Soil Survey- How do they do it ?

[sprucebunny]

My land only has about 40 acres that are "moderately well drained soil" according to the soil maps. Right in the middle of this area, at the edge of an old log landing, I started clearing and moving away the top soil to put up a pole barn and had a wet spot that wouldn't go away so I dug down some more... And the soil I hit sure doesn't seem good draining to me !!! It's on top of a slight rise so I've dug a channel/ditch. It's not flowing but the underlying soil has a major clay component and seems in no hurry to dry. This same clay-ey layer is under every place I've dug up in this area.

It's got me wondering about the accuracy of the soil surveys and how they were done. Anyone know ?

ETA: The soil type is Sunapee #169 B+C slope.

[SwampDonkey]

Depends on how fine the "resolution" of the sampling. By this I mean how far apart between sample locations. Could be 1 mile or 10 miles on grid, in ditches of roads, open pits or fields or any fresh earth works. It's unlikely the soil was sampled specific to your woodlot. Not enough $$ and man power. But they sampled enough of the geography to catch most soil types common to the area. I've always found with our system of classification in NB or even in BC, the vegetation, forest floor, and drainage will usually be enough to key out the soil type specific to your region. We go by eco-regions or micro-climates here. In fact with the keys you can pin point pretty precisely what you have with the three factors mentioned.  Poorly drained ground will have standing water or very muddy year round, imperfect will have pits and mounds where the pits have seasonal water and usually no ground veg in the pits. Well drained will be vegetated and not a lot of micro relief, more undulating. As far as forest floor: a Mor has a litter layer (L), fermentation (F), and humous layer (H) with a matted  F layer of fungal mycelia, mushroom smell; a Moder has L,F, H well defined, but the F horizon is loose and less mycelia , fresh potting soil smell; and Mull has a very thin F and H layers, very friable F and a good rich top soil layer, granular peds with earth worms.

[WDH]

They are typically mapped by elevation and slope position.  Generally, they are amazingly accurate, but any micro-site may have variation like you experienced.  Has that site been compacted from use in the past?  The logging could have compacted that area and caused the drainage to suffer.

[sprucebunny]

How far down would the compaction have an effect ? There is more than a foot of mineral soil of some kind over the layer that is causing the problem.
Many places that have been skidder trails have a barrier of clay on each side that blocks water drainage. When I dig around and break the barriers ( often more than a foot down) the drainage improves through the soil.

[WDH]

Studies have shown that it only takes a couple of passes by heavy machinery like skidders to significantly compact the soil.  That is why most Best Management Practices say to contain the skidding to as few trails as possible so that you minimize the compaction over the entire site and concentrate it in as few trails as possible.  Also, the soil is a dynamic system, so even a little compaction in the upper surface can change how the soil behaves.  I bet that is what happened in you case. 

[Roxie]

This is a very interesting discussion.  :P

[SwampDonkey]

The compaction can cause what we call hard pan to develop and a heavy clay layer is about the worst. In the north we do get some relief in the upper layer of soil because of hoar frost. The worst I see in the woods is deep ruts that never heal. They fill up with water on wet sites because they tend to be flat and a lot of slag areas acting like a bowl. The ruts basically cut off the rooting layer, the roots can't go deep and under the rut because of sub surface water and compaction. Roots need oxygen to live.

[JV]

If you can find someone who has a soil probe and pull some core samples you can see the areas compacted.  Better yet  if someone has a penetrometer you can quickly determine the areas compacted.  Possibly your local NRCS office could have some suggestions.  A one shank subsoiler or V-ripper used during dry soil conditions would lift and shatter that compaction.  Normally permeable soils can become very troublesome with even minor compaction.  Some surveys were done years and years ago and haven't been updated.  Here is the NRCS web soil survey site that might be of interest:

http://websoilsurvey.nrcs.usda.gov/app/HomePage.htm

[sprucebunny]

I could make a soil probe, JV, that's a good idea. Thanks. I'd love to use a penetrometer: there are some very strange places out in the woods. The land has been logged several times over the years but not for the last 30 or so. Much of it is ready again but poor wood and long skidding distance make it not worth it.

Here is a map from the WSS site.


 
Right to the left of center above the 'road' is where I've dug some holes.
Until recently, there were 5-6" diameter white birch and some cherry growing there and also some spoil piles from clearing the landing.
I'm thinking of calling the extension service and see if there is a soil expert similar to getting the state forester to visit... eventually I want a septic system and I need some better clues for where to look for good soils !

