Manufactured Soils

AFSFIRST_300dpi_rgb.jpg

What are Manufactured Soils?

Manufactured soils are blends of soil, soil components and soil-like material used in horticulture/landscape applications and site restoration.  Using manufactured soils allows for “tailoring” of soil properties to specific needs.  For example, a potting mix for seedlings may need to be light and well drained to accommodate seedling roots and reduce transport costs.  In contrast, a landscape blend may need to be heavier and have a larger mineral component to provide long-term support and nutrition for trees and shrubs. 

Soil blending is performed at a production scale across the United States, generating millions of cubic yards of product annually.   Manufactured topsoil is used on residential, commercial, and industrial sites for establishing healthy lawns, gardens, parks, sports fields, and green space.  In urban and suburban areas, native topsoils tend to be lost or contaminated due to past and current human activities that cause erosion or degradation of soil.  Production level soil blending takes in available sub-soils, sand, and composts to generate a soil that performs equal to or better than native topsoils.

All soils, natural or manufactured, require mineral components to provide structure and organic components to provide essential nutrients and water-holding capacity.  Components of manufactured soils range widely.  Typical components include: Compost, sub-soil, dredge, sand, shredded bark, and other organic materials.

The demand for manufactured soils, and thus for sand or sand-like materials, is huge.  The U.S. Department of Agriculture estimates that more than 50 million cubic yards of sand are used annually in the horticulture market, which consists of a combination of plants grown in nursery beds and plants grown directly in pots.  Many nursery operators blend their own soils and customize them to meet the needs of the plants.  Others purchase manufactured soils from commercial soil blenders.

Manufactured soils are also widely sold through landscaping companies, nurseries and retail establishments.  Landscape contractors account for a large percentage of soils sales volume, due to the demand for topsoil and landscaping soils on residential and commercial properties.  All bagged topsoils and gardening soils sold in commercial establishments are manufactured soils. 

How is Foundry Sand used in Manufactured Soils?

Soil texture is an important component of the overall soil quality. Different proportions of sand, silt and clay comprise the basic soil textural classes, as defined by the United States Department of Agriculture (USDA) in “Textural Triangle for Soil.” The particle size distribution of foundry sands generally fit into the “Sand” to “Silt loam” portion of the soils triangle.

Clay-bonded foundry sands (“green sands”) are primarily comprised of the same mineral components that make up native soils – sand, clay and water.  The sands used in metalcasting processes are natural sands, and the clays are mixtures of natural clays, primarily western and southern bentonites.  These ingredients make up more than 95 percent of the green sands on a dry weight basis.  Foundry green sands are also dark in color and generally uniform in sizing and chemistry from individual sources.  Although foundry resin sands lack the clays and are lighter in color, they behave like virgin sands in providing bulk and porosity for manufactured soil products.

Foundry sands are of interest to soil blenders for a number of reasons.  First, sand is an essential ingredient of manufactured soils, where it provides filler, weight and drainage to a blend.  Second, the dark color of foundry green sand is an advantage, producing a soil blend that physically resembles natural soil.  Third, the clay content in foundry green sands provides additional water-holding capacity, an essential characteristic for good soil.  And lastly, foundry sands provide a number of macro- and micro-nutrients that are essential for plant growth. 

Foundry sand has been used in manufactured soil blends developed for horticultural, landscaping and turf applications.  There is commercial experience using foundry sand in a number of different manufactured soil products, including general landscaping soils, all purpose topsoil, nursery bed mixes, and container mixes.  Foundry sand has also been blended with peat to produce mulch products, where the sand reduces the tendency of the loam to clump in the presence of moisture.  Soil scientists have also developed custom blends containing foundry sands for specialized applications such as baseball diamond infield soils, green roofs, and soils for bio-retention and rain garden projects.

How do Manufactured Soils perform using Foundry Sand?

