Soil scientists have only recently started to comprehensively understand the extraordinary web of life that inhabits healthy, undisturbed soil. We now know that plant roots, fungi, and countless soil-dwelling organisms form a vital underground ecosystem that builds and sustains soil structure over time. So regardless of a soil’s physical and chemical composition, its makeup and overall health can almost always be improved by adding living plants, supplementing organic matter, minimizing disturbance, and allowing the underground community to heal and rebuild itself. However, some soil health challenges call for more specific solutions. Here are a few examples of stressful situations that could occur in your garden, and the steps you can take to remedy them.
Alleviating compaction will help improve air and water movement
Pores are tiny, interconnected pathways between soil aggregates that allow air and water to move freely, sustaining plant roots and soil life. Soil with good structure typically contains about 50 percent pore space. Compressing or overworking soil, particularly when it is wet, can disrupt and destroy pores, leading to slow drainage and reduced soil respiration. Clay soil is especially vulnerable.
Surface compaction is usually caused by foot or vehicle traffic, while subsurface compaction from cultivation often occurs just below plowing depth. In some regions, naturally compacted subsoil layers formed by glacial activity or other geological events are found relatively close to the surface. Regardless of the cause, the effects of soil compaction on plant health are worsened if the soil remains wet for extended periods. Plants growing in dense, waterlogged soil experience low nutrient availability, poor root development, increased susceptibility to root diseases, and stunted growth.
Avoiding unnecessary soil disturbance, particularly tilling, is the best way to prevent compaction. For seriously compacted soil, a mechanical soil-loosening tool like a broad fork or aerator can be useful for opening up some channels to allow water to drain and air to reach plant roots. Growing cover crops can be an effective strategy for reducing surface compaction, while deep-rooted cover crops such as daikon radish are sometimes useful for relieving subsoil compaction.
When soil compression occurs in the root zones of trees, shrubs, and perennials, it is often not possible to aerate the soil, incorporate cover crops, or replace the plantings. Fortunately, top-dressing the soil surface with compost or organic mulch will usually be sufficient to restore and improve structure. To prevent tree roots from being smothered, keep mulch away from trunks and do not apply a layer more than 3 inches deep.
Design roadside plantings to tolerate heat and salt
There’s a reason landscape professionals call a planting area next to a road or sidewalk the “hellstrip.” In northern climates, salt applied to roadways inevitably ends up in these beds, causing decreased fertility, poor drainage, and increased erosion. Summer heat can also become a serious threat to curbside plants. Most species do best when the soil temperature is under 90°F, and the majority prefer much lower temperatures. Soil that is too hot can slow or stop root growth, decrease flowering, or cause permanent wilting of leaves.
Properly preparing roadside soil will greatly improve the health and longevity of plantings. Spring rains—or deep, deliberate watering—can wash salts out of the soil, significantly reducing their impact on plants and soil organisms. Well-drained soil can handle the amount of flushing a roadside planting generally requires, and it may be worth building a raised bed with amended soil to improve drainage. Incorporating mulch will help keep the root zone cool while increasing natural drainage and organic matter within the soil matrix. Choosing climate-appropriate plants and trees that thrive under curbside conditions will improve your chances for success.
Repairing heavily disturbed soil takes time and patience
Establishing plants in a highly disturbed area, such as a recent construction zone, will require much more planning and effort than planting in undisturbed soil. Contractors frequently remove topsoil with heavy equipment, compacting the subsoil (photo p. 22). After construction, it is important to ensure that the subsoil is loosened before improved topsoil is installed across the site.
To develop a planting plan following new construction, the first step should be to collect several soil samples from the areas where you wish to garden, have them tested, and address any deficiencies identified in the soil test report before planting. An agricultural professional or extension agent can help determine what types of soil improvements may be needed, and which plant species will do best in your soil.
Restoring organic matter to disturbed soil increases drainage and moisture retention, offers a food source for soil life, provides nutrients for plants, and protects the surface of the soil from erosion and extreme heat. Incorporating different types of organic matter before planting will provide a strong source of energy for plants and the life that inhabits the rhizosphere (root zone). Because organic matter does not bind to sand particles as readily as it does with clay or silt, you’ll generally need more mulch, compost, or manure to improve nutrient availability in loose, sandy soil.
Another method for improving soil in established gardens is planting a living mulch. Living mulches are plants that create a protective surface over the soil and help address issues in the soil through their deep roots and natural addition of organic matter. It is best to choose native plants that grow easily in your climate, or non-invasive species that thrive in your area. These could include ground covers or wildflowers.
While there are methods available for improving soil in challenging situations, taking care to avoid soil issues before they start is always the preferred approach.
Kirsten Kurtz is assistant director of the Cornell University Soil Health Laboratory in Ithaca, New York.
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