Maintaining proper soil moisture levels is essential to keep your plants healthy, and a major factor in achieving this is your soil’s drainage. Soil that drains too quickly will need watered more frequently if not improved. Soil that drains too slowly can cause a variety of problems including compaction, poor aeration, stunted and shallow root growth, poor anchoring, and disease.
Measuring Your Soil’s Drainage
Test your soil’s drainage by digging a hole that is one foot deep and fill it with water. As soon as it has drained completely, fill it again. Set a timer, and check how much has drained after 15 minutes. Multiply this by four to calculate the inches that would drain in one hour. A good rate of drainage is 2 to 6 inches per hour. Less than 2 inches is considered poor drainage, and should be improved to prevent problems caused by soggy, waterlogged soil. More than 6 inches per hour is excessive drainage, and should also be improved to prevent problems such as leaching and drought conditions.
Correcting Drainage Problems
To improve soil that drains too quickly, add organic matter. This acts as a sponge to hold water in the soil, and can be compost, peat moss, coconut coir, or a green manure cover crop. A good layer of mulch helps prevent evaporation; while this doesn’t affect the drainage, it will help your soil stay moist longer. To improve soil that drains too slowly, adding organic matter can also help improve the soil structure. Planting a cover crop can help break up heavy or compacted soils and allow water to drain more easily.
You may also consider some landscaping solutions that can help the water flow away from your plants faster: French drains are trenches filled with gravel or rocks that are dug to help divert water away from planted areas. It should be constructed at a 0.5 to 1% grade to provide adequate slope to move the water downhill. Line the trench with landscape fabric to keep the dirt out of the gravel, as it is the gravel that will allow the water to flow away and not get bound up in the soil. You can either leave the gravel exposed to the air, or you can cover it with more landscape fabric to seal in the gravel, and cover it with 4” of soil so the water will flow underground. Berms and mounds provide an elevated area to promote excess water to drain away from trees. Since the root crown, located just under the soil surface, is the most vulnerable part of the root system, the raised planting area will help to protect it. Build your berm or mound 6 to 12 inches high, and 2 ½ to 4 feet wide. The slope should be gradual to prevent erosion from destroying your hard work. Raised beds work similarly to berms and mounds, by providing elevated ground to assist the drainage. Your raised bed can be constructed from wood, rock, or concrete blocks. Raised beds can be decorative and won’t erode like berms and mounds. They should raise the soil level inside the bed to at least 6 inches above ground level.
I am curious what reference text are you citing when you state that “good drainage” for growing is between 2 inches and 6 inches per hour? I have been reviewing U.S. internet sources over the last week or so for scientific references to back this up, as I previously knew that about 4 inches per hour was considered “good”, but had no references to back up my own claim. Arboriculture 4th edition says 0.5 inches per hour perc rate is the minimum required, but that is for ornamental tree culture as opposed to edible fruit tree culture.