Agronomic benefit is gained from the use of foundry sand to blend soils with a Sandy loam texture.  Soils with a Sandy loam texture tend to drain easily, allowing ready access; are non-plastic, unlike clay soils; and don’t tend to form a hard, impenetrable crust that is hard for plants to grow into.  A review of scientific literature and golf groundskeeping magazines will show that Sandy loam is the texture that supports plants well in many settings.  Foundry sands also darken soils, adding to aesthetic appeal.

Foundry sand soil blends have been sold commercially for more than a decade in states that allow this use.  There is no difference in performance between manufactured soil blends containing foundry sand in comparison to those with virgin sand.  In fact, several plant uptake studies have shown that many foundry sand blends can improve plant growth, probably due the presence of calcined clay and micro-nutrients that are contributed to the blended soil from foundry sands.

Soil scientists and landscape architects develop specifications for soil blends in order to meet the needs of the specific use for the resulting soil.   For example, the American Society of Landscape Architects (ASLA) has recently developed guidance specifications for custom soil solutions that include manufactured structural soils; manufactured light weight soils; manufactured moisture retentive soils; and manufactured layered soils.
 
Foundry sands have been extensively used for decades in structural fills, embankments and landfills that are later vegetated.  In some instances, structural soils for construction projects are manufactured using foundry sands.  In addition to their use as the sand substrate in landscaping and horticultural blends, foundry sands are also good materials for use in stormwater management systems such as rain gardens and bio-retention cells. 

What are the technical issues associated with Foundry Sand in Manufactured Soils?

Manufactured soil blends have performed successfully using foundry sand ranges as low as 5 percent and as high as 50 percent, depending on the soil specification being met.  Soil blends for high volume projects such as structural soils or large landscaping projects often incorporate low-grade sub-soils, which typically contain elevated levels of clay.  Clay is difficult to work with, so foundry sand is added to push the texture toward Silt loam.  Compost or other organic matter is then added so that the final blend is a Sandy loam.  Soil blenders need to experiment with different percentages of foundry sand and organic matter, depending on the type of soil they are manufacturing. 

Two of the most important technical considerations when using foundry sand are particle size and pH.  Many foundry sands are fine sands, and the other useful additions to foundry sand such as clay and organic material are also fine.  Particle size is easily measured.  In general, foundry resin sands are coarser than foundry green sands because they do not contain clays. 

Most often, the largest technical consideration for foundry sand in manufactured soils is the pH of the sand.  A strongly acidic soil (pH < 4) will be phytotoxic to most plants.  A strongly alkaline soil (pH > 9) will cause micronutrient deficiencies in the plants.  Most foundry sand has a fairly neutral pH (6.5 to 8), however some are higher (>8).  Considering the low percentage of sand typically used in a soil blend and the fact that the foundry sands are not well buffered, it is unlikely that this would pose a problem for the finished blend.  It is also easily measured and a blend can be adjusted as needed.

When designing soil blends, soil scientists consider other characteristics such as bulk density; organic carbon; cation exchange capacity; and plant available water.  A typical good native soil will have a bulk density of 1.2 to 1.4 g/cm3#.  Foundry sands on a stand-alone basis have a slightly higher bulk density, but that is seldom a factor in a finished soil blend.  Soil organic carbon is also important because it increases aeration, drainage, water holding capacity and nutrient retention.  Foundry sands have low levels of organic carbon compared to what is needed in the finished soil product.  However, manufactured soil blends do not depend on foundry sand to contribute most of the organic carbon or nutrient requirements; compost or other organic matter is an important part of manufactured soils.  

Are there any specific QA/QC issues that suppliers and/or end users need to be aware of?

