Solving drainage problems in turf, tree and landscape plantings is not a difficult task if you understand the underlying cause of the problem and you are able to redirect the excess water to a lower area such as a drainage ditch, dry well or catch basin. It is much easier and less costly however, to correct soil drainage problems during the initial design and construction phases of a project rather than afterwards. Unfortunately, we often “inherit” poor drainage problems on many sites simply because essential soil testing to determine soil texture, structure, and drainage was not previously done.
Although excessive drainage can be a problem on some coarser textured sandy soils and slopes, poor drainage is a much more common landscape problem and is considered by many to be one of the biggest killers of landscape plants. Pore space (the air space between soil particles) controls soil drainage characteristics and in soils dominated by large pore space like sands and loamy sands, water moves rapidly. To help solve water retention problems in these excessively drained soils, managers will routinely incorporate organic matter in the form of peat moss or compost into the soil via tilling or topdressing. This will increase the water-holding capacity of the soil resulting in less drought stress for the plants. In poorly drained soils dominated by many small pore spaces (such as compacted soils or those with greater than 20 percent clay); however, water is slow to move, and soils easily waterlog causing root decline, root rots, and eventual death of all but the most tolerant wetland species.
Identifying poorly drained soils is not difficult if you know what to look for. Typical soil symptoms associated with poor drainage include surface puddling or ponding, blue/gray soil matrix colors, foul “anaerobic” odors, distinct soil mottles (grey/red/orange spots) within the top 24 inches of the soil surface, percolation rates less than 1 to 2 inches of water drop per hour, and surface and subsurface impermeable crusts, hardpans, corepans and or plowpans.
IMPROVING SURFACE DRAINAGE
To improve surface drainage problems, you must first identify and then correct the contributing factors leading to the problem.
IMPROVING SOIL STRUCTURE
Poor surface drainage in many landscape beds may be the result of excessive tilling, which many believe is the only way to “control weeds and aerate the soil.” Ironically, excessive tillage does not improve drainage, but rather hinders it by destroying soil structure and the aggregated soil particles that help transport the necessary oxygen to a plant's roots. On these garden sites, correction of the problem involves reducing the number and frequency of tillage operations and incorporating a 1 to 2 inch layer of organic matter into the soil. This helps to aggregate and “glue” the finer clay particles into larger aggregates, creating more macropore air space and therefore improving aeration and drainage.
Organic amendments such as peat and compost aid in adding nutrients and improving soil “structure” via particle aggregation, but it decomposes with time and may cause settling. Usual recommendations warn not to add more than 20 percent by volume (1 inch peat per 5 inch root zone) in any one growing season. Levels should also not exceed 12 percent organic matter by weight (obtained from soil test reports), or excessive settling, moisture and aeration problems will result.
Most organic matter will increase the water-holding capacity, so beware of the amount applied. The dose can make the poison! Also beware of salt and ammonia problems if you use animal manures unless they are well composted, aged or leached by rainfall or irrigation. Tilling 1 to 2 inch organic matter layers into beds in the fall will help prevent nitrogen immobilization the following spring if high carbon to nitrogen ratio organic amendments are used such as sawdust or high wood content mulches.
You can also add inorganic amendments such as sand, porous ceramics, expanded shale and calcined clay to poorly drained surface soils to increase soil aeration. If you use sand to improve aeration and drainage, it is crucial to know its particle size distribution and its acidity, or “pH.” Adding a small amount (<50-80 percent by volume) of a high-pH limestone sand (pH 7.5 to 8.5) dominated by “fine” particle sizes (<0.5 mm grain size) to a bed destined for acid-loving plants is a recipe for disaster and will usually result in micronutrient deficiencies unless you supplement with chelated micronutrients. You may also add Epsom salts (magnesium sulphate) to restore a better calcium-to-magnesium ratio if you've already used limestone sands, but it may still be very hard to change pH once limestone sand is applied. Silica sand, unlike limestone sand, would be a much better alternative to use than limestone sand because it has little buffering capacity (defined as a “resistance” to a change in pH) and will assume the pH of the soil it's mixed with or if used alone, may assume the pH of the irrigation water used during irrigation.
It is important to avoid using mixed particle size sand with lots of fine sand because finer particles will occupy existing macropores air space if it exists producing a denser, less porous mixture. This will cause the greatest change in beneficial physical properties with the least amount of added sand. In fact, <5 percent should be comprised of fine sands passing a 100-mesh sieve size. Not until sand constitutes 45 percent or more of the volume of soil will the soil begin to have some of the beneficial properties of sand. Hence, additions of 50 to 80 percent by volume are usually required to achieve any beneficial effects and that amount of sand can be very expensive in large-scale operations.
SOLVING GRADING PROBLEMS
Surface water will move quickly down a sloped surface and is typically referred to as “surface drainage.” Visual observation of the landscape topography will usually indicate where surface drainage problems may occur. Many drainage problems exist simply because soil grading and leveling was inadequate. Subsurface drains are not a replacement of surface grading. Water can be removed from a site more rapidly by surface drainage, allowing use within hours after a rain. This is especially true for golf courses and sports fields.
For proper surface drainage on poorly drained clay soils, a minimum 2-percent slope is usually recommended, which requires a 2-foot vertical drop for every 100-foot horizontal distance or a 0.2 foot drop (roughly 2.5 inches) over a 10-foot horizontal distance. By properly grading the site, surface water is directed to the lowest spot on the property, which will then empty into a drainage ditch, catch basin or dry well. It is always best to check local and regional drainage laws and ordinances to avoid potential lawsuits.
Turf or landscaped areas should have a minimum surface slope of 1 to 2 percent so water flows to a lower elevation away from the site or to a grated, subsurface (sunken) catch basin or dry well. On new sites with many low spots, it may be best to remove the topsoil and stockpile it out of the way. Then establish the subsoil grade with a 1- to 2-percent slope. When adding or moving soil, always allow for about 15 to 20 percent settling on fine-textured soils and less for coarse soils. Another less costly option than topsoil removal and subsoil grading is to plow the soil as deeply as possible and then grade the site to a 2-percent slope. Surface grading, though sometimes costly and disruptive to an existing landscape, may be one of the best solutions for poor soil drainage problems and it may be the only practical and economical means of improving drainage for poorly drained or compacted clay soils.
In open turf areas, you can create a drainage “swale” by digging or grading a sloping ditch where downhill flowing water will be “intercepted” and flow to lower ground. An alternative would be to install French drains, which are simply ditch-like trenches dug and filled with rock or gravel to the soil surface and crowned to prevent soil covering the drain. To be effective, French drains must also slope at least 1 to 3 percent and flow to a discharge outlet.
SOLVING ISOLATED LOW SPOTS WITH NO OUTLET FOR RUNOFF
Identify low spots and break up the underlying compacted soils with a tiller or backhoe and fill the low area with soil of the same texture as existing soil. This helps prevent water movement problems at the soil interface between the two soils. The same recommendation applies for raised or “mounded” beds. Do not add finer-textured soils over coarser-textured soil or perched water tables will result predisposing plants to needless root stress, rots and death.
Chris Carlson is director of horticulture technology at Kent State (Salem, Ohio).
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