The benefits from fines are added to soils via a variety of mechanisms which allow the nutrient elements in the rock to become available to the plant root system. These mechanisms include dissolution, ingestion and digestion by macro-organisms (earthworms and moles, etc.), microbial attack on the surface on fine particles, and direct ion exchange.

Particle size considerations

Because of the nature of these transfer mechanisms, the material size is an important factor in determining its applicability for soil remineralization. Typical aggregate products available that are too coarse for this application include concrete sand, abrasive sand, filter sand, mason sand, blow sand and screenings. The fines referred to are pond settlings, rock dust (product from dust collectors), rock flour, and classifier tailings.

For best results, at least 75 percent of the rock dust should pass a #200 mesh (75 micron) sieve. Application rates should be between 2 and 20 tons of fines per acre and would be tailored to soil deficiencies if known.

Mineralogy considerations

Mineralogy of the material is as vital as size in determining the success of soil remineralization. It is best to tailor the elemental analysis of the fines to provide elements deficient in the local soil, if known.

Typically, the best suited aggregate types include:

1. glacial sand and gravel that contain a myriad of heterogeneous combination of various rock types and mineralogy;
2. metamorphic or igneous rock such as basalt or rhyolite; or
3. sedimentary material, such as limestone and dolomite, which can be used to balance pH and provide for calcium and magnesium deficiencies.

Comparative tests are still needed, however, to evaluate the performance and costs of different types of rock in the vast varieties of soil types and conditions that exist.

Existing research

The concept of remineralization is much further advanced in the European community and Australia than in the U.S. Their current practices include hydroseeding of fines to forests dying from acidic atmospheric conditions. Similar conditions exist in a few forest areas of the U.S. It is encouraging, however, that interest in remineralization is growing with a variety of studies being conducted.

Dr. Robert Bruck at North Carolina University has an on-going project involving planting Fraser fir and red spruce trees in the Appalachian Mountains. Trees in the upper reaches of the mountains have been dying from acidic atmospheric conditions and the depletion of minerals in the soils necessary to sustain survival. Young trees have been planted and studied with and without the application of fines to the soils. Those in soils with added fines have a noticeable increase in the survival and growth rate.

Recently the U.S. Department of Agriculture at the Beltsville, MD Agricultural Research Station has funded a three-year pilot project to study the affects of various aggregate industry by-products as an agricultural soil amendment. This study includes glacial rock, granite and basaltic fines.

Corn was grown the first summer, then winter wheat, and the USDA now plans to plant soybeans. The demonstration plots were on the tour for the agency's sustainable agricultural field day. USDA is planning more rigorous experiments for next year using mineral fines in some composting studies.

Growing interest in the remineralization concept

There is a growing public concern about agricultural soil and ground water in regards to both quality and quantity. There is a general awareness that food products are tainted by fertilizers, pesticides, herbicides and fumigants utilized by the agricultural industry. This, along with the thrust for sustainable agricultural practices, has driven an interest in the concept of remineralization.

Fertile regions of the Nile River Basin, Matanuska Valley in Alaska, and the Mississippi River Valley area give testimony to the effect of aggregate mineral fines (generated by the grinding action of stones in a moving water system) when flooding deposits new, enriched mineral fines to lowland areas.

By taking an active role in promoting the use of these by-product fines, both publicly and through government agencies, the industry can forge new markets for products that currently have very limited market application. For those seeking further information on the subject, contact:

Rick Meininger, PE.
National Aggregates Association
900 Spring Street
Silver Spring, MD 20910. P0

Ronald Korcak (left) from the Beltsville, MD. Research Facility of the U.S. Department of Agriculture stands with Robert Able at the agency's Agricultural Research Station.

The U.S. Department of Agriculture at the Beltsville, MD Agricultural Research Station has funded a three-year pilot project to study the affects of various aggregate industry by-products as an agricultural soil amendment. This study includes glacial rock, granite and basaltic fines.