Bulletin #2283, Tips for Purchasing Soil for Gardens and Landscape Projects

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By Bruce Hoskins, Maine Soil Testing Service, University of Maine, Katherine Garland, Extension Horticulture Professional, and Richard Brzozowski, Extension Professor, University of Maine Cooperative Extension.

Adapted with permission from UNH Cooperative Extension

For information about UMaine Extension programs and resources, visit extension.umaine.edu.
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a shovel stuck in dirtSince Maine has no legal definition of soil (also sometimes sold as “loam”) and no regulations governing its sale, consumers are often left wondering how to assess soil quality prior to purchasing. Or, if they’ve purchased soil sight-unseen, they may find themselves with a pile of inferior material in their driveway wondering if it’s even worth using for their landscape projects. This fact sheet is designed to help you make wise decisions about making this very important horticultural purchase.

Before deciding to purchase soil, consider using the soil that exists on the site already. Purchasing soil involves mining it from one site for use at another. Even the best quality purchased soil will likely need some adjustment to pH, organic matter content, or nutrient levels before use. You will often be spending as much time, effort, and money on purchased soil as you would on simply improving the soil you already have. For example, the drainage properties of moderately sandy or clay soils can often be gradually improved by adding organic matter over time. Sites with coarse gravel or soils with especially high silt and clay content may benefit from bringing in high-quality soil. You’ll also want to bring in soil if you’re attempting to grow food crops at a site with known contaminants.

What is loam?

Loam is often used as a term for any purchased soil. However, loam is also a technical term. As defined by the USDA Natural Resources Conservation Service (NRCS), Loam is a category of soil based on the portions of sand, silt, and clay content as follows: less than 52 % sand particles, 28 – 50% silt particles, and 8 – 28 % clay. Technically, a true loam has too much silt and clay to drain readily and therefore sandy loam is preferred for turf or garden use.

Questions to ask vendors prior to purchasing soil

Where did the soil come from?

  • Occasionally, soil stripped off farmland may contain herbicide residues that could hinder seed germination or growth. If you have any concerns about residues, take a sample home, plant a few seeds in it and see if they germinate okay. Try a variety of different seeds, such as oats and beans, especially if you’re planning to plant a vegetable garden in this soil. Herbicide residues can affect some crops, but not others. The process of test-germinating seeds will also help you, as a prospective buyer, determine if the soil is infested with difficult-to-control annual or perennial weeds, such as galinsoga, quackgrass or thistles.
  • Be aware that most garden centers purchase the topsoil they sell from one or more sources by the truckload. Each batch will likely be different.

What’s the “recipe” or mix of ingredients? (for blended soil products)

  • If compost is one of the ingredients, ask what the compost is derived from. You may find that some suppliers use compost made from municipal sludge or biosolids. Although registered products are deemed by the EPA as safe, many people have reservations about using this type of product for food crop production.

May I see the soil?

  • Be wary if the vendor will not let you see a sample of the soil you’re going to be purchasing.
  • Assess the soil texture by feel and by performing a ribbon test.
  • Sniff before you buy. Don’t buy a product that has a chemical odor or other off-odor.

Can you share soil test results from the batch I’d be purchasing from? Or May I have a small sample to test myself? (if they don’t have soil test results)

  • A soil test will alert you to the existing make-up (and shortcomings) in a stockpile or batch of soil. Without a soil test, you will just be guessing on important chemical factors such as pH and nutrient availability. In addition to those items, a soil test will offer information about organic matter content and presence of lead. The report includes specific recommendations on what to add in order to create an optimal foundation for plant growth.
  • To collect a sample to have tested yourself, mix 5 samples (approx. ¼ cup each) collected from various areas of the vendor’s pile.
  • Test kits can be requested online, by calling 207.581.591, or are available at your county Cooperative Extension office. Be aware that soil test results will take approximately 2 weeks after submission to the soil testing service.

Is the soil screened?

  • Unscreened soil can be full of rocks, roots, wood chips, bark, plant rhizomes or other debris. You may be willing to rake them out yourself if the price is right, but it’s definitely something you’ll want to know upfront.

How do I determine how much soil I need for my project?

  • This resource can help you determine how much soil to purchase.

Best practices for using purchased soil

Whatever its source, most unamended soil is low in organic matter. Organic matter is important for holding moisture, improving soil structure, and retaining plant nutrients. Add plenty of organic matter (up to 10% by volume) to purchased topsoil in the form of compost (20-30% by volume) and/or animal manure. Because of food safety issues, don’t plant vegetable crops in soil recently amended with fresh animal manures. Food crops should not be harvested for 90 – 120 days after applying raw or uncomposted manure. Amending purchased soil in the fall season will be the best approach for spring garden planting.

For in-ground garden or lawn applications, be sure to incorporate both the organic matter and the purchased topsoil into the top few inches of your existing soil rather than simply spreading the new soil on top. This can be done by rototilling. Plant roots grow best in a single zone of uniform topsoil (to a depth of at least 4-6”).

Newly established raised beds being placed on areas previously covered in vegetation or directly onto an impermeable surface, such as pavement, should be lined before filling with soil. The lining can be biodegradable, such as cardboard or a thick layer of newspaper, or a permeable more long-lasting material, such as landscape fabric.

What about bagged soil?

Most bagged soil products do not contain true mineral soil (sand, silt and clay particles), but are a mixture of various animal manures, aged bark, peat, and compost. While many bagged soil products may work ok in the short-term as is, they will not have enduring qualities for use in raised bed gardens as the sole planting substrate over the years. Mineral soil is a superior source for many of the essential micronutrients.

Summary

The soil that currently exists on your property may provide approximately what you need with a few adjustments. However, if purchased topsoil is needed, you’ll want to be an informed consumer before diving in. It pays to ask questions and do your research!

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Information in this publication is provided purely for educational purposes. No responsibility is assumed for any problems associated with the use of products or services mentioned. No endorsement of products or companies is intended, nor is criticism of unnamed products or companies implied.

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