Rocks in Pots: Drainage or Perched Water Table Problems?

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Many of us have grown plants in pots, planters or other containers. It’s a different environment. Not only is the planting location unique – isolated from the ground and open at both the top and some on the bottom, the container soils are different from ground bed soils.

The soil’s most important function regarding plants is to store and supply the water and minerals essential for plant growth and survival. Soil aeration is a critical element for plant growth as it supplies oxygen to removes carbon dioxide from the plant roots.

Image of rocks in potsThe soil water and mineral “tank” available to container plants is much less than for those growing in the ground that can tap into resources further from the plant. That being the case this reservoir must, therefore, be replenished by frequent irrigation and fertilization, thoughtfully managed.

Think of a flat sponge to picture the effect of container soil depth on how much water is available to the plant from the container soil. The sponge, like the soil, is full of pores which are occupied by water when the sponge is saturated. After water stops dripping from the flat sponge, then is stood on end, more water will drain out of it. Just increasing the height of the sponge by turning it up on end decreases its water content.

Container soil behaves the same way. When we water our plants correctly it soaks into the soil. Each particle although small, actually has lots of surface area from tiny uneven ridges and valleys. Through surface tension, a certain amount of our applied water is held by the soil pores and the remainder gradually percolates downward to the depth of the pot where all the soil is filled with water.

Actually, a perched water table (where the water “perches” or gathers) forms at the container soil bottom where the drainage level is, even though it is open at the bottom. This saturated water level is called a water table. This happens in outdoor soil too, not just in our containers. The water table is the dividing line separating the unsaturated zone from the saturated zone. The soil is saturated because the pores are filled with water. The area above the water table is the unsaturated zone and is where the plant’s roots have space to grow well. If gravel is added to the bottom of the pot, the perched water table area of saturated soil without aeration is above that in the container, so even less room for the roots to grow and be healthy in. Root rot diseases can be the result of roots remaining in waterlogged soils.

Although container smallness and shallowness create problems for growing plants, these problems can be minimized through proper irrigation, fertilization, and use of soil amendments.

Remember, CONTAINER SOILS ARE DIFFERENT and therefore require different care than garden or field soils. Skip the gravel in the bottom of the pot. It can create a result completely contrary to the drainage you want to achieve.

Minda Daughtry is Horticulture Agent for North Carolina Cooperative Extension in Lee County.