The grades differ in particle size and therefore in bulk density, water holding capacity, and aeration performance. The 1-3mm grade is finer, denser, and holds more moisture per unit volume – ideal for seed germination and propagation. The 3-6mm grade is coarser, lighter, and provides more aeration – better suited to rooting media, hydroponics, and construction applications. The 2-4mm grade performs between the two and is the most versatile for general horticulture.