Plant!t Vermiculite 10L or 100LIncreases the Water and Nutrient Retention of the Substrate Vermiculite (magnesium aluminum iron silicate) is a popular soil additive. It's lightweight and completely inert, making it very similar to perlite but it has much greater levels of water retention. It also tends to hang on to nutrients, releasing them slowly and providing a sustained source of nutrition. Vermiculite is pH neutral so it won't affect the acidity alkalinity levels of the