Search for the “Perfect” Soil Specification

Imported soil specifications have evolved over the last 30 years to become quite specific about products. Frequently, they appear to be iron-clad descriptions of the most optimum soil; many specification writers would like us to think that even minor deviations from these documents will result in horticultural disasters. But “standard” specifications are not one-size-fits-all.

Many standard specifications were developed for projects centered in the central and eastern United States, where development projects have stimulated budgets and motivation to standardize the soil purchasing process. This region is blessed with a huge expanse of relatively similar and good quality loam soils that form the basis for standard soil product descriptions, which were made for plants typically used in the agricultural regions where they were developed (other products in a typical soil specification have also become standardized).

This limited approach does not take advantage of regional soil and product availability that, in many cases, can be adapted to produce acceptable soil. Remember, these recommendations were made for plants typically used in the agricultural regions where they were developed. It is never a requirement to follow standard soil specifications if your regional resources, site soil, or plant material are better suited to something different.

Quest for the perfect specification

In a standard specification, minor differences in available soil products can be adapted. For example, the proportion of the sand can be adjusted to attain the required drainage rates (the most critical part of the soil specification equation). Soil blends that drain too fast can simply be irrigated more frequently. Deficiencies in nutrients can be resolved by adding fertilizer. But these fixes may not always be in the best interest of other project goals.

Something not generally mentioned in specifications is soil structure. Since most soil specifications require screening or pulverizing the soil, the presence of soil structure, a principle factor in soil drainage, is no longer an issue, instead, large amounts of sand compensate for lack of structure. The importance of a soil profile, how the soil layers interact, is also often not considered in the design.

Using unscreened or loosely screened soils to preserve structure (peds or clumps) makes relying on standard specifications more difficult, particularly for projects in regions without fine grained, strongly-structured, loam soils or regions without loam soil resources. This article is directed at designers who are interested in using more natural soil and less sand, or even no sand, and less compost.

The product equation

There are three (with an optional fourth) primary products that come together in a soil specification: (1) some type of natural, harvest soil, to which is added (2) coarse sand, and (3) some type of composted organic material. For rooftop applications, (4) a lightweight material such as expanded shale, clay, or slate (ESCS) may be added to reduce soil weight.

In a traditional soil specification, the soil component is typically loam, sandy loam, or sandy clay loam soil. The soil should have sufficient clay and silt to have good water holding capacity, but not so much that the soil cannot be easily blended. This limits the areas where suitable soils can be harvested.

The sand is typically a coarse and angular material (avoiding limestone sand and round sand). Deposits of these sands are more limited than the suitable soil deposits in many regions. Long Island, in New York, has large deposits of ideal sand for soil blending, and it is no coincidence that many specifications are based on Long Island sand.

Compost and other organic products vary by region, and most eventually decompose to be similar compositions. Most are locally produced.

Lightweight material (ESCS) sources are limited to a few large processing plants, which rely on large-scale distribution networks. The plants are primarily located in the eastern and southern United States, where sedimentary rock deposits are suitable for processing. The farther a project is from the ESCS plant, the higher the cost of the material due to shipping fees.

A new part of the soil equation motivating landscape architects to use more local and recycled products is the sustainability factor. The new Sustainable Sites Initiative puts a premium on working with local soil and materials rather than trying to attain a perfect and standardized soil. They use 50 miles as the maximum distance that soil should be imported to a site.

Regional examples of soil products

Regional conditions will impact the type of soil products and options to use a standardized specification. The following are a few examples of these – while some people may view these conditions as problems, in reality none of them preclude designing soils suitable for planting. In fact, by adjusting our strict approach to soil specifications, many of these regional products can be used to create excellent growing conditions.

High pH Midwest soils

In the western Great Lakes region are large deposits of soft limestone underlying layers of silty loam with high pH. The silty soil does not form as strong a structure as loam with greater clay content and blending these soils tends to require more sand to attain the same drainage rate. The pH is often well above most specification limits, and the available sand also has a higher pH. Lower pH materials are available but have increased cost. On the other hand, local plants are well adapted to the higher pH.

High pH soils in Marquette County, MI. Flickr credit: Kerry Woods.

High pH soils in Marquette County, MI. Flickr credit: Kerry Woods.

