The root systems of the plants in our gardens are very sensitive to environmental conditions. Heat, moisture, aeration, soil composition and drainage all have a dramatic effect on the health and development of our plants. A garden bed with poor drainage can create an environment that is simply impossible to grow in. It is vitally important to the health of the soil and the health of the plant’s root system that proper drainage be maintained in our planter beds.
Drainage is, simply put, the process by which water moves through the soil. When watering, we want a certain amount of water to move completely through the soil and wash out. For this reason it is important that our raised planters be designed to allow water to drain from the soil. Our raised planters at the Edible Learning Lab are specifically designed with a waterproof membrane and a sloped floor to facilitate proper drainage.
Plant roots need to breathe. Without proper drainage our soil will become waterlogged causing air pockets in the soil to fill with liquid and suffocate our root systems by restricting the availability of gaseous oxygen, essential for healthy soil microbe populations and normal root development. As available oxygen decreases, carbon dioxide in the soil will increase creating an unbalanced microenvironment promoting the growth of bad bacteria.
This new low oxygen, high carbon dioxide environment leaves our plants exposed to root rot, fungus and bacterial problems. Poorly drained soils also limit nutrient bioavailability by slowing or ceasing the decomposition of organic material. Root development can be stunted and the overall health and development of the plant will be restricted.
The rate at which water filters through the soil has a lot to do with soil composition. For a successful garden our soil should have enough loamy organic material to resist compaction and create air pockets but have enough mineral and clay to have some structure to it. Without a good compositional balance we will end up with highly compacted, poor draining soil or overly loose, quick draining soil – neither are good for growing.
What to do
If your soil seems to be draining too fast or too slow, it will be important to do some work to improve your soil composition. Any amendments added to existing soil need to be chosen very carefully. It is important to create a balanced medium in the planter, provide good water drainage and use an amendment that will allow for air space.
Balanced Soil: Ideal soil consists of 25% air, 50% solid matter and 25% water.
Clay Soil: The addition of sharp sand and some form of organic matter is recommended when amending clay soils. Every 6 inches of clay soil needs 3 to 4 inches of organic matter. Adding gypsum (calcium sulfate) at the rate of 50-100 lbs/1000 ft2 may also improve internal drainage of clay-based soils. Gypsum provides a soil with the abundance of calcium ions which subsequently lowers the concentration of sodium.
Sand or Loamy Soil: If amending a sandy or loamy soil, add organic matter only. Every 6 inches of sandy or loamy soil needs 1 to 2 inches of organic matter.