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In the last 6 months the level of the soil in my back yard has dropped 4-6 inches exposing all the sprinkler heads and tree roots. Whats going on?

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Where are you and what's your climate like? Has it been raining lately? Do you know of the ground geology in your area? Is there a water table underneath? Have there been any recent drillings for whatever reason in your city/town/area? Please update your question with more information. –  Lorem Ipsum Dec 20 '11 at 0:47
    
I'm probably the most geological minded here, and I would echo yoda's comment. Not knowing anything else, water is probably implicated, but it could be too much, too little, or something completely different like mining or karst related subsidence, or decomposition, or compaction. –  winwaed Dec 20 '11 at 1:58
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Was your house just built? Are you on a steep hill? Do you live in South Dakota or Wyoming? –  Ed Staub Dec 20 '11 at 2:35
    
I'm thinking the same as Ed Staub. Probably your soil is settling following some kind of works (e.g. house just built) where the soil was messed around and/or replaced. It's natural for soil to settle by compacting against itself like this. A rather morbid example are sinking graves, caused by the soil not being compacted enough when filled (image). –  Lisa Dec 21 '11 at 3:59

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It could be one or more of several things. First, rain is a main source of erosion. erosion from rain occurs when there is a dry period, drying the soil down several inches. The rain then lands on the dust, splashing mud and dust and traveling downhill as the amount accumulates. When rain erosion removes a layer of topsoil, there should be a level area on the uphill side of roots or large rocks where the mud pooled on its way downhill, and drained. Second, If your soil was very dry and dusty, wind can quickly move large amounts of topsoil. The result usually looks like the ground is sunken between roots and rocks, and where there are no healthy root systems. Third, soil compaction can put moist soil down several inches in a relatively short amount of time.

Here are some good tips to reduce soil erosion:

  • Keep your soil in a healthy condition, adding plenty of organic matter (like compost) to your soil. This keeps it springy and holds moisture, greatly reducing erosion.
  • Grow something. A cover of plant growth and a good system of plant roots works with healthy soil to prevent erosion.
  • Where you can't grow anything, mulch. Mulching with organic matter protects the soil from the elements and feeds it at the same time.
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+1 For adding plenty of organic matter. We have transformed our backyard over the past year by replacing the worst clay with sandy loam and prevented that new soil from compacting by turning in loads of pea straw (other mulch would work too). And the mulch has also improved the drainage and root-friendliness of the remaining clay soil. –  Lisa Dec 21 '11 at 4:01

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