Plants to help maintain soil nutrient balance

Something I've always been really fascinated by is how plants just seem to have endless applications and uses. So when it comes to maintaining a good soil nutrient balance for growing my crops, how much help can I get from companion planting or from growing plants specifically to use as a nutritious mulch or to turn into liquid fertiliser?

Comfrey is an obvious one to use, and seems to work well as a tomato mulch as it's high in potassium (good for fruiting plants) but having done some more investigating, here are my top 3 alternatives:

1. Nettles
Ok, so part of my reason for including these is simply that I have absolutely loads of them and it'd be good to put them to some good use rather than just swear at them for stinging me!

But apart from that, they're also quite handy. They're a good source of nitrogen, so great for leaves and stems and for young plants needing to grow quickly. They also contain magnesium (needed for photosynthesis), sulphur (for photosynthesis, root and seed production) and iron (a trace element also used in photosynthesis).

It's best to crop nettle before it flowers and it's usually recommended that you make it into a liquid fertiliser. I wonder if you can use it as a mulch too...

2. Bracken
This is the only plant I can find that is said to be high in phosphorous (so good for root growth). It's also a good source of potassium and it either lowers or maintains your soil pH, so should be generally good for improving nutrient availability if you've got quite an alkaline soil (which I do).

Bracken can be used as a mulch (this is said to be particularly good around trees to promote tree root growth). I can also be made into a compost - best made with green summer bracken. This is good for ericaceous plants (blueberries and heather, for example) as, of course, it helps maintain a slightly acid soil. It'll also help prevent potato scab as this is caused by a bacteria that likes limey soil.

3. Clover
Famously nitrogen fixing, I definitely intend to grow clover as a ground cover plant. It can also be cut and dug into the soil or used in the compost as a good source of nitrogen and trace elements.