[snowstorm]

if nh is anything like maine. the guy that draws up the sepic plan will do the soil test. not everwhere will pass usually you can find some ground that will. i know the guy pretty well that i have do them for me so i tell him where i would like to have it. a coulpe times we have had to use the ...there was a house here once ..... in that case they have to design something. dont be to conserned with the clay the sepic will be on not in the ground. i try not to do the stone beds anymore i like the plastic coulverts as i call them. look up presby they are in nh. he is a bit of a salesman and then some but his product works good. have over a dozen systems in the ground with his coulverts all working fine

[sprucebunny]

Thanks, snowstorm. I guess I should have said that I've only dug down 2-3 feet and hit water . This is the third place it's happened.
For the perc test they want a 4 foot deep hole. Pretty sure it's not going to perc when it's filling with water ! I'm going to clear another half an acre over the edge of the knoll and see what's under that.
Maybe I should pick a spot for a dug well... that might be dry  :D

[Autocar]

Here in Ohio they sometimes make a above ground leach feild and what I mean about that the feild it is a elavated feild that is 40 feet wide by 80 feet long built at ground level then the water is lifted from the septic tank and pumped up into the feild to drain into the soil. Theres a float on the motor so it pumps when the water level is at a certain place in the tank. Don't know for sure but have heard that water will leach out of the side hill when theres heavy uses of water from the house, but don't reaily if thats truthful.

[snowstorm]

Thanks, snowstorm. I guess I should have said that I've only dug down 2-3 feet and hit water . This is the third place it's happened.
For the perc test they want a 4 foot deep hole. Pretty sure it's not going to perc when it's filling with water ! I'm going to clear another half an acre over the edge of the knoll and see what's under that.
Maybe I should pick a spot for a dug well... that might be dry  :D

the well may be a good idear. in some places a dug well works just fine. dig down at least 16' use 4' well tiles do not mortor them together fill around the tiles with gravel. dig the hole and let it set overnite what looked like a dry hole will be full the next day compacting the clay from digging will close off the vein of water for a while.

[SwampDonkey]

I'd be looking for a dome shaped knoll and some seepage down hill at it's base. Of course sample it to see it's not an up-cropping of shale. It will usually be the best spot for well and house. I would never build under a hillside, asking for water troubles.

[sprucebunny]

As WDH mentioned, just a pass or two with a skidder will change the soils. I've got several areas of 'seepage' but when you look close, they are old skidder trails. So much of the land was run over that after 30-50-80 years, it's hard to tell what's what.
For now, I'd like to understand the soils better and figure out if the soil survey is correct without having to cut down all the regrowth and dig the whole place up !!

Which makes me wonder: if they do a soil survey based on vegetation, wouldn't logging throw a wrench in the works ?

[WDH]

Vegetation is not one of the critical factors in defining or describing a soil.  However, some vegetation is soil specific because of things like drainage, pH, etc.

[SwampDonkey]

Sprucebunny, they look at the climax community and the ground veg (moss/herbs/ferns/shrubs/heath). The vegetation is a good indicator of the site and soils in the way we classify soils. I have keyed out the soils many times with the vegetation. The best thing is to dig a pit to define the depth of the horizons, drainage (we have keys) and to see if evidence suggests the water table fluctuates (mottling), depth to compaction and parent material that the soils were derived from.

With the keys to soil type and drainage a soil pit is always used for sure. But with being familiar with the geography in the area the vegetation, forest floor and drainage indicators will tell me what I need to know. In my site region we only have 11 soil types. And for instance a decent natural mature cedar stand will be on ST3 and if more upland and ST4 unless it's a real poor cedar stand, which can be on and ST2. Never find a forest of tolerant hardwood there. Might find yellow birch and red maple mixed in though (ST3 and 4).

Now we have a national soils classification, probably more like what your referring to. Like for instance, in my area it would be classified as a humoferric podzol on the rich hardwood sites that are turned into farm fields. In NB I never here anyone mention soil classification out side of academics and research. When they mark out a harvest block, they go by what they can see on top. Building a house here doesn't involve any soil sampling of any kind. ;)

All this is specific to NB.

[sprucebunny]

My water problem turned out to be a combination of soil compaction and poorly placed, very big rocks. There was a 4" diameter w. birch tree on top of the rocks. I got 2 rocks out and I'm going around the third. The bigger of the 2 was in at an angle; >3'x8'x1.5'.
Think I'll put dug well in nearby to try to keep the soil saturation down.

Thanks for your responses.

[WDH]

I have not been able to get the web soil survey site to work for me for some reason.  I have used it a bunch of times in the past, but I am getting an error message when I try to access it on this computer.  I will try on another computer.