Is it a good idea to add forest soil to your garden and raised beds?
Using forest soil, especially soil from a deciduous forest, is a great way to inoculate your garden soil or increase mycorrhizal fungi in your garden and subsequently improve plant growth, especially for trees and perennials that thrive in fungal-dominated soils.
In this article, we will look at the differences between garden soil and forest soil and how forest soil can benefit your raised beds and garden.
Forest soils differ from garden soils in appearance, microorganisms, structure, and pH.
Garden soils are generally existing urban or agricultural soils that have been mixed with compost to improve characteristics like nutrient content, water holding capacity, drainage, and aeration so that they can improve plant growth.
Forest soils are naturally occurring soils that have been influenced by vegetation over time.
Other differences between forest and garden soils are in the pH, microbial diversity, and texture of the soil.
Garden soils usually have a mid-brown color, while forest soils are darker in appearance.
Forest soils have a dark brown color from the abundance of decayed organic matter or humus. In contrast, garden soils are a lighter brown to mid-brown color, depending on the color of the original soil, and the quantity of compost added.
Forest soils have more fungi than bacteria, while garden soils are the opposite.
Every soil contains a combination of microorganisms like fungi and bacteria, and no soil is entirely fungal or entirely bacterial.
Forest soils are typically fungal-dominated, with considerably more fungi than bacteria. This is due to the abundance of “brown” materials, or carbon-rich organic matter such as twigs and forest floor litter that encourages the growth of fungi.
On the other hand, garden, urban, and grassland soils are bacteria-dominated, with more bacteria than fungi. The higher bacterial content is due to the abundance of “greens” or nitrogen-rich organic materials such as fresh leaves, kitchen waste, or compost added to the garden soil.
In terms of structure, forest soil stick together and is less crumbly than garden soil.
Fungi, particularly arbuscular mycorrhizal fungi, produce a substance called glomalin which acts as a glue, binding soil particles together.
Since there are more fungi in forest soils there’s also more glomalin in forest soils than in garden soils. Additionally, the humus and decaying matter in the forest soil also helps to bind the soil particles together.
While garden soils contain decayed organic matter in the form of compost, they have little or no glomalin. As a result, they are more powdery and loose than forest soils.
Forest soils are often acidic, and rarely alkaline. But garden soils can be either alkaline or acidic depending on the mineral content.
Research by the US department of agriculture shows that most US forests have acidic soils, with varying degrees of acidity (Perry and Amacher, 2002). The low pH of forest soils is attributed to the continuous additions of organic acids and carbon from decaying fallen branches and trees.
Unlike forest soils that become acidic from the constant input of organic acids, garden soils don’t have a set pH range and could be neutral, alkaline, or acidic, depending on the type of chemical fertilizer used, and the mineral content of the soil. More calcium, sodium, and magnesium make soil alkaline, and fewer of these minerals make the soil acidic.
Forest soils can improve the growth of garden vegetables, and annuals, as well as trees, and perennials by providing beneficial microorganisms and essential nutrients.
Forest soils contain beneficial microorganisms within the decomposing litter that can stimulate plant growth.
Forest soils have beneficial microbes, often called “mountain microorganisms”, living within the decomposing forest floor litter. They can benefit plants in the garden by stimulating growth and triggering plant immunity to pests and diseases.
The forest microorganisms are not only compatible with garden soil microbes, but they can also work together providing synergistic results.
For instance, bacillus bacteria (which is abundant in garden soils) and mycorrhizal fungi (from forest soils) have been shown to work synergistically producing better results than either one alone.
A study found that inoculating marigold plants with a fungi species (Glomus fasciculatum) and the Bacillus subtilis bacteria produced nearly 20% more flowers than uninoculated plants (Nanjundappa, et al 2019).
In general, a shovelful of forest soil contains a lot of life, such as nematodes, collembolans, arthropods, maybe some earthworms, which can all enrich your garden ecology.
Forest soils have mycorrhizal fungi, which can improve plant growth by increasing water and nutrient supply, and improving disease resistance.
Forest soils contain a great variety of fungi including plant-beneficial fungi like the Trichoderma and Glomus species. When this soil is used for gardening, the fungi in it colonize the roots of the plants. Glomus fungi are Arbuscular mycorrhizal fungi that send out long thread-like filaments (called hyphae) into the surrounding soil to pull water and nutrients to the plants. The increased water and nutrients translate to improved plant growth and yield.
Trichoderma fungi are present in all soils. But forest soils typically have more fungal activity, and thus contain more Trichoderma than regular soils. This fungus doesn’t supply water and nutrients like the glomus Mycorrhizal fungi. But when used in gardens it helps plants by killing pathogens in the soil and stimulating plant disease resistance.
Although some garden favorites like cabbages and Brussels sprouts aren’t compatible with forest soil fungi, most others are. Over 80 percent of all plants are compatible with soil mycorrhizae and will benefit from having fungi in the soil.
Some forest soils contain essential plant nutrients that can improve the growth and yield of plants in the garden.
Forest soils aren’t the same everywhere. Some are more nutrient-rich than others. Generally, soils from deciduous forests (forests with trees that shed leaves like spruce needle, maple and oak) are the most fertile forest soils. While rainforests and coniferous forest soils are not nutrient-rich.
The soil from a deciduous forest is known as Alfisol and the high fertility is due to the constantly decaying leaves on the forest floor that return nutrients to the soil. These soils contain high amounts of Nitrogen, Potassium, and Phosphorus and also have high concentrations of Calcium, Sodium, and Magnesium (Bekele, 2021). Alfisols also have a high water and nutrient-holding capacity.
The high fertility and nutrient-holding capacity of Alfisols are great for plants in the garden, as the nutrients promote photosynthesis, root and leaf growth, pest and disease resistance, and fruit/crop yield.
The acidity of forest soils can be beneficial to garden plants, especially acid-loving plants that thrive in acid soils.
Forest soils are typically acidic, and a large number of garden favorites like tomatoes, potatoes, alliums, blueberries, radishes, and pumpkins are acid-loving plants. Even cabbages, Brussels sprouts, and other brassicas are acid-loving plants, so even if they don’t form mycorrhizal relationships with forest soil fungi, they can still benefit from the acidity of forest soils.
Garden trees such as citrus, apple, peach, apricot, and mangoes also prefer slightly acidic soils, and as such will benefit from the low pH of forest soils.
Problem with using forest soil in the garden?
There is virtually no problem with using forest soil in your garden. The only concern would be if you are removing a lot of soil from the forest soil which could affect forest ecology.
Also, it should be noted that the soil taken from the forest is not necessarily fertile or contain rich fungal communities.
Taking some soil from the forest, especially deciduous forests, to add to your compost pile or to your mulch of 1 to 2 inches deep is a great way to increase mycorrhizal fungi in your garden soil and reap the benefits. This can help increase the microbial activities in your garden and ultimately improve plant growth.
Nanjundappa, A., Bagyaraj, D. J., Saxena, A. K., Kumar, M., & Chakdar, H. (2019). Interaction between arbuscular mycorrhizal fungi and Bacillus spp. in soil enhancing growth of crop plants. Fungal Biology and Biotechnology, 6(1). https://doi.org/10.1186/s40694-019-0086-5
Perry, C., & Amecher, M. (2002). Chemical Properties of Forest Soils. Department of Agriculture, Forest Service. https://www.srs.fs.usda.gov/pubs/gtr/gtr_srs104/gtr_srs_104Chapter9.pdf&ved=2ahUKEwjQj52u-e78AhX8VKQEHRQzBl8QFnoECA8QAQ&usg=AOvVaw2hVi7PBlmQO5txva_PS2UA
Bekele, D., & Birhan, M. (2021). The Characteristics, Distribution and Management of Alfisols. International Journal of Research Studies in Agricultural Sciences, 7(6). https://doi.org/10.20431/2454-6224.0706001
Kadoya, T., Takeuchi, Y., Shinoda, Y., & Nansai, K. (2022). Shifting agriculture is the dominant driver of forest disturbance in threatened forest species’ ranges. Communications Earth &Amp; Environment, 3(1). https://doi.org/10.1038/s43247-022-00434-5