Environmental Factors and a Healthy Landscape: Soil and Drainage
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By Deepa Sanyal
Understanding the environmental factors that exist on a site is critical to designing a functional and healthy landscape. Before you set out to plant your garden, get to know your soil type and condition, water flow and drainage, landscape topography, and climate and microclimate. The value of knowing these factors is that you can then amend them if possible and/or adjust your landscape design and plant selection accordingly.
Soil is not dirt! It is made up of weathered rock (minerals), organic matter, microorganisms, air, and water. Ideally, soil should be 45% mineral, 25% water, 25% air, and 5% organic matter. The mineral components in soil are sand, silt, and clay to varying degrees. In Central North Carolina, clay is the predominant mineral in the soil so we say “we have clay soil.” While soil can be improved, its predominant property whether sand, silt, or clay cannot be changed.
Clay soil does present some minor and not so minor garden drainage issues because it is dense and slow to filter rainwater. Drainage properties of clay soils can be improved by adding organic matter like compost or pine bark fines to the soil. When creating a new bed, incorporate 25% organic matter by volume 6-8 inches into the soil. For established beds, annual surface application of a few inches of organic matter (like compost or mulch) will slowly incorporate and improve soil structure. Organic mulches such as triple shredded hardwood mulch, pine bark, and pine straw will decompose over time (at varying rates) and add organic matter to the soil. Annual crops such as vegetables will need more frequent amendments with organic matter, while ornamental perennial and woody beds do well with just surface applications of compost or mulch.
In addition to amending clay soils, you can also improve drainage by filling in the selected garden area with new, high-quality soil to build raised ground beds that have better soil structure and drainage compared to ground-level gardens. There are several ways to build raised beds and the two that are most effective in dealing with drainage issues are flat-topped mounds of additional soil, six to eight inches high, and berms, raised mounds of additional soil, 18-24 inches high, built up in poor drainage areas to help redirect the water flow. Here are some photos of a flourishing garden on berms.
For more serious soil drainage issues, French drains, a gravel filled covered ditch, and drainage wells provide a place for rainwater to flow after a rainfall. Ponds or rain gardens in a low lying part of the garden can also help with drainage and are lovely features in any garden. Rain barrels, attached to a downspout, collect rainwater that can be used for watering.
These are some of the simple ways to improve soil structure and drainage. Next month we will look at gardening on slopes and low lying areas.