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[reposted from Earth Science since more relevant here]

I am at a farm where previous tenants left a non-biodegradable plastic film roughly 10-15 years ago. It is inconsistently spread over a few acres and was not discovered until we attempted plowing the fields. The fields are overgrown with weeds (grasses, thistles, small shrubs, etc.) about 1m tall and this growth has fragmented the plastic considerably (see image, left). The plastic must be removed in order for food to be safely grown there, and to meet the organic standards of the farm. The fragments are in a wide range of sizes and easily breaks when handled.

What method and equipment (such as an excavator) would best remove the plastic and top layers of soil, with minimal damage to the remaining soil (i.e. minimal degradation)? What types of remediation could help deal with microscopic remnants of plastic?

close-up of black plastic, left, and view of field growth, right.

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    The first thing that comes to mind is to mow everything short, and then use a sod cutter. Seems like that might remove the top layer without disturbing sub-layers. And you'd just roll up the plastic with the weed roots. Regarding the microscopic bits, I'd think bioremediation would be the best way. BUT you might need to know more about the plastic to know which bacteria to use.
    – That Idiot
    Aug 17, 2015 at 16:03
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    I've only ever done this manually, and without soil remediation. It was tedious, and we started by using a defoliant to kill the vegetation which you'll probably not want to do either. "a few acres" suggests more than a hectare (10,000 square metres). If you remove 5cm (2") of soil from 2 hectares that's 0.05m * 20,000 = 1,000 cubic metres (In imperial units, 28,000 bushels), about 3000 tonnes (100 truckloads). Which is a lot of soil.
    – Móż
    Aug 20, 2015 at 11:21
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    Are you sure it's really toxic for food growth? You don't say exactly what kind of plastic it is, which would affect that question. It sure sounds like a large-scale intervention with high environmental and financial costs would be needed. Maybe the ability to grow food organically or at all does not outweigh these costs.
    – Reid
    Aug 25, 2015 at 2:59

1 Answer 1

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Get the plastic itself analyzed. It is likely Polyethylene. PE itself is harmless -- chemically it is essentially a very long chain wax. If there were any solvents in it, by now they have leached into the soil (rueful grimace). In many jurisdictions using PE mulch is allowed in organic farming. Get a ruling from your local authority.

Long exposure to sunlight will cause PE to cross link. This reduces it's strength and makes it brittle. It gradually becomes PE confetti. If you have sandy soil, this actually helps with water retention. If you have clay soil water movement can be slowed too much.

The problem with removing the topsoil to get rid of it, is you are throwing the baby out with the bathwater. You want that soil for growing your crops.

I would look at doing this:

  1. Modify your plough by removing the plough shares, leaving the coulters in place.

  2. Go over the field with coulters several times. You want to cut the plastic every 4 inches in two directions.

  3. Put the plough shares back on.

  4. Plough and disk the land.

  5. Try harrows of various types. Chain, bar, and spring tooth. These may catch the plastic and flip it to the surface.

  6. Try using a hay rake to see if it will gather the bits that are on the surface.

  7. If #6 didn't work, try using a hay bine with the blades removed. (Essentially vacuuming the field.)

  8. This will remove only the plastic that is mostly above the surface. Disk again. Repeat whatever worked before.

  9. Rototiller may work to bring deeper stuff up.

  10. Rock rake may lift stuff up.

response to comment on burning.

This will be largely ineffective. The grass will have created a sod above the plastic (I have had this happen in my tree farm). Grass fires generally don't get hot enough to kill the roots, let alone burn the plastic 1/2 inch to 2 inches below.

You say that it's fragile. Cheap non-stabilized plastic then.

You could make a machine to do this.

During a dry summer work the dirt up. Plough disk, harrow, rototil.

Use a haybine or peatmoss harverster and go over the surface, You want some device like a vacuum cleaner brush that lifts the top half inch and flicks it into the air. The vacuum pulls the flakes of plastic (high area, low density. high area/mass ratio) as well as a lot of fine dust (low area, but even lower mass, high area mass ratio). A correctly designed cyclone separator should get most of the plastic separate form the dust, which you return to the soil.

Weigh the plastic you retrieve. Calculate the mass of plastic originally in the field.

Use the least energy intensive way to stir the soil to get the next harvest of plastic.

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    I read here that plastic mulch/covering is allowed in the US but only if it is removed at the end of the growing season. Not sure where the OP is from though.
    – THelper
    Feb 18, 2016 at 15:11
  • To remove the plastic that my previous owners left behind for the small crops that is in bad condition and it breaks up when touching, Would it help if I burn that area considering the plastic is only on top of the dirt but over grown by grass
    – Darryl
    Jan 31 at 21:49

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