Grow your own plant nutrients

for this:   Plant these:

Sulfur: mustard, fennel, and plantain 
Magnesium--mullein, sow thistle, carrot leaves 
Silica (not an established plant nutrient, but interesting)--borage, horsetail, bamboo
Zinc--vetch, alfalfa
Molybdenum--vetch, peach tree clippings, cornstalks
Iron--groundsel, strawberry leaves
Cobalt--vetch, most legumes
Boron--sweet clover, muskmelon leaves
Copper--dandelions, spinach
Carbon--sunflowers, beets, mangles, daikon 
Potassium--fennel, borage, chamomile, yarrow
Phosphorous--marigold, chickweed, lemon balm, buckwheat, mustard
Nitrogen--legumes, stinging nettle, kelp, cattail reeds, Elaeagenus,
fenugreek, carob
Manganese--rhododendron, bracken
Calcium--chicory, arrowroot

In addition to these nutrients, oxygen, hydrogen, and (in some desert and
salt-marsh species) sodium are required. Under most conditions, plants do not seem to have much trouble getting oxygen and hydrogen.