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Understanding a soil survey

Soil mapI've been writing recently about some of the steps you should take to learn about a property you're interested in before buying it --- a title search, figuring out where the boundaries are, and seeing if the listed acreage is accurate.  Assuming you're planning to use the property for homesteading purposes, it's also essential to get an idea of your soil type.

In the United States, the NRCS has laboriously mapped every little pocket of earth, so you can learn a lot by simply looking at maps.  The modern way to do this is to go to the Web Soil Survey and find your property using their interactive mapping tool, but that website doesn't seem to be playing nice with my computer and I have an ancient copy of my county's soil survey, so I decided to go old school.  (You can probably get a free copy of your region's soil survey at the closest NRCS office, and some of you can download a scanned version online.)

Soil boundaries on mapMy county soil survey is an ancient book with four maps stuck in a pocket inside the back cover.  The hardest part is locating your property on the soil survey maps since they only include roads, houses from whenever the map was produced (1951 in my case), and large rivers and streams.  I did this the easy way by photographing the relevant section of the map and then scaling it in the Gimp until the shape of the nearby river fit with the same river curve on my aerial photo.  Since I had the boundaries as a separate layer in my map file, I could soon see which soil types were found on the property and how they related to the one cleared field (the irregularly shaped blob in the northwest corner of the image above).

There's a key on the side of the soil map that helps decipher the odd codes on the soil survey.  We live in a karst region, so there are lots of sinkholes (the ovals with pointy teeth), and people have farmed our hilly land too hard so there are gullys (red Gs).  The other letters on the map refer to the soil type: Hsn for the pink zone that includes most of the current field, Ws for the paler area south of that, and so forth.

In our soil survey, there's a separate fold out sheet of paper that turns the soil codes into real words, then you can look up that soil type in the book to learn much more about it.  Here's what I discovered about the cultivated areas on the property shown above:


Soil type

Slope range

Internal drainage

Parent material

Productivity and class

Acres per animal unit

Hsn

Hagerstown silt loam; Rolling phase

8-15

Medium

Residual material from weathered limestone

High; First

1.5-2.4

RoL

Rolling stony land (limestone material)

3-30

Medium to slow


Medium to moderately high; Fourth

4.0-5.8

Htz

Hagerstown stony silt loam; Steep phase

15-60

Medium

Residual material from weathered limestone

Medium; Fourth

2.3-3.3

Ws

Westmoreland silt loam

30-60

Medium

Residual material from weathered limestone and shale mixed

Medium; Fourth

2.4-3.8


Of course, all of this mapping should be taken with a grain of salt --- or rather, a shovel and a day on the ground.  For all I know, the prime topsoil in the Hagerstown silt loam has all eroded away, but if it hasn't, that area is clearly the best spot for vegetable gardening, while the poorer soil on the east and south ends of the field could use some soil-improving grazing.  Taking a look at the real state of the ground is a good thing to add to the list for a site visit!

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It never ceases to amaze me how many things are named after our town!
Comment by Angela Sat Aug 11 08:51:33 2012
Angela --- It does seem a little strange to name a soil that comes day down here after a town in Maryland.... :-)
Comment by anna Sat Aug 11 10:06:27 2012
When you see the sinkhole symbol, could you you also then look at wet water springs (creeks) to see about drainage? That is, does the soil survey map give any topological info, too? At the Roan Mtn Rally the end of July there was a geologist who said he could find out info about the substrata by just using the coordinates, or even just using an address. I was hoping to check this out, lmaybe for friends who live in the Abrams Falls, Va. region, to see what risks there would be for hydrofracking there, to people who did have a series of sinkholes: for ex. if the hydrofracking might cause the sinkholes to collapse more. So my question to you is about other geological info that the soil survey shows. Thanks!
Comment by adrianne Sat Aug 11 12:13:43 2012

Mom --- Where it says "internal drainage", you can see how likely a certain patch of soil is to get swampy. If you combine that with a topo map showing that there's a stream there or nearby, you can get a good estimate of where swamps might be. I'll be posting about how to find topo maps of your region next week. :-)

You can find geological information on different maps too. Around here, limestone = caves.

Comment by anna Sat Aug 11 14:41:18 2012

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