Gardeners and farmers constantly battle with the weeds, but weeds can have a useful purpose. We should be using weeds as soil indicators. Simply by observing the most prevalent weeds that are growing in a specific area, they can indicate if the soil is acidic or alkaline, whether the soil is a healthy, balanced soil, or if it’s depleted. Weeds can indicate a poorly draining soil, or a soil that is unable to retain moisture. Weeds can even indicate if the soil is unbalanced, being overly rich in one nutrient and deficient in others.
When using weeds as soil indicators, observe several of the most prevalent types of weeds to get an accurate soil assessment. For example, the dandelion and common mullein both indicate an acidic soil, but common mullein can also mean a low fertility soil, so if you see it alone, it could mean several things, but seeing it along with dandelions would indicate an acidic soil.
Pay attention also to the health of the weeds, a healthy stand of clover could indicate a soil that lacks nitrogen, while the same weed will grow in soil that had sufficient nitrogen, but will appear much less vigorous. It should be noted that some weeds like purple nettle (Lamium purpureum) and Shepherd’s purse (Capsella bursapastoris) will grow on most soil types and so are not reliable indicators. Cornflowers (Centaurea cyanus), and Hydrangeas are excellent indicators for a soil’s pH, the flowers will be pink in an acidic soil and blue in an alkaline soil.
Specifics on Using Weeds as Soil Indicators
An acidic soil is a soil with a pH below 7.0. Look for these weeds as an indicator of an acidic soil: eastern Bracken (Pteridium aquifolium), Buggenum buttercup (Ranunculus spp.), Chamomile-German (Chamomilla pecutita), Curly Dock (Rumex crispus), English Daisy (Bellis perennis), Ox-Eye Daisy (Chrysanthemum leucanthemum), Dandelion (Taraxacum officinale), Hawkweeds (Hieracium aurantiacum and pratense), Knapweeds (Centaurea species), Lady’s-Thumb (Polygonum persicaria), Mayweed (Arthemis cotula), Mosses (Musci class), common Mullein (Verbascum thapsis), Nettles (Urtica dioica), Wild Pansy (Viola sp.), Pineapple Weed (Matricria matricariodes), Pinks (Dianthus sp.), Plantain (Plantago major), Prostrate Knotweed (Poly-aviculare), Wild Radish (Bapranus raphanistrum), Rough Cinquefoil (Potentilla monspeliensis), Sheep Sorrel (Rumex acetosella), Silvery Cinquefoil (Potentilla argentea), Sow Thistle (Sonchus species), Corn Spurry (Spergula arvensis), and Wild Strawberries (Fragaria species). Plants that grow well in an acidic soil are azaleas, blueberries, endive, hydrangeas, rhododendrons, rhubarb, potatoes, shallots, sweet potatoes, and watermelons. Adding lime or using woodstove or fireplace ashes can raise the soil’s pH to the desirable pH range.
Alkaline soil has a pH higher than 7.0. Weeds that indicate an alkaline soil are: Bellflower (Campanula sp.), Bladder Campion (Silene iatifolia), Wild Carrot (Daucus carota), Field Peppergrass (Lepidium virginicum), Goosefoot (Chenopodium species), Gromwell (Lithospermum officinale), black Henbane (Hyoscyamus niger), white Mustard (Brassica hirta), Pennycress (Thlaspi arvense), Salad Burnett (Poterium sanguisorba), Scarlet Pimpernel (Anagallis arvensis), Stinkweed (Thlaspi arvense), Nodding Thistle (Carduus nutans), and True Chamomile (Anthemis nobilis). Asparagus, broccoli, beets, lettuce, muskmelons, onions, and spinach do well in alkaline soil. Sulfur can be added to a overly alkaline soil to lower it’s pH.
A healthy, fertile soil will have a pH of 6.2 to 7.0. Weeds indicating a fertile soil are: Burdock (Arctium minus), Butter Print (Abutilon theophrasti), Chickweed (stellaria media), Chicory (Cichorium intybus), Dandelion (Taraxacum officinale), Fat Hen ( Atriplex hastata), Groundsel ( Senecio vulgaris), Lamb’s-Quarters (Chenopodium album), Pigweeds (family Amaranth), Pokeweed (Phytolacca americana), Purslane (Portulaca oleracea), Queen Anne’s lace (Daucus carota), and Velvetleaf (Abutilon thoephrasti). Broccoli, corn, lettuce, melons, peppers, squash, and tomatoes are all heavy feeders and will thrive in a fertile soil.
