A self-watering planter requires only minimal irrigation through the growing season. Water sits in a reservoir below the plants, where it is slowly drawn into the potting mix as necessary. The only watering required is periodic refilling of the reservoir. The best potting soil absorbs and distributes the moisture consistently and evenly through the growing medium.
Self-Watering Soil Needs
Self-watering containers draw moisture up into the soil from a water reservoir positioned beneath the soil. The soil inside the container remains moist as long as there is water in the reservoir. Dense soil, such as that found in most garden beds, compacts in the small confines of the container. Compaction prevents the soil from drawing water up to the plant roots properly. A self-watering container requires a loose, well-aerated soil that can consistently draw in water without suffering compaction.
Soilless Mix vs. Potting Soil
Soilless mixes container compost and peat combined with perlite or vermiculite in varying combinations. These nonsoil materials provide a loose, well-aerated medium for container growing. Peat naturally wicks moisture, making it especially suitable for a self-watering container as the peat will draw the moisture up from the reservoir and into the potting mixture. These materials are typically sterile, which prevents most disease problems in the container. General potting soil mixtures typically contain some soil, which makes them more prone to compaction in the self-watering container. The soilless medium provides the optimum growing conditions in the self-watering planter.
Commercial or Homemade Mixes
Commercial soilless mixes provide a simple option for a self-watering container. The ingredients are listed on the bag, allowing you to choose a mix that contains no soil. Alternatively, create your own homemade mix by combining equal parts sterile compost, peat moss and vermiculite or coarse builder's sand. You have the most control over the contents of the mix if you make it yourself, making it the preferred choice, but a commercial soilless mix works well if you don't have access to the necessary ingredients.
Soilless mixtures have very few nutrients on their own, so they require fertilization. A self-watering container works best with a single fertilizer application applied at planting. A granular, slow-release fertilizer leaches into the soil slowly over the entire growing season. The slow water movement through the potting mix prevents too much fertilizer nutrients from being released at once. Sprinkle 1 to 2 cups of a balanced fertilizer in a strip on top of the soilless mix before you plant. Cover the strip with a thin layer of the mix. Plant to either side of the strip. The strip dissolves over a period of months, providing a constant supply of nutrients to the otherwise sterile potting mixture.
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