Fraser River sands and Canadian topsoil restrictions

The Fraser River, which flows to Vancouver, BC, produces a seemingly endless supply of river sand, but its particle size and angularity are often out of the range of the eastern United States soil specifications. Yet these sands can be effective in improving soil drainage. The river valley also has wonderful topsoil, but there are restrictions on development and harvesting soil in that area, such that topsoil is often harvested from rocky upland development sites. Rock and the roots from upland forested sites make these soils difficult to use without extensive screening. By increasing the permitted rock and root fragments, good soils can still be produced.  The sand, while not perfect, can still perform well, particularly in unscreened soil.

The Fraser River in British Columbia. Flickr credit: Tjflex2

The Fraser River in British Columbia. Flickr credit: Tjflex2

Oklahoma red clay region

These high clay soils are difficult to blend. They turn to dust if worked when too dry and become plastic when too wet. Think of the dust bowl. Pulverized clay clogs sand pores in soil blends. With less screening and blending, these soils can still work well.

Oklahoma red clay soil. Flickr credit: Franklin B Thompson

Oklahoma red clay soil. Flickr credit: Franklin B Thompson

Florida sand soils

The soil in much of south Florida’s coastal regions is mostly deposits of high pH, limestone sand and decomposed coral sand. There is almost no loam soil, as it is normally understood. However, there are large deposits of Florida reed peat, not actually a real peat soil, but layers of silt and fine sand with black organic coatings from decomposed wetland reed grass. Lower pH silica sand is available but at much higher cost. Pine bark from timber in the north part of the state is also available. Local coastal plants are adapted to high pH sandy conditions. A mixture of local sand or the more expensive silica sand with the Florida peat and pine bark make soils that support most south of Florida plants.

Florida's sandy soil. Flickr credit: GreenNetizen

Florida’s sandy soil. Flickr credit: GreenNetizen

Urban soil

Urban soil is a diverse set of mineral soils that is often over looked or disparaged, and sent to the landfill or used as fill soil. Much of the time these soils simply are heavily compacted, and with low organic matter. They can often be rehabilitated for use as planting soil by adding compost. New York City Parks is doing just that, excavating urban soil from tree spaces and using it in other tree spaces after they amend it.  At the project level there are challenges to writing a specification, but it is not an insurmountable problem.

These are only a few of the types of regional soil conditions that make complying with standard soil specifications difficult – but none of these are conditions that preclude designing soils suitable for planting.

So, what should you do when the natural soil profiles or other local products do not fit the standard soil specifications?

What to do when natural soils don't fit the specifications

Get local information

The process of modifying standard specifications starts with understanding of what products are available in the regional market.

All metropolitan areas have soil suppliers, but the quality of their product and knowledge about soil will vary. Track down the most reliable sources and talk with them about the kinds of finished products they supply, and particularly the types of source soils, sand, and compost they typically work with.

Ask for any chemical and physical testing data and see how they may respond to the testing and submittal requirements in your standardized soil specifications. Do your own testing if the suppliers do not have reliable testing of their own (soil testing is actually fairly inexpensive). An actual soil sample is often the best way to gauge the quality of the product.

Be sure to tell the soil supplier what you are looking for and ask about the cost implications of using locally sourced material versus products imported from outside the region to meet your requirements. A review of the USDA web soil survey can enlighten you on the different types of soils in a region. Look at the soils in areas currently under development pressure to see what kinds of soils may be coming on the market.

Evaluate substitutions

Once you understand the ranges of soils, sands and compost available, compare them to the product descriptions in your specification.

Many parameters in specifications set the range of minimum and maximum units that are far inside the range where plant performance is impacted (this will vary depending on the plant material). In some parameters, upper or lower numbers may be quite close to impacting some plants, while others may be just fine above or below the established range. Maintenance can overcome some issues; for example, more water can be added to overly dry soils, or drought tolerant plants can be used. On the other hand, too much irrigation can kill plants in soil that otherwise may be fine if little or no irrigation is applied.

Consider the pH requirements for the plants you are specifying and match them to the local soil pH. PH can often be raised, but is difficult to lower – and there are many good plants that tolerate high pH. Local sources of compost feedstock may create compost with different pH levels that can significantly impact the pH in the final soil blend. Compost made from bio solids often has very high pH.