A poor or depleted soil will have weeds such as: Broom sedge (Adropogon virginicus), Dog fennel (Eupatorium capillifolium), Wild Radish (Bapranus raphanistrum), Sheep Sorrel (Rumex acetosella), Wild Parsnip (Sium suave), Biennial Wormwood (Artemisia bennis) and Yellow toadflax (Lindaia vulgaris). Beans, beets, carrots, legumes, parsnips, peas, radishes, sage, and thyme will tolerate poor soil conditions and perform well in depleted soil.
A heavy or Clay soil will have Bradleaf Dock (Rumex obtusifolius), Wild Carrot (Daucus carota), Chicory (Cichorium intybus), Creeping Buttercup ( Ranunculus repens), English Daisy (Bellis perennis), Dandelion (Taraxacum officinale), Mayweed (Arthemis cotula), Milkweed (Asclepius syriaca), Plantain (Plantago major), Canada Thistle (Cirsium arvense), and Wild Garlic (Allium vineale).
Weeds that indicate a wet, poorly draining soil are: Hedge Bindweed (Convolvulus Sepium), Bull sedge (Carex lasiocarpa), Canada goldenrod (Solidago graminifolia) Cattail (Typha latifolia), Coltsfoot (Tussilago farfara), Creeping buttercup (Ranunculus repens), Curly dock (Rumex crispus), Ox-Eye Daisy (Chrysanthemum leucanthemum), Docks (Rumex sp.), Foxtail (Hordeum jubatum), Goldenrods (Solidago sp.), Groundnut (Apios americana), Poison Hemlock (Conium maculatum), Horsetail (Equisetum arvense), Jewelweed (Impatiens pallida), Joe-pye weed (Eupatorium purpereum), Lady’s thumb (Polygonum persicaria), Marsh Mallow (Althaea Officinalis), May apple (Podophyllum peltatum), Meadow pink (Lychnis floscuculi), Meadow Sweet (Astilbe sp), Mosses (all species), Stinging Nettles (Urtica urens), Pennsylvania smartweed (Polygonum pensylvanicum), Ragwort, Tansy (Senecio jacobaea), Sheep sorrel (Rumex acetosella), Silvery cinquefoil (Potentilla argentea), Sweet flag (Acorus calamus), Tall buttercup (Ranuculus acris), Thyme-leafed speedwell (Veronica serpyllifolia), Black Willow (Salix sp.) If you see Dock, Horsetail, Foxtails, Willows, Ox-eye Daisy, Goldenrod, Poison Hemlock, Rushes, Sedges and Joe-pye you can expect soil in that area to experience soggy or swampy conditions at some time of the year. Wet spots are obvious during the rainy season but could appear fairly dry at other times. These weeds are excellent indicators that the area will be soggy at some time during the year.
Weeds that grow in sandy soils are: Arrow-leafed Wild Lettuce (Lactuca pulchella), Field Bindweed (Convolvulus arvensis), White Cockle (Lychnis alba), Cornflower (Centaurea cyanus), Dog Fennel (Eupatorium capillidolium), Goldenrods (Solidago sp.), Maltese Thistle (Centaurea melitensis), Sandbur (Cenchrus species), Small Nettle (Urtica urens), and Yellow Toadflax (Linania vulgaris).
Weeds that indicate a hardpan soil are: Field Mustard (Brassica nigra), Horse Nettle (Solanum carolinense), Morning Glory (Ipomoea purpurea), Pennycress (Thlaspi arvense), Pineapple Weed (Matricria matricariodes), and Quack Grass (Agropyron repens). Bok choi, broccoli, cabbage, cauliflower, and mustards grow well in this type of soil.