The most important quality control issue for foundry sand to be used in manufactured soils is that the sand must be segregated and screened to remove all extraneous material, and must be homogeneous from load to load.  Some foundry processes will yield excess sand that is at grain size and can be used as is.  More commonly, discarded foundry sand also contains pieces of broken molds and extraneous metal.  Broken molds and cores need to be crushed to grain size and blended with the sand.  In foundries that have not previously had markets for their sands, the sand piles may also be contaminated with simple garbage such as floor sweepings, gloves and soda cans.  All of these materials, including metal pieces, must be removed from the sand before it can be used in soil blending. 

Screening foundry sand is a very simple process that can be done with common equipment.  Removing extraneous ferrous metals (iron and steel) from the sand can be done with magnetic heads on the screening equipment.  Part of the business relationship between a foundry sand marketer and a soil blender is to determine who will be responsible for screening the sand and delivering it to the blending site in market-ready condition. 

On the quality assurance side, foundries and foundry sand marketers should be prepared to provide evidence that their particular foundry sand meets the necessary state regulations.  Most states have guidelines and may require permitting for land application uses of municipal or industrial byproducts.  Often soil blenders are familiar with local regulations as they may use byproducts other than foundry sand.  Foundry sands are typically “cleaner than soil” and are an excellent candidate for recycling.  However as with any industrial byproduct, it is appropriate that a beneficial use program include screening for total elemental content, and for potential organic contaminants. 

Are there any specific environmental issues associated with the use of Foundry Sand in Manufactured Soils?

Because foundry sands are comprised of naturally occurring materials – sand, clay and carbon – their elemental profile is similar to natural soils.  The elemental concentrations in ferrous and aluminum foundry sands are generally similar to or lower than concentrations found in other industrial and municipal byproducts, such as biosolids, FGD gypsum, manure and yard waste composts.

Native soils contain micronutrients that are essential for plant growth. Among the essential micronutrients are manganese, iron, molybdenum, zinc, boron, copper and chloride.  These are also micro-nutrients that are essential for human health, and are commonly found in multi-vitamin products.  Many of these micro-nutrients are present in discarded foundry sands. 

In 2002, the U.S. Department of Agriculture (USDA) initiated the national Foundry Sand Initiative, pulling together the expertise of soil scientists in the Agricultural Research Service (ARS) and several prominent land grant universities.  The goal of the Foundry Sand Initiative was to produce peer-reviewed, journal-published research that would conclude with a comprehensive risk assessment of foundry sands in soil-related applications. 

The ARS research program concluded that sands from ferrous and aluminum foundries contain trace elements and organic compounds at concentrations generally lower than background soils.  The research program also demonstrated that foundry sands can be successfully blended with other byproducts to create manufactured soils.  The foundry-sand soil blends successfully supported plant growth, and tissue analysis showed no elevation in trace element concentrations.  

USDA next collaborated with U.S. EPA to expand the research into a comprehensive risk assessment that considered multiple exposure pathways.  These included inhalation of fugitive dust; leaching to groundwater; and a home gardener scenario including ingestion.  At the time of this writing, the final U.S. risk assessment document has not been released.  However, the draft circulated for peer review, as well as several public presentations by U.S. EPA managers, stated U.S. EPA’s belief that non-olivine iron, steel and aluminum foundry sands have constituent concentrations similar to that of native soils and do not pose a threat to human health or the environment when used as an ingredient in manufactured soils or as an ingredient in soil-less media.

Several earlier research studies had also shown that foundry sands from iron, steel and aluminum foundries have similar environmental profiles to native soils and pose no more risk to human health and the environment than background soils.  These metalcasting types represent more than 93 percent of metalcasting volume.  Sands from brass and bronze foundries may --but do not always--have levels of copper and other elements that could exceed regulatory limits so these are generally not good candidates for soil blending.  Some other sand types, such as olivine sands, have not been extensively studied in soil blends.

Although limited in scope, some research has also shown that foundry sands alone and in manufactured soils often have lower elemental characterization profiles than off-the-shelf bagged topsoil sold in major retail outlets.

For more information on the Environmental Profile of foundry sands, please visit the Environmental Profile section of this website. 

Soils Research