Consider how much more or less clay and silt in the local soils might impact the mix parameters. How will screening and blending these materials change the final mix performance? Are the chemical and physical characteristic differences really critical to the projects needs of the soil, or can the soil blend be modified to compensate for the difference? If this is feeling confusing, try to think of it like cooking: cooking is full of modifications. If the brownie recipe calls for unsweetened chocolate and you only have sweetened chocolate, add less sugar to the specified quantities to achieve roughly the same result. The same is true, to some extent, with soil.

A note on manufactured “topsoil”

Suppliers may manufacture “topsoil” to meet the requirements for the soil part of your specification, adding compost or sand to make the soil meet the soil texture and organic matter requirements. Remember, though, that these soils do not necessarily share similar characteristics with natural topsoil even though their test results may appear similar or the same. Manufactured “topsoil,” where washed sand and compost is added to poor quality soil, is never a good substitution for the specified topsoil in the soil mix.  This soil is already a mix, and adding more sand or compost may not be advisable. In a soil mix, washed sand and compost is distributed in between small aggregates of soil.

In natural soil, the clay, silt and organic matter coat the larger mineral particles, binding them together to create larger aggregates and a healthier soil. The organic matter in natural soil is more stable than the same amount of carbon in compost. Recycled soils where small amounts (10 to 15% by moist volume) of compost have been added may be a reasonable substitution for the topsoil portion of a soil mix. A good way to check if you have been sent a soil mix as a substitution of for natural topsoil is to find small soil lumps in the sample and break them open.  They should be the same color on the inside as the outside.

Get a soil test

When working in an area where local materials may be quite different, you or the soil supplier can make up small test batches of soil to get a feel how the products blend and work together. You can obtain samples of the material from the supplier.  Send out these test soils for chemical, particle size distribution, and drainage rates.

If you are working with non-screened soil, be sure to tell the lab not to screen the sample before testing for drainage rates or organic matter. Lab screening significantly impacts the results, lowering the drainage rate and organic matter.

Test several different mix blends to learn how minor changes in proportions change the results.  Save a sample of the test batches to use when samples come in from the contractor during the submittal process. Visual analysis is a critical first step in determining if the product you are buying is what the specifications were supposed to define.

Don’t overthink soil

Thirty years ago, soils were not nearly as controlled by specifications as they are today.  I think our specifications tend to emphasize controlling the wrong soil characteristics. Compaction during installation, the degree of screening, methods of blending, and the design of the soil profile layers are likely the most significant factors in soil quality, not nutrient levels, texture, and organic matter that specifications typically worry about.

I am not suggesting that we abandon changes to soil specifications, but in my experience we often overthink soil, relying too much on those properties that can be lab tested and ignoring more important factors, particularly soil structure, profile, and compaction. How a soil is mixed and installed is often more important to soil performance than the kind of soil being used.

Learning how soil should look, feel, and smell is a critical first step in improving the quality of soils, particularly at the edges of our standardized specifications. We currently have a very narrow definition of good soil, treating it as a universal solution regardless of climate, geography, and plant material. This limited approach does not take advantage of soil and product availability that in many cases can be adapted to produce acceptable soil.

 

James Urban, FASLA is the principal of Urban Trees + Soils and the author of “Up By Roots” (ISA).

 

Tflex2, Kerry Woods (CC BY-NC-ND 2.0), Franklin B Thompson, GreenNetizen (CC BY-NC 2.0)

4 comments

  1. Thanks for your post. I didn’t know that there were so many specifications for what kind of topsoil I could get. However, I do stand behind the idea of getting natural stuff if you can, and of testing the soil. My mom is huge gardener and has some topsoil brought in now and again, but she always tests it to see if she needs to add anything.

  2. You are very right that you should not overthink it. There are many options to choose from in regards to soil. I’d be sure to get the right type for the environment that I live in.

  3. I think it is awesome that New York City is starting to use their urban soil. It is a much better idea to fix the soil for use as topsoil than to have better stuff shipped in. It is also great because the soil is already somewhat ready for use in that environment. It is one of the reasons I always try to get any topsoil I need from a local company.

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