Have you ever wondered what exactly potting soil is made of? Not all soil is created equal, and since you've invested a lot of time and money into your garden, make sure you're also investing in the quality of soil that you're using. Cheap soil is often only worth what you paid for it!
Be a Label Snob
Take a moment to read the label and see what the ingredients are. Potting soil and compost mixes come in many different blends and mixes, including organic mixes which I prefer. This can also help you find out what the nutrient value is of the compost or soil is, and if any extra nutrients have been added. The three-digit NKP number (nitrogen, phosphorous, and potassium) on the bag shows you the nutrient value. A bag of soil that is 20-20-20 will have double the amount of nutrients as a bag that is 10-10-10, and is better value for your money!
Personally, I prefer buying potting mixes that don't have added nutrients, and then I add fertilizer specific to the plants that I'm growing. This ensures the individual health of my plants. As well, most potting mixes with added nutrients still require you to fertilize the plant within three weeks anyway, as the nutrients are not sufficient for the long term.
Let in the Air
Good potting soil will be free draining and airy. You can test this by squeezing some of the soil in your hand. If it bounces back and puffs up again, then it is free-draining and will allow your plant roots to breathe. If it stays compressed, then it's structurally not useful for your gardens or posts, as it won't drain well and your roots will end up soggy. Historically, bagged mixes use peat moss for retention of water, then add in other ingredients like bark or perlite in order to create a mix that adds air into the soil.
Storing your Soil
If you have a bag of soil that is half used, make sure to store it in a dry space, out of the sun and light. It will stay fresh as long as the bag is closed, rolled, and taped. Bags of compost or manure generally need to be used right away, or if that's not possible, stored in air-tight containers like Tupperware.