing crops. Current soil classification is based on particular properties of soil. These properties are so specific that in some cases they can be used to pinpoint the exact location that soil came from. Types of soil are named with the suffix –sol. For example, the top photo to the right represents a mollisol, a fertile, thick grassland soil. The bottom photo is an oxisol, which is a highly weathered and oxidized soil with low fertility. It can be found in a tropical rainforest. While you do not need to know the names of the different types of soils, you should know that words ending in – sol are particular types of soil. Soil Horizons One of the ways that we classify soils is by the layers, or horizons. Not all locations will have all horizons so the number of horizons present and the thickness of the layers in certain areas is one way of characterizing soils O horizon: O stands for organic. This layer is sometimes called humus and consists of decaying organic material. It is most pronounced in forest ecosystems. Below the O horizon, the layers go in alphabetical order with some exceptions.
The A horizon is generally below the O horizon. The A horizon is a zone where minerals and organic material have been mixed together. Another name for it is topsoil. In some ecosystems, there will be no O, so the A horizon will be at the top. Below the A horizon is the B horizon, also called the subsoil. This layer contains mostly inorganic material and is where nutrients will accumulate. The B horizon is present in all soils. The C horizon is below the B horizon and is the least weathered layer of soil. It consists of pieces of the parent material. The R horizon is below the C horizon and is bedrock. R stands for Rock. There are some soils that can have extra horizons because of particular properties of the soil. The E horizon is a zone of leaching that can sometimes be found above the B horizon in acidic soils. (E stands for eluviation)This is a zone of leaching. Due to the soil’s acidity, material is more likely to dissolve and be transported down to the B horizon. Properties of Soil In addition to looking at the horizons present in certain areas, soils can be described and classified based on a number of physical and chemical properties. This can tell us its suitability for agriculture and help in remediating the soil to rehabilitate damaged ecosystems Soil Texture Soil texture is determined by the amount of sand, silt and clay. These three types of particles differ in both size and composition and determine many other properties of soil. Sand is the largest particle, followed by silt and then clay. Soil scientists can use a soil triangle to classify soils into groups. Try the following examples: What is a soil called that is 40% sand, 40% silt and 20% clay? Loam (very fertile soil) What is a soil called that is 60% sand, 10% silt, 30% clay? Sandy clay loam What is a soil called that is 33% sand, 33% silt, 33% clay? Clay loam Porosity & Permeability Both porosity and permeability are properties that are directly determined by the texture of the soil. Porosity is the amount of space in between the grains of sand, silt and clay that
make up the soil. Porosity is important because it determines the ease with which water, oxygen, and nitrogen can work their way down between soil particles to the root zones of plants. It also determines how quickly & easily water can soak in and become groundwater in an aquifer. Permeability describes how easily water passes through soils. It is influenced by the porosity and can be measured with a percolation test. If the permeability is too low, the soil can become water-‐logged, but if it is too high, the soil can’t retain enough water for plant growth. As a result the best soils for plant growth have medium-‐size pores or a mixture of pore sizes. Chemical Properties of Soil Plants need three main nutrients from the soil: nitrogen, potassium and phosphorus. On fertilizer labels you will see three numbers indicating the ratios of these nutrients. Deficiencies in one or more of these nutrients can limit plant growth, so soil scientists and farmers will monitor the levels of these three nutrients and add them in the form of organic or chemical fertilizers if necessary. Soils can have a variety of pH levels which can influence the types of plants that can grow there and the nutrients available to those plants. As I mentioned previously, highly acidic soils can exhibit leaching of minerals from the O and A horizons so they will not be available to plants. Soil Erosion Environmental Scientists study these properties of soil because it tells us about the health of the ecosystem. In many ecosystems the soil is being eroded or degraded which will have future ramifications on the health of the ecosystem. Erosion can be measured by inserting fixed pins (see picture) for a certain amount of time and then measuring the amount of soil loss. This technique has also been useful in quantifying strategies to reduce erosion. According to these measurements, 5-‐7 million ha (12-‐17 million acres) of productive cropland are lost annually worldwide. Causes of Soil Degredation Over the past 50 years, soil degradation has reduced global grain production by 13% Currently 70% of the world’s rangeland is classified as degraded
We will address these causes later in the unit as we talk about specific ways that we use the land Promoting Soil Conservation Legal measures can be used to prevent the degradation and loss of soils. The US government enacted the Food Security Act of 1985 to preserve soil. It authorizes farmers to receive price supports and other benefits if they adopt soil conservation practices. The Conservation Reserve Program (1985) is similar in that farmers are paid to place highly erodible land into conservation reserves. Essentially, they are paid to stop farming on some of their land and plant trees and grasses. This program saves an estimated 771 million tons of topsoil per year while generating income for farmers and promoting biodiversity by providing habitats for native wildlife.
