Urine is an excellent compost heap activator (that great pioneer of organic gardening, Laurence D. Hills, called it euphemistically "liquid household activator") but you shouldn't be using it as a fertiliser. Grow a patch of Bocking No 14 comfrey instead. Take a couple of crops off it each year (it will last forever) and soak the leaves and stems for a good two or three months in a water butt. Even leave it until next year, which is what I do. Dilute the resulting rather smelly liquid about 4:1 and water your crops with it about once every couple of weeks in summer. You'll be amazed. It's especially good on potatoes, tomatoes and roses, but it's good for everything.

As for Epsom salts, don't use it. You don't need it. Magnesium deficiency in soil is fairly uncommon, mostly found in acid, sandy soils, and, in any case, if you're gardening properly by feeding your soil with lots of compost or well-rotted manure you will automatically be correcting pH and magnesium issues. Epsom salts is a harsh inorganic chemical that will damage the soil ecosystem and hurt your plants' roots.