LECA is an acronym for Lightweight Expanded Clay Aggregate. They are baked clay balls that expand. Due to the expansion, they can soak up water. The porous surface allows roots to have something to grip onto. As they are made of clay, they do not contain any nutrients. You will need to add nutrients for the plants into the water.
LECA is most commonly used for hydroponics. Due to its size, it ensures good root aeration. It can be mixed into soil to promote drainage or be simply put at the base of the pot before adding soil on top as a space filler to reduce overall pot weight. It can also be added on top of soil to reduce moisture loss and reduce evaporation. As LECA is non-organic, it's a sterile medium which reduces pests.
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