Perlite

 

Perlite, of volcanic origin, is a siliceous, sterile, spongelike, amorphous glass mineral. Free draining with low water retention properties, perlite is commonly mixed in a 1:1 ratio with vermiculite to create a substrate that maintains the right amount of water while having good drainage.

Perlite is a natural heat expanded  volcanic mineral that is chemically inert, and will not alter the nutrient quality or the Ph of the material with which it is mixed. The particles provide excellent drainage, while trapping air and water on their irregular surfaces. Perlite is available in fine, medium, and coarse grades.  

The higher the total porosity of the mix, the greater the potential for providing water and air. Oxygen is essential to healthy root activity, but not more essential than moisture. Practical boundaries are 75% perlite/25% peat moss for lowest wet bulk density and 25% perlite/75% peat moss for highest wet bulk density. Finer grades of perlite decrease porosity of the mix, while coarse grades increase porosity.  For ease of transplanting tender seedlings, adequate levels of aeration and moisture, I have been using the coarse grade of perlite. It provides the additional benefit of maximum root oxygenation.

Added to your germination mix, perlite will improve drainage and may reduce the cost of the overall mix. Outdoors, perlite performs similarly to help hold nutrients from leaching in sandy soil and to improve aeration and drainage in heavy clay soils.  It can also be used as an insulating winter mulch if covered with about 2 inches of soil.

Bottom Line: We do not recommend perlite. It tends to float to the top of soils, and it crushes to powder easily. It doesn't really have space where the roots can access air. We recommend coconut coir instead.