Bottom watering, the technique of hydrating plants by placing the pot in a tray of water, is a popular method for ensuring thorough soil saturation without disturbing the delicate foliage or soil surface. The answer is yes, bottom watering can be an effective way to deliver nutrients, but it requires careful execution and specific attention to fertilizer concentration. This approach is particularly helpful for plants that dislike having wet leaves or for soil mixes that have become so dry they resist absorbing water from the top. However, because the plant sits in the solution for an extended period, the margin for error with nutrient concentration is small.
Understanding Capillary Action
The foundation of bottom watering lies in a scientific principle called capillary action, which allows water to move upward against the force of gravity. This phenomenon occurs because water molecules are attracted to the small pores and channels within the potting soil particles (adhesion). Simultaneously, the water molecules stick to each other through cohesion, pulling neighboring molecules along as they climb.
This combined action draws moisture from the reservoir at the bottom of the pot up into the root zone, much like a sponge soaking up a spill. Finer-grained soils, which have smaller spaces, generally exhibit stronger capillary action than coarser mixes. This mechanism ensures that the entire soil column is evenly moistened, providing a consistent supply of water and dissolved nutrients directly to the roots.
Step-by-Step Guide to Bottom Fertilizing
Integrating fertilizer into the bottom-watering routine requires selecting a liquid or fully water-soluble powdered fertilizer that is completely dissolved before use. Use a solution significantly more diluted than the manufacturer’s directions for standard top watering. A general guideline is to use half-strength or even quarter-strength, especially for young plants or frequent feeding, due to the prolonged contact time with the roots.
Pour the diluted fertilizer solution into a tray deep enough to reach about one-third of the pot’s height. Place the container directly into the solution, ensuring the drainage holes are submerged so capillary action can begin. Monitor the soil surface; once it appears visibly damp (usually 20 to 30 minutes), the plant is fully saturated. Allowing the plant to soak for too long risks waterlogging the roots.
Once the soil is fully saturated, immediately remove the plant from the nutrient solution to prevent the roots from sitting in stagnant water. Discard any excess fertilizer solution remaining in the tray to avoid over-fertilization. Allow the pot to drain completely for several minutes before returning it to its usual location, ensuring proper aeration and reducing the risk of rot.
Managing Salt Buildup and Concentration Risks
The primary challenge with bottom fertilizing is the potential for mineral salts and fertilizer residue to accumulate on the soil surface. As the water wicks upward and evaporates, non-volatile mineral salts are left behind. This accumulation manifests as a visible white or tan crust, which can inhibit nutrient uptake and cause chemical burn to the plant roots.
To counteract this concentration risk, periodically flush the soil with plain water. This involves thoroughly drenching the soil from the top with a large volume of water until it drains heavily out of the bottom holes. This top-down flushing action dissolves the accumulated salts and washes them out of the potting medium. Performing this leaching every few weeks, or at least every third or fourth watering session, maintains a healthy balance and minimizes salt damage.