Previously cultivated soil will have theses weeds predominately: Carpet Weed (Mullugo verticillata), Chickweed (Stellaria media), Dandelion (Taraxacum officinate), Lamb’s Quarter (Chenopodium album), Plantain (Plantago major), Purslane (Portulaca oleracea), Ragweed (Ambrosia artemisiifolia), and Rough Pigweed (Aramanth family).
Individual weeds that indicate a soil’s nutrient values are useful in determining if the soil is unbalanced. Annual Bluegrass (Poa annua) indicates very low calcium, low humus, low bacterial count, and high magnesium levels. Burdock grows in soils very high in iron and sulfate, and very low levels of calcium and manganese. Buckhorn Plantain indicate very low levels of calcium, low humus levels, and very high in chlorine, magnesium, potassium, and sodium. Common Chickweed and Mouse Ear Chickweed indicate very low calcium and phosphorus levels, and very high potassium and sodium levels. Crabgrass indicates very low levels of calcium and phosphorus, low pH, low humus, very high chlorine levels, and high levels of magnesium and potassium.
Dallisgrass indicates low calcium, very high magnesium, and high potassium levels. Dandelions indicate very low levels of calcium, and very high levels of chlorine and potassium. Hop Clover and Oxalis indicate very low levels of calcium and high levels of magnesium. Prostrate Spurge indicates low calcium levels and very high levels of chlorine, magnesium, potassium, and sodium. Purslane and Mustard indicate an abundance of phosphorus. Red Clover indicates an excess of potassium. Redroot Pigweed indicates an abundance of nitrogen. White Clover indicates very high levels in chlorine, magnesium, and sodium. Wild Garlic indicates very low calcium and bacterial count, and very high levels of chlorine, magnesium, potassium, and sodium. Yarrow (Achillea millefolium) indicates low potassium.
Soil can be depleted of a needed mineral or have an excessive amount of a mineral and need to become more balanced, eliminating many growing problems. Calcium doesn’t move freely within the plant, so the first symptoms of low calcium will appear in new growth. Chlorosis begins first at the leaf edges and then moves inward. Terminal buds become distorted. Young leaves will first turn yellow, then brown. Low calcium levels cause tomatoes to develop blossom-end rot and lettuce tip-burn. Low calcium is found in acidic soils, sandy soils, soils that contain excessive levels of magnesium or potassium. Temporary problems may be due to drought or excessive moisture. Eggshells or oyster shells will strengthen plants in low calcium areas.
Low copper levels will cause young leaves to become chlorite in a strange way. Leaf center yellows while the veins and leaf margins remain green for a while. Shoot tips die, terminal leaves become brown or leaves may fail to develop. Common in muck or peat soils, soils with too much lime, nitrogen, phosphate, phosphorus, or zinc.
Low-iron chlorosis begins at the top of the plants and works it’s way down. Shoots may die back and the fruit become discolored. Alkaline soils or soils with excess aluminum or phosphorus can cause low iron levels. Iron is important in photosynthesis and is a catalyst in plant respiration and iron utilization.
Magnesium moves freely within plants, so a magnesium deficiency will start in the lower leaves, discoloring the veins. First they turn yellow, then orange, and finally brown. Leaves will feel brittle, thin, and sometimes cup upward. Magnesium deficiency is found in wet, acidic, or sandy soils, also in soils with high concentrations of calcium, fertilizers, and potash. Magnesium is vital for photosynthesis, facilitates the use of nitrogen, phosphorus, and sulfur. It cleanses the plant of toxins that happen as a by-product of its own metabolism, and it’s needed in the formation of proteins.
Manganese deficiency may be hard to diagnose because it’s similar to iron deficiency. Chlorosis is most severe at the top of the plant, with yellowing of the leaves first appearing near the leaf margins and developing into a V-shaped pattern. Leaves will then develop tan or gray spots. These spots are the major difference between manganese and iron deficiency. Manganese deficiency most often occurs in alkaline soils high in humus or soils with a pH of 6 or more. Manganese is a catalyst in the process of plant nutrition and encourages the growth and maturation of plants.
Nitrogen deficiency will cause plants to turn pale green, then yellow. It begins at the tip of leaves at the bottom of plants, especially older leaves, and works its way in the direction of the main stem. Yