Placing a tarp over sand, lime, and other aggregates that aren't being stored in a container or silo is a great way to protect them, but a tarp really offers a lot more protection than only keeping the sun and rain off the materials. When you get a custom-fitted tarp that covers all of a loose pile of sand, for example, you also help the environment, the surrounding farms and subdivisions, and the condition of the aggregates in the face of specific threats.
Reducing Wind- and Rain-Driven Pollution
Aggregates where the materials are lighter, like lime, can be blown away by strong winds and carried off by rain runoff. If a strong storm blows through, any pile of material is in trouble if it isn't protected. You can reduce rain runoff issues by locating the pile in an area farther up a slope or on land designed to carry the runoff away from the pile. But wind is another matter; it can sweep across open land but also strengthen if it's forced between buildings. It can seem like there's no way to win against wind unless you cover the aggregates with a well-fitted tarp.
Reducing the amount of aggregate that can blow away helps protect the land around you from unintended pollution. You reduce the risk of lime affecting soil on a neighboring farm, for example, and you stop extra salt from hitting roadways where it could damage landscaping.
Improving Relations With Neighbors
Keeping a tarp over aggregates also helps relations with your neighbors. No one wants materials from another property blowing over onto theirs, and while other farm owners might understand why it's happening, it doesn't make it any less annoying to find out the place where the pollution originated hasn't taken steps to prevent all that material from blowing over.
Reducing Mold Promotion
While materials like salt and sand might not be edible crops (this is not table salt under those tarps, remember), and they might not seem like things that can grow mold, if the pile is contaminated with soil or other mold-prone materials, then if the pile gets wet, mold and mildew can grow. This is a risk with anything you store outside, but you can reduce the risk with a custom-fitted tarp that protects the whole pile.
Tarps come in different materials, and you'll need a very good idea of how big the tarp needs to be for the eventual size of the pile you need to cover. Speak with manufacturers and sellers to find the best options for creating a custom tarp.