Soil is one of the basic resources that we have when beginning to work with land. Along with water, climatic patterns, and existing ecosystems, soils form the canvas on which we paint our agro-ecological life support systems.
In the US the Web Soil Survey (WSS) managed by the US Department of Agriculture’s Natural Resource Conservation Service operates one of the largest soil resource information systems in the world.
Soils of more than 95% of the counties in the continental United States have been mapped as part of the National Cooperative Soil Survey. That data is available online through an easy to use map-interface, and a wide range of data is freely available for download as a (well formulated) PDF or as tabulated and spatial data for Geographical Information Systems (GIS) program.
In this article I’ll show you how to navigate the WSS interface, and where to find soil data which is most relevant for initial site assessments for permaculture design.
The map is not the territory, but it’s a good place to start
There is no substitute for being directly on the land, and taking samples from the individual places you’re developing. However, many designers work out of an office, and conduct initial site assessments before making an on-site visit.
The soil information resources described here are helpful when needing to garner a rough understanding of soil profiles, texture, pH, ecosystem type, slope, hydrology, and potential factors which may enhance or limit the functionality of a site.
With this data, you can provide a valuable block of information for your clients — and quickly understand the basic conditions you’re working with as a designer.
Getting soil data
From the home page of the Web Soil Survey find the right side-bar and click the “Start Web Soil Survey” link.
On the left hand side you’ll see various ways to quickly navigate from the national map to your desired area of interest. The quickest way is usually to put in the address. You can also use the cursor to zoom in from the national map to the area you want soil data for.
WSS area of interest
Use the “AOI” feature (circled in the above picture) to define you Area of Interest. This will be the perimeter of the area for which soil data with be calculated.
Once you have your AOI, the general soil type and classifications, along with slope and total acreage selected will be generated on the left hand side bar.
WSS soil unit description
You can click on the linked soil classifications and get a more in-depth look at the conditions.
Exploring the soil data
Armed with just the soil classification, slope, and acreage, you could go far with an initial assessment. However, there are a variety of other in-depth variables at your fingertips.
Under the “Soil Data Explorer” meta tab there are a handful of sub-tabs which give more detailed information and explanation of the dynamics affecting and being effected by the soil of the AOI.
Intro to soils tab
WSS intro to soils
The first tab on the left provides some easy to read definitions of soil science jargon.
It also includes discussion of how and why soil data is relevant to certain enterprises: from structural engineering concerns, to cropland and forestry soil science, and urban planning. The Info tab provides relevant discussion on many aspects of land-use planning affected by the nature of the soil.
Suitability and limitations tab
WSS suitabilities and limitations
As it sounds, the next tab outlines the general suitability of the soils within the Area of Interest for particular purposes such as the suitability of mining sand or gravel for industry, crushed rock for road base, or building a waste water treatment facility.
Under this tab is also metrics for determining the productivity of forests in cubic feet/acre/year of wood growth of select timber species grown in these soil conditions.
The information provided is in-depth, however vaguely useful in the context of an early site assessment.
Soil properties and qualities tab
The next sub-tab is the “Soil Properties and Qualities” — a store house of practical information for designers resides here.
Not all the data is available for every site, and certain ratings are simply irrelevant to most sites. However, much of the useful data for permaculturists doing an at-a-distance site assessment are available.
WSS soil properties example — an example of data, this one concerning clay content of soil.
Note the data is displayed below the map.
For instance, the Soil Chemical Properties section gives information on the expectable range of pH, salinity, and percent-carbonates in the soil — all of which are relevant for designers to know up front, but which may be prohibitively expensive to discover via initial tests on a site.
The Physical Properties section gives reliable information on the percentage of sand, silt, clay and organic material throughout the soil types of the property.
This section also has some obscure, but relevant information on the available water supply (AWS) in the soil. AWS is the total volume of water (in centimeters) that should be available to plants when the soil, inclusive of rock fragments, is at field capacity — relevant for understanding how much water is functionally available to plants within a given root-depth.
The Soil Qualities and Features section gives data on the depth of soil to a restrictive layer (such as caliche or bedrock), soil susceptibility to frost-action (frost-heaving), and the classification of the parent material from which the subsoil was made.
Ecological Site Assessment Tab
WSS ecological site assessment
When you click this tab, the ecological site classifications are automatically pulled up for the Area of Interest. As per usual, the ecology of a site is more or less defined by the dominant over-story vegetation. Hence, it does not give you a lot to go on.
However, some types of forest are better documented (such as eastern hardwood forest), making this feature a lot more useful. For much of the western US, however, the data is pretty basic and says little about the specific lay of the land.
In the above screen shot, it says the forest is of two types — Ponderosa pine/Oregon white oak/elk sedge forest complex, and the Douglas Fir/Ponderosa pine and elk sedge complex. These are two slightly different forests, dominated by either of two overstory trees, which generally indicate the transition of one soil type to another.
Exporting soil data
Under the “Download Soil Data” and “Shopping Cart” tabs you will find a few options and formats for how to extract the data.
If you want to get GIS-ready data for the Area of Interest you selected, go to the Download Soil Data tab, stay in the “your AOI” section and click the “create download link” button.
At the bottom of the section a link to a .zip file should appear. The .zip file contains the tabular and spacial meta-data formatted for GIS software. Those familiar with GIS should easily be able to extract these files into the appropriate folders for use in GIS software.
The Tabular Data can also be used independently of GIS software. Data for larger swaths of the US are available from the soil data download page.
WSS soil resource report — close up of a high resolution soil resource map,
including the GPS points.
To get a consolidated PDF document with selected soil data and maps and descriptions, go the the “Shopping Cart” tab. On the left you will see all the ways which you can customize the PDF, including the dimensions of the document, and what data you want to be displayed in it.
A high resolution aerial map with soil overlay up to 22×17 inches can also be incorporated into the document as well.
The PDF is well formatted, and represents an elegant and easy addition to an initial site assessment. Even if you never make it onto a property, you can hand people a lot of valuable information about their site with very little time and effort.