To forget how to dig the earth and to tend the soil is to forget ourselves.
- Mohandas K. Gandhi
By definition, restoration ecology is the process of improving degraded land through the removal of invasive species and improving soil conditions to create a stable, bio-diverse ecosystem. In the previous post, Restoration in Progress, I cited Glass’ post, Thoughts on Restoration Management, where he suggests restorers tackle the underlying cause(s) for the habitat’s invasive species by tending to the soil. Tending and understanding the soil in a restoration area requires a soil analysis.
The primary factors affecting plant growth in a restoration garden are 1) soil conditions, 2) sunlight, and 3) site slope. Evaluations of all three factors are essential in the creation of a viable restoration plan. Within each of these primary factors, there are subcategories. A soil’s texture and drainage, structure, pH, and nutrient levels are the four factors that comprise its soil conditions.
A soil’s texture and drainage falls into one of four categories, sandy, loam, clay, or organic. In the field, soil texture can be determined using the Feel Test. The Feel Test is accomplished by rubbing the moist soil between the thumb and fingers several times. The test requires that the experimenter determine whether the soil holds together when moistened, as well as describe the soil’s texture.
Sand as identified by the Feel Test has course particles that will not stick together, even when moist and feels gritty. Soil that falls under the Sand heading is made up of large mineral and organic particles. This course textured soil is typically of poor quality, well-drained, dry, acidic, and low in nutrients and water-holding capacity. Only Shortgrass prairie plant species will thrive under such arid conditions. Prairie Moon Nursery has put together a prairie seed mix for this type of soil, one having sandy texture and fast drainage.
Loam as defined by the Feel Test forms clods and feels like flour when it is dry. When moist, the loam feels silky and easily sticks together. It is a medium textured, rich soil that has good water holding capacity and good drainage. Loam soils provide an excellent medium for growing a variety of native trees, shrubs, flowers, and grasses.
Clay as characterized by the Feel Test feels slick and smooth, lacking a gritty texture due to its small, smooth particles. When clay is wet, it can be shaped into the form of a ball because of its high water holding capacity. However, when a silt component is present in the soil mixture, some clay may feel floury. Clay is a heavy soil type that tends to have poor drainage and air movement but is usually quite fertile.
Organic soil as classified through the Feel Test feels spongy and has a fibrous texture. Organic soils exhibit a very distinctive odor and color. Decaying vegetation and other organic matter are responsible for the soil’s odor and color. Organic soils contain a high proportion of muck or peat, which occur naturally in wet areas like swamps, bogs, and marshes. Due to its fibrous nature, the soil holds large amounts of water and nutrients. Even when drained, the soil can still hold water like a sponge. However, once dried, the soil can be difficult to “re-wet.”
Sand, silt, clay, organic matter, minerals, air and water particles are the primary building blocks of soil. Soil structure is the arrangement or combination of soil particles into porous compounds called aggregates. Pores and cracks separate the aggregates. The soil’s overall structure is determined by the aggregates shape, which in turn affects water and air movement through soil. Air, water, and humus are necessary components of a soil that is to sustain life and perform other vital soil functions. The four basic aggregate shapes include granular, blocky, prismatic, and platy structure. Granular soil structure arrangement, the structure of choice, provides adequate water flow, which promotes seed germination. Soil structure, unlike texture, is not permanent. Cultivation practices such as plowing and tilling can help the gardener obtain a granular topsoil structure in the beds.
Another alterable soil condition is pH. The acidity or alkalinity of soil is measured in pH units. In chemistry, as well as in other applications, pH is defined as the negative logarithm of the hydrogen ion concentration. The pH scale ranges from 0 to 14. The neutral point of the pH scale is pH 7. As the amount of hydrogen ions in the soil increases, the soil pH decreases becoming more acidic. Soil is increasingly more acidic from pH 7 to 0 and more alkaline or basic from pH 7 to 14. Soil pH, a critical soil condition variable, controls many chemical processes that take place between the earth and plants.
