Liquid garden fertiliser
"He who is afraid of every nettle should not piss in the grass" - Thomas Fuller
Not to be confused with the delicious and mostly-beneficial herbal infusion, the organic gardener's nettle tea is a form of fast-acting plant food, after the fashion of compost tea. It is easy to make, versatile and rich in nutrients.
The first thing you will need is nettles. If you have a patch in your garden (as you might, given that it's a food source to encourage butterflies], please take care not to cut it all at once, and preferably, harvest once the caterpillars have pupated. You will also need a large container. I was fortunate enough to have an old 9-gallon water butt in my allotment garden, which I half-filled with loosely compacted nettle leaves and stems, which I covered by six inches with rainwater, and left to stew.
I have heard many recommendations for the best maturation time, but my own experience is that three weeks is adequate in a temperate zone in the spring or autumn. However long you leave it, be careful to cover it as best you can - mosquito and midge larvae seems to love it as a playground!
Once it is finished, strain off the liquor into jars or buckets, and sling the nettle sludge on the compost heap. (There is little risk of confusing the gardener's nettle tea with the cook's - there are few people who would find the aroma in any way appetising enough to drink.) I also found that the solids are good for starting a small compost heap - it seems to have a good balance of the relevant bacteria and nutrients to kick-start the anaerobic composting process.
Using the fertiliser
Using the "tea" is simplicity itself. For ground feeding, it can be used undiluted, but if you want to use it as a foliar feed, dilute it by at least half before using it, as it can damage some more sensitive foliage such as lettuce, carrots and some annual herbs.
The tea is rich in iron, potassium and nitrates, and has a variety of positive effects. It certainly provides a fillip to weak or straggly plants, can help to reduce rot and prevent leaf rust, and also encourages the growth of soil microbes. My neighbour, an allotment gardener of many years standing, also used it in the greenhouse, as he claimed that it discouraged whitefly.
Finally, it is possible to add plant material other than nettles. I was fortunate enough to have a lot of comfrey in my patch, and added that to the mix. This cocktail is allegedly good for fruits, including tomatoes, capsicum peppers and soft fruit such as strawberries and raspberries (be careful not to apply as a foliar spray within a couple of days of picking though).