After reading the following two pamphlets many years ago I decided there was no such things as weeds they can either indicate whats wrong with the soil or place needed trace elements and minerals back into it. I never looked at farming and agriculture the same again.

Three Major groups of weeds. The first comprises of those that live in acid soil and therefore indicating increased acidity. To this group belong the Sorrels, Docks, Finger-leaf weeds, Lady’s Thumb, and Horsetail.

The second major group indicates crust formation or hard pan in th soil. Belonging here are the Field Mustard, The horse Nettle, Penny Cress. Morning Glory, Quack Grass, The Camomiles and the Pine Apple Weed.

The third major group consists of those weeds which follow human cultivation, spreading out with compost and manure, wherever man walks. Here belong the Lambs Quarters, Plaintain, Chickweed, Buttercup, Dandelion, Nettle, Prostate knot Weed, Prickly Lettuce, Field Speedwell, Rough Pigweed, Common Horehound, Celandine, Mallows, Carpetweed.

The acid soil groups tell us the best which changes are occurring in the soils. acidity in the soil increases with lack of air, standing water in surface layer, cultivating in too wet a soil, insufficient cultivation, insufficient drainage, one-sided cultivation, wrong kind of fertilizers used, excess adic fertilizers, increasing sheet erosion and loss of humus.

Indicates –
Slightly acid soil due to insufficient cultivation ; Daisies, Horsetail, Field Sorrel, Prostate Knotweed.
Very acid Soil due to Wrong cultivation or insufficient drainage ; Cinquefoil, Swampy Horsetail, Hawkweed and knapweed.
Salty soils ; Shepards Purse, Russian Thistle, Sea Plantain, Sea Aster, Artemisia Maritima.
Hard Pan soils ; Wild Mustard and Horse Nettle.
Dry soils ; Mustard, Silvery Cinquefoil, Russian Thistle, Agrimony, catchfly, Broom Bush, Crotalaria, Dyers Greenweed, Rabbits Foot clover, Sweet clover, ST barnabys, Malthease Thistle, Common speedwel, Prostate Pigweed, Spurge, Shrubby ST Johnswort.
Sandy Soils : Most Golden rods, flowered Aster, Arrow Leaved wild lettuse, Yellow toad flax, Ononis, Partrdige Pea, Broom Bush.
Steepe Formation ; Russian Thisstle, Sage, Loco Weed.
Alcaline soils ; Sage brush, Woody Aster.
Lime Stone Soils ; Penny Cress, Field Peppergrass, Hares Ear mustard, Wormseed, Canada Blue Grass, Cornelian cherry, Barnabys Thisstle, Field madder, Mountain bluet, yellow Camomile.
High potasium in Soil ; Marsh mallow, wormwood, knapweed, Fumatory, Opium poppy. (Red clover disappears with lack of potasium)
Absence of lime ; Yellow Clover, Rabbits foot clover, fox glove, wild pansy, garden sorrel, sundews, white mullen, scotch broom, black vetchling.
Gypsum ; Common burdock
Moist badly Drained soils ; Smartweed, Mild water pepper, hedge bindweed, silverweed, white avens, swampy horsetail, meadow pink, hedge nettle, stinking willie, canada and narrow leaf goldenrod, tradescant, purple stem aster, joe pye weed, march foxtail, rice cut grass.
On Pastures ; Buttercups, Dock, knotweed, fingerweeds, white avens, grass leaved stickwort, ST Johnswort, pokeweed, Milkweed, Wild garlic, Briar, Thistles.

Sources of Minerals and Trace Elements –

bulbous Buttercup ; cobalt, copper, iron, manganese, phosphorus, potasium
broad leaved dock ; phosphorus, potasium
chamomile ; calcium, potasium,
coltsfoor ; calcium, copper, iron, magnesium, potasium, sulphur.
Sheperds purse ; calcium.
chickweed ; manganese, potasium
comfrey ; calcium, nitrogen potasium
creeping thistle ; calcium, copper, iron, potasium
daisy ; calcium, magnesium
dandelion ; calcium, copper, iron
fat hen ; calcium, iron
groundsel ; iron
pimpernel ; calcium
cleavers ; calcium
great pantain ; calcium, copper, iron, potasium, silica
horsetail ; calcium, cobalt, iron, magnesium, silica
silverweed ; iron, calcium, magnesium
sow thistle ; copper, potasium
stinging nettle ; calcium, copper, iron, nitrogen, potasium
sun spurge ; boron
valerian ; silica
yarrow ; copper, nitrogen, phosphorus, potasium,
tufted vetch ; cobalt, copper, phosphorus, potasium.