Plants need soil nutrients in order to thrive. Soil pH controls decomposition activity by nitrogen producing soil microorganisms in addition to regulating nutrient availability by controlling a mineral’s chemical form within the soil. The optimum pH range is between 6 and 7 for most plants. Soil pH that is either too acidic or too alkaline, is not conducive to nutrient transfer therefore, the soil nutrients remain undissolved and are not absorbable by the plant. Optimal soil pH levels provide nutrient rich soil that produces not only faster growing plants, but ones that are more pest and disease resistant. Adjustment to the soil’s pH is necessary when the pH level is greater than a plants’ preferred range. Guidelines for adjusting soil pH can be found here. Keep in mind, changing the pH depends on a number of factors including current pH level, your soil’s texture, and the material you are using to amend the soil.
All restoration sites must have a minimum threshold level of nutrients for vegetation to establish and become self-sustaining. Seventeen essential plant nutrients are required self-sustaining growth. The three most important nutrients, nitrogen, phosphorus, and potassium can be restored by treating the project site with a variety of treatments, including topsoil, mulch, compost, as well as organic or commercial fertilizer.
Most native plant species prefer soil conditions with limited nitrogen availability, whereas, invasive species thrive in soils with increased fertility (Morgan 1994). Site soil analysis will help the prairie restorationist decide what repairs are required. The goal of soil repair is to make the soil ideally suited for native plants and undesirable for invasive species growth. Ultimately, knowledge of the site’s soil texture, composition, drainage, acidity, and mineral density, will help prevent the disappointing restoration results that can occur when a site’s soil is inappropriate for a native plant garden.
How to Take a Soil Sample
Step 1: Obtain representative soil samples from the site areas where plants are to be grown. The soil should not be overly dry or wet. Each test area should be representative of a region’s unique growing characteristics, which may include soil type, drainage, slopes, and/or sunlight conditions.
Step 2: Take a garden trowel and go down 6 to 8 inches, in a garden area measuring 3 ft. by 3 ft. In plots greater than 10 ft. by 10ft., featuring the same growing characteristics, multiple soil samples from 6 to 10 different areas of the garden should be collected. These soil samples should be mixed together in a large ziploc bag and labeled for easy identification.
Step 3: Empty an area’s bagged and tagged soil contents into a clean container. Remove the plant debris and mix the soil together, crushing any lumps larger than pea size.
Step 4: Spread the soil out on a sheet of paper and let it dry overnight.
Step 5: Obtain a Mosser Lee Soil Master or another soil testing kit from your local garden center. (Sending soil samples to a local University extension office for evaluation is an alternative to do it yourself soil testing kits.)
Step 6: Run the pH, Nutrient, nitrogen, phosphorus, and potassium tests on the soil samples. Obtain and record the test results on the Mosser Lee record sheet. Check the Mosser Lee’s vegetation pH and nutrient reference guide for species-specific nutrient and pH preferences. For more detailed information regarding the appropriate soil remediation recommendations, contact a nursery professional, or garden Cooperative Extension Service with the site’s soil testing results.
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Curtis, Peter. Restoration Ecology, Peter Curtis Group. Ohio State University. N.D. Web. 23 Mar. 2012.
Diaboll, Neil. “Step By Step Site Analysis Procedures for Developing a Native Landscape Plan.” Prairie Nursery, The Productivity Source, LLC., 2012 Web. 24 Mar 2012.
Dunne, Niall. ed. Get To Know Your Soil. Landscape For Life: based on the principles of the sustainable site initiative. Brooklyn Botanic Garden N.D. Web. 25 Mar. 2012.
Morgan, J. P. Soil Impoverishment: A little-known technique holds potential for establishing prairie. Restoration and Management Notes 12 :55-56.1994.
“When Good Soil Is Bad.” Wildtype: design, native plants, & seeds, ltd. N.P. 2011 Web. 26 Mar. 2012.
“Understanding Your Soil.” Prairie Nursery, The Productivity Source, LLC., 2012 Web. 24 Mar 2012.