Soils. Slide 1. Slide 2

Transcription

1 Soils Slide 1 Slide 2 In this section we will review basic definitions of soil, describe common visual features found in soil, briefly discuss soil genesis or formation, and introduce some valuable online resources you can use to learn more about soils in your area. Slide 3 Let s begin with a definition of soil. Wikipedia, the web-based encyclopedia, states that soil is natural, composed of unconsolidated rock particles and organic materials, and capable of supporting life. This definition only scratches the surface. Soils are also dynamic systems in which energy (sunlight, heat), carbon (plants, organic matter), gasses, water and other materials constantly enter, move through, and exit. Soils are also teeming with life from large animals to microscopic bacteria. Across a region or even a small landscape soils can vary in physical, chemical and biological properties. Finally, soils are manageable. Humans can alter either positively or negatively many soil properties. This Master Gardener module emphasizes understanding and improving soils as media for plant growth. Slide 4 As a medium for plant growth soil has four main roles. Soil is a reservoir of mineral nutrients and water. Soil also supplies air or, more importantly, oxygen to plant root systems. Finally, soil provides a base for the physical support of plants. The quality of soil as a medium for plant growth depends on how well soil fulfills these basic roles. Slide 5 One convenient way to think about soils is on the basis of their composition. This pie chart shows the composition in percent by volume of an idealized or textbook soil containing 45% soil mineral particles, 5% organic matter, 25% air and 25% water. Slide 6 The four main roles of soil as a medium for plant growth can be related to each component of soil. The soil mineral fraction is composed of small inorganic particles that constitute the primary architecture, or texture, of soil. These particles retain and release nutrients for plant growth. Soil organic matter contributes to the formation of soil structure and also provides nutrients for plants. Soil air supplies plant roots and other soil organisms with oxygen. Soil water transports nutrients to roots and supplies plants with water for transpiration. Each component of soil will be discussed in more detail later. Page 1 of 6

2 Slide 7 Note a few important implications of this diagram of an ideal soil. First, the ideal soil is composed of only 50% solid material by volume. Collectively, the air and water volume is called pore space and together makes up the remaining 50% of the soil volume. Thus, soil is a very porous medium. Second, the line between the solid and pore space in soils can shift if the soil is compacted. This has significant implications for plant growth, since compaction reduces pore space capable of containing air or water. Third, the line between the air and water components is also dynamic and will shift as rain or irrigation events occur. If soil pores are filled with water for an extended period of time, plant roots have limited access to oxygen, resulting in increased incidence of disease and other growth problems. Slide 8 Soils commonly vary with depth in horizontal layers called horizons. A soil profile includes all of the horizons from the surface down to bedrock or other material from which the soil formed. The horizons are given letter designations beginning with A or O (organic) at the surface, followed by E, B and/or C horizons below. Topsoil is the A horizon and subsoil the remaining horizons below A. Slide 9 Soil formation is influenced by five factors: time, parent material, organisms, climate, and topography. Each factor is complex and can have several states or conditions. There are also many interactions among the factors. For example, consider the diversity of climates in terms of temperature and moisture from eastern to western Washington. Now consider how the diversity of climates will influence the organism or vegetation factor. We will briefly review each factor to help you understand their influence on soil formation. Slide 10 A soil profile may take thousands or even tens of thousands of years to form. This is important to keep in mind when you want to alter major soil properties in an afternoon of gardening, or even a single growing season. Generally, older soils are more developed with more horizons and complex features within the profile. This is not always the case, however, since soils in very cold or dry regions develop at a slower pace than soils in warmer, wetter regions. Slide 11 As the name implies, parent material is the material from which soil formed. Parent materials are altered or weathered through both chemical and physical processes to form soils. The main influences of parent material on soil are through the size of particles and their mineral composition. In general, soils that form from coarse parent materials like glacial till or colluvium tend to be composed of larger inorganic particles or a coarse texture. Soils that form from small particles like wind blown loess or lake sediment tend to be composed of smaller inorganic particles or a fine texture. Many different types of Page 2 of 6

3 parent materials are found across Washington due to the complex geological history of this state. Slide 12 Organisms influence soil formation mainly through the location and amount of organic matter added by plants, the decomposition of this organic matter, soil mixing, and weathering of rocks and minerals. Consider how much organic matter is added annually and where this organic matter is added in soils from grassland ecosystems compared to forests. The surface of soil profiles from grassland ecosystems commonly have a thick, dark A horizon indicating large amounts of organic matter have been added to this layer by plants over a long period of time. In contrast, forest soils typically have an organic layer (O horizon) at the surface where forest litter falls and collects. Slide 13 Climate influences on soil formation are related mainly to temperature and moisture. Together, temperature and moisture drive most chemical, physical and biological processes or reactions. Water is also responsible for moving materials through soil. For example, in warmer, drier climates, horizons with high concentrations of lime (carbonates) are found near the surface due to inadequate movement of water and soluble carbonates through the soil profile. The presence of lime buffers arid land soils in the alkaline ph range. In areas with higher precipitation, the depth to the carbonate layer is greater. In areas with high rainfall, there is sufficient water to leach lime completely out of the soil profile and soil ph is commonly neutral or acidic. Slide 14 Topography influences soil formation mainly through slope and the potential for erosion and deposition of materials; and aspect, which moderates the effects of climate. Steep slopes often lose soil material through erosion. This material may then be deposited at the base of the slope. Consequently, surface horizons are thinner on slopes and thicker at the base of slopes. South-facing aspects are often warmer and drier than north-facing aspects, particularly in the northern hemisphere. Slide 15 Now let s look at a few soil profiles from different locations in the United States. The tape measure pictured in each profile indicates the depth in feet from the surface. For each profile, where do you think soil is from, and what do the visual features indicate about the quality of each soil? For this profile, consider the depth and color of the A horizon. Is this a high quality soil? What visual features lead you to this conclusion? Slide 16 Relative to the previous example, consider the depth and color of the A horizon in this Page 3 of 6

4 profile. What are the white deposits found in this profile? At what depth do they occur? Slide 17 Again, relative to the previous examples, consider the depth and color of the A horizon in this profile. A continuous layer of white material has replaced the discrete white deposits. Note the depth at which this layer occurs compared to the previous slide. Compare the quality of this soil to the previous examples. Slide 18 Finally, relative to the previous examples, consider the depth and color of the A horizon in this profile. A cemented layer of lime or caliche has now replaced the layer of white material, and this is found very near the surface. Slide 19 These four soil profiles represent a climate transect from the Western to Midwestern U.S. The influence of precipitation on soil formation is clearly evident in these profiles. The A horizon is much more developed in the high precipitation area due to more vegetation and organic matter inputs. The carbonate layer is much closer to the surface and more developed in the Western U.S. profiles. Slide 20 Let s turn our attention to Washington State. Due to diverse climate, parent materials, vegetation, topography and age, Washington has a wide variety of soils. Use the interactive map included as part of this module to view examples of profiles and learn more about the history and basic properties of Washington soils. Other print and Internetbased resources on soil are also available. See the list of these resources at the end of this session. Slide 21 To conclude this module, we introduce you to a valuable resource through which you can learn more about soils in your area. The Web Soil Survey was created and maintained by the USDA Natural Resources Conservation Service (or USDA NRCS). Using this database, you can identify a specific location virtually anywhere in the U.S., determine which specific soil types exist at this location, and learn more about chemical and physical properties of the soil as well as the suitability of the soil for a variety of uses from agriculture to building construction. Slide 22 The Web Soil Survey contains information previously found in printed soil surveys created by the USDA NRCS. This information is now available in an easy-to-use interactive format available to anyone with a computer and Internet connection. Three tutorials one each featuring examples from western, central and eastern Washington are provided to assist you in navigating the Web Soil Survey. Page 4 of 6

5 Slide 23 The USDA Natural Resources Conservation Service or USDA NRCS created Web Soil Survey to enhance access to soils information formerly contained in printed survey documents. With this database you can identify a specific location anywhere in the U.S., determine which soil types exist at this location, and learn more about chemical and physical properties of the soil as well as the suitability of the soil for a variety of uses from agriculture to building construction. The purpose of this tutorial is to guide you through the basic navigation steps necessary to identify and obtain information about soils using Web Soil Survey. To begin, open your internet browser and type in the address: websoilsurvey.nrcs.usda.gov or search for Web Soil Survey. The opening page contains background information as well as several how to and tips for using Web Soil Survey. When ready, click Start Web Soil Survey to begin your search. The first tab in the upper left corner is Area of Interest and is used to locate a site. This can be done in various ways, including zooming in on an area using the interactive map, or entering the exact physical address or longitude and latitude coordinates for the area. We ll use the physical address for this exercise and focus on the Washington State University main campus near Pullman. Once you ve entered the address, an aerial map appears with a symbol identifying the exact location of the address on the image. The next step is to define your area of interest. Use the rectangle or polygon tool to outline the area for which you are interested in obtaining information. Once outlined, the program calculates the number of acres in the area of interest. If you need to clear the area of interest and define a new one, you can do so at this stage. Next, click on the Soil Map tab to generate an overlay map in the area of interest. Specific soil types are identified by map unit symbols. A summary table titled Map Unit Legend is also generated, showing the name of the soil series for each map unit symbol as well as the number of acres and percent of the total area occupied by each soil series in the area of interest. Once you have identified the area of interest and map unit symbols, you are ready to continue to the Soils Data Explorer tab, where specific information about each soil is located. You have several options in Soils Data Explorer. If you want to learn more about a general topic, click the Intro to Soils tab. Here, click the topic of interest and then click View Selected Topics to learn more. To obtain information about the suitability of soils for various uses, click on the Suitability and Limitations for Use tab. The series of lines in the left column identifies the types of information accessed through this tab. If, for example, you want to know which soils in the area of interest are suitable to construct a dwelling with a basement, click Building Site Development and Dwellings with Basements. If you want to know more about how this suitability rating is defined, click View Description. To view ratings for soils in the areas of interest, click View Rating tabs. A color-coded overlay appears on the soil map for the area of interest. To interpret the codes, click the Map Page 5 of 6

6 Legend tab. A summary table of suitability for each soil series in the area of interest also appears at the bottom of the map. To view information about other soil properties, click on the Soil Properties and Qualities tab. Data for several soil chemical and physical properties are available under this tab. For example, let s assume we are interested in information about ph. Click the Soil Chemical Properties line and then ph. Enter the depth to which you want to map ph in the area of interest. In this case I ve just marked surface layer. Similar to the suitability information, a map overlay appears on the soil map for the area of interest. To interpret the codes, click on the Map Legend tab. Again, a summary table of ph for each soil in the area of interest appears at the bottom of the map. As you navigate through Web Soil Survey all maps, summary tables, and narratives can be added to a shopping cart and later downloaded for free in a PDF format as a custom soil resource report. This concludes this brief introduction to Web Soil Survey. We hope you will use this tool to learn more about soils in your area and as you assist others in your role as a Master Gardener. Slide 24 Your assignment is to use the Web Soil Survey to discover information about the soils in your home landscape, farm, or local city park. Identify the soil series name and read more about the horizons and soil profile, chemical and physical properties of the soil, and limitations on its use. We think you will find this to be a valuable resource for personal use and as you assist others in learning about soils in your role as a Master Gardener. Page 6 of 6