Lava Rock is a naturally occurring volcanic cinder that has a porous structure which makes the material lightweight and highly moisture retentive. Incorporate into your bonsai soil mix to increase drainage (aeration). It will absorb up to 40% of its weight in water. Small lava rock (1/4 to 3/8 inch) is the best “sand” component around because it is “sharp” and has very irregular surfaces.
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