Research results

Techniques for Greening Slopes with Exposed Highly Acidic Soils

Hokkaido Acid Sulfate Soil Location Map

Some soils generate sulfuric acid upon contact with air or water, causing them to oxidize and become highly acidic with a pH 3.5 or less. These soils are characterized by a high content of iron sulfide, iron disulfide (pyrite) and other sulfur compounds, and have distinguishing colors ranging from dark olive to blue-gray. They are generally referred to as acid sulfate soils.

Such soils that become exposed during construction or repair work on roads and rivers can prevent vegetative cover from growing on man-made slopes built using the cut-and-fill method. For this reason, they are covered with a thick layer of ordinary soil or treated using other countermeasures on these slopes. However, this work cannot be performed on some construction sites due to local factors, and also involves high construction costs. Against this background, the present study involved the testing of a technique in which a 1.3-cm-thick layer of mainly calcium carbonate (as an alkali that neutralizes acidity) is spread over the surface of an acid sulfate soil layer and materials are sprayed on it to create a vegetation bed (known as the neutralization method) and an approach in which waterproof resin sheets are used to inhibit acid sulfate soil oxidation and protect plant roots (known as the impermeable sheet method).

With the neutralization method, thick vegetative cover began to grow immediately after the work was performed, and stable growth was maintained over the subsequent 16-year period. With the impermeable sheet method, on the other hand, growth and establishment of vegetative cover failed. These results confirmed that the neutralization method could be used as a technique for greening slopes with exposed acid sulfate soils.

In Hokkaido, acid sulfate soils are found mostly in land areas that were previously shallow bays in ancient times. In some cases, rocks and soils located under sulfurous hot springs for long periods have turned into this type of soil. When civil engineering and construction projects involving large-scale earthwork are conducted in areas with such soils, and when soil to be used for soil dressing on farmland is selected, due caution is required and appropriate countermeasures should be taken. In this context, CERI’s Rural Resources Conservation Research Team organizes and stores information on locations where acid sulfate soils are found. As the discovery of this type of soil at other locations is also expected, such information needs to be updated on an ongoing basis.

In order to determine whether soil is of the acid sulfate type, it is necessary to check its pH under conditions of forced oxidation, analyze its sulfur compound content and examine its color and odor. For related technical support, including inquiries about the specific test procedure, contact the Technical Consultation Department at CERI.

(Contact: Rural Resources Conservation Research Team, CERI)