Geog 317 - Geography of Soils
Soil Survey Assignment

The purpose of this assignment is to become familiarized with County Soil Surveys and their uses while making a practical assessment of the various soil forming factors, and the types of soils they produce, in a chosen area of California. The information needed to write this report should come from (1) a single soil survey, best derived from downloaded SSURGO data, but some are available in the map library in printed form (2) field work (sampling and other observations during a visit to the area), (3) your textbook, (4) lecture notes, and (5) other maps (you might find road maps, topographic maps, and geologic maps useful).  

This is a group project. You should work with two other students on this. You should preferably have one member who is reasonably proficient in ArcGIS -- having completed or currently in Geog 603 should be enough. A good group then might have other members who are primarily responsible for other needs, such as photography, soil color and sample analysis, planning field excursions for the group, poster design, etc.

The first thing you need to know is what soil surveys are available.  Some areas you might be interested in are not, though they are increasingly becoming available as a download, both the scanned documents with all of the interpretative narrative, and as SSURGO data and maps.  You will need to download your soil survey, or try to get a printed document. We have some printed surveys in the map library (here's a list but it needs updating), but they are not for checkout, or Government Documents in the main library. Note that many of the printed surveys are currently out of print, though many have been scanned. If you really want a printed version, for the county you choose, try the phone book for a county office of the NRCS (part of USDA), and you might be able to get one for free, especially if it's a recent survey, but I expect they'll refer you to the digital sources. Otherwise, the USGS can sell you one, or the state office of the NRCS in Davis may also. You can check out the ones in Government Documents in the main library, but the collection in the map library is for use only in the library.   

Soil Data: Surveys and SSURGO data

sonoma.overall.thumb.jpg (10065 bytes)
A (kind of long) soil profile poster

However you gain access, you'll need to sign up for a given soil survey to make sure each group does something different from the others.  I'll put a signup sheet on iLearn where you should list all of your names.  Do not choose a survey already chosen.  (If two groups really want to work on the same soil survey, you'll need to make sure you are doing different areas before getting approval.)

You should then read through and take notes from several sections of the soil survey: (1) the section on the General Nature of the Area (Physiography, relief, drainage, geology, geomorphology, development, water supply); (2) any sections on climate; and (3) the section discussing the Formation, Morphology, and Classification of the Soils. Refer to the generalized soil map provided in your survey (either bound or loose, often in a back sheet folder). Glance through the other sections of the survey to see how it's organized. You will find that information related to soil forming factors and properties is scattered throughout the survey, but much is repetitive. The intended users of these surveys differ sufficiently that much of the information is repeated, with the emphasis of specific characteristics varying (e.g., presence of shrink-swell clays may be mentioned in several sections, but it is emphasized in sections dealing with the use of soils in building or engineering).

Please note: some soil series classifications have changed, especially with older surveys (pre-1970) or in areas of volcanic rock (a new order -- Andisol -- was added just a few years ago, and there are andisols in California), so you will need to refer to a source of the current classification. The best source of information is the Natural Resources Conservation Service's Soil Series Descriptions web site at NATIONAL OFFICIAL SOIL SERIES HOMEPAGE. Make sure to see the current descriptions for all of your soil series. Download all of your soil series from this site, even if your survey is recent.

Once you have a general understanding of the patterns, characteristics, and causes of the soil types in your area, you are ready to plan a more detailed look. What you will do is discuss the variations in soils in the area as they occur on a transect through the area. I would recommend a straight-line transect, but you might choose a road or trail which traverses the area, but avoid roads that follow river valleys because the soils vary little along them.  The direction of the transect or route will vary with the area chosen, but for most California areas an E-W transect is most interesting for soil variations. Note: crossing a stream valley -- especially a wide valley with a flood plain -- often creates a nice variation in soils, as you progress from residual to cumulic alluvial soils and back.

For this project, you'll create a poster to display in our classroom on the last day of class.  The poster should include the following (each is described further below):

  1. The transect displayed as a 2-dimensional section, with soils and features labeled
  2. A map showing the transect
  3. A one-page analysis of soil variations along the transect
  4. Photographs of landscapes and soil profiles along the transect
  5. Soil samples (in small bags attached to the poster)
  6. Other poster elements:  title, your names, etc.

Transect

sisk.overall.thumb.jpg (9631 bytes)Your transect (see figure) should show the variations in soil types along your transect.  Use the soil survey and the NATIONAL OFFICIAL SOIL SERIES HOMEPAGE to identify the extent and types of soils along the transect. 

In order to complete these transects, you will need to refer to the photomaps supplied with the survey (either loose or bound), the table which explain what the map symbols mean, the table showing the Soil Taxonomy classification for these soils, the individual soil descriptions (in which are discussed the typical parent material, natural vegetation, relief controls, climate, and uses of the soil series), and the discussion of major soil associations also found in the survey. You will find the seeming complexity of the area starting to fall into place as you progress along the transect. I would recommend using a piece of paper or a notebook to record soil symbols, their names, typical factors, and classifications of soils as you find them along the transect. You will find they repeat quite often, so if you keep a list of just those you've seen so far you won't need to keep flipping back and forth to the various sections.

The photomaps provide quite a bit of land use/cover information, but you may find reference to topographic or geologic maps useful. Unfortunately there is nowhere near as complete geologic coverage of the state as topographic, but the geomorphic information in topographic maps should be sufficient for most purposes. Some geologic information can be derived from the soil descriptions in the survey.

Make sure to indicate where the transects begin and end.  You might indicate direction (N -->,  E--->) on the transect.  You should also include where likely roadside stops (or side trips) might provide a look at the soil associations.

Your transect should minimally include the following elements:

Example:  one method of constructing a transect

The following is more detailed than above, but is just one suggestion, if you're looking for more specific instructions. The length of a transect can vary significantly. If a short transect is very interesting, it's enough. Depending upon the length of your transect, your scale may differ.

  1. Estimate the length of the transect. Most of these will be between 20 and 60 mi (appr. 30 to 100 km). Since these soil surveys have been produced over the last 20 years in the U.S., we will use English units (miles, feet, inches) to make our job easier. Don't feel like your transect has to be this long -- the point is to illustrate interesting changes in soils, and for some areas this can happen over a short distance.
  2. Using a reasonable scale, which for longer transects shouldn't be larger (don't forget what "larger" scale means) than 1:63360 (1" to 1 mile), produce a baseline on your graph paper. You may need to divide your transect into two parts, if it is too long. Note that the scale of your transect will be smaller than that of the maps you'll be working from. Note: precision is not critical for this -- do it carefully and neatly, but the main point is to communicate the soil variations that occur and the factors that influence their development.
  3. Get the topographic maps which cover your transect (use these in the map library).
  4. Start at the western or southern end and work east or north. Find the first segment to analyze. The easiest way to do this is to find identifiable landmarks (e.g. road intersections, ridgelines, streams) that you can identify on both soil and topographic maps. A segment would then extend from one landmark to the next, and probably cover a few miles at a time.
  5. For this and subsequent segments:
    1. Measure the distance on one of the maps, and mark the ends on your baseline.
    2. Using a vertical scale of 1"=1000' (vertical exaggeration = 5.28) and defining your baseline to be sea level, mark the beginning and ending elevation on your diagram. You should add other spot elevations as needed to help you produce a more accurate shape. Note: this scale is just one example. Depending on the length of your transect and the relief that exists, some areas need very different scales.
    3. Using the topographic map as a guide, roughly (but neatly) sketch the topographic profile of the segment on your diagram.
    4. Label any feature of interest on this segment (on the top side). These might include road intersections, streams, and other significant topographic or land use features apparent on the map.
    5. Using the soil map, mark off soil map units on the profile, and label them below. As in d above, label features of interest apparent on the air photos (e.g., forest to grassland, grassland to orchard, forest to vineyard) - label these with topographic features above the profile, since they are surface features. If your transect follows a stream for a while, it may be more interesting to indicate what can be seen in the uplands above the floodplain.
    6. continue with the next segment.
  6. Now go to the classification table, and label each of these series with their classification to the Great Group level (i.e., Arbuckle is a Haploxeralf). If complex areas occur (rapidly changing back and forth among 2 or 3 series), use arrows or something.
  7. Color a 0.3" thick line below the profile according to soil order, given the following system
    order color
    alfisol green
    andisol black
    mollisol dark brown
    ultisol red
    vertisol purple
    inceptisol light brown
    entisol yellow
    aridisol orange
    histosol gray
    spodosol white
    oxisol paisley
    gelisol electric blue

     

    Map

    You don't need to make a map from scratch.  One approach is to make a color photocopy of the "generalized soil map" in the soil survey you choose, and draw your transect on it.  Do not cut this out of your soil survey, even if you own it -- I will grade projects down if I see you have done this -- these soil surveys are valuable documents and go out of print.  Your map should be clearly related to your transect, and show locations of samples, photos, etc. map.thumb.jpg (5524 bytes)

    Analysis

    Your paper should consist of an introduction to the general trends of soil-forming factors and the resultant soil distributions. This is a challenge, to get the important characteristics all on one sheet of paper. Do not copy what it says in the soil survey; it's obvious when you do and you'll never get it on one sheet of paper anyway. You must spend time understanding the soil changes, understanding the factors that cause the soil variations, then write in your own words what's happening along your transect. This is probably the hardest part of this assignment, and is crucial.

    Photographs

    At some point during the semester, you will need to make the time to thoroughly visit the study area to take pictures and soil samples.  Make careful notes where you take the pictures -- mark them on a map -- because you will need to record where you took them.   Hopefully you'll find some roadcuts with exposed soils where you can clean a new face to take a picture of representative profiles.  But landscape views are also useful.sonoma.stops.thumb.jpg (9149 bytes)

    Samples

    At a few sites, collect soil samples of important soils.  Make careful notes about where you took them, what depth and horizon they are from, and any other observations about the profile that is significant to the soil type.  You don't need a lot of material.  Try to get both A and B horizons -- a Bt horizon is critical in identifying an alfisol or ultisol for instance. If you sample at a road cut, you may not be seeing the A horizon -- so you might need to get above the more eroded road cut itself. A small vial or very small sample bag would be best, since you are going to attach them to your poster.  When you mount them, make sure we know what they represent, and where they came from.samplepix.thumb.jpg (3664 bytes)vials.thumb.jpg (10450 bytes)

    Everyone enjoys this, but it takes time, so start early.