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This is a coarse sand that birds eat to help with their digestion. You can normally find it at farm supply stores.

This article may help: https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/nitrogen-plant-fertilizer.htm

You can move the mulch and mix composted manure and lime, but I would only recommend adding lime if you have had a soil test that recommends that the soil pH is off.

One of the fastest ways to enrich your soil before planting your garden is to mix some good compost. Here are a couple links that may be of interest to you: https://www.gardeningknowhow.com/edible/vegetables/vgen/vegetable-garden-soil.htm, https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/what-is-soil-made-of-creating-a-good-garden-planting-soil-type.htm

This article will help you with amending the soil: https://www.gardeningknowhow.com/edible/vegetables/vgen/vegetable-garden-soil.htm We have put together a guide for new gardeners too, if you are interested in that: https://www.gardeningknowhow.com/edible/vegetables/vgen/starting-vegetable-gardens-guide.htm

You shouldn't have to add soil each year so long as it's healthy, though refreshing the soil with compost may help. If it's filled to capacity, simply remove some to allow room for your mulch.