Removing agricultural plastic film waste after 10+ years degradation

Question

[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?

Answer 1

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.