Order vermiculitefrom One Garden in 10L bags for a better price than you can get in the stores.
 
Vermiculite added to the garden or vermiculite in potting soil increases water and nutrient retention and aerates the soil, resulting in healthier, more robust plants.

Read more at Gardening Know How: What Is Vermiculite: Tips On Using Vermiculite Growing Medium https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/vermiculite-growing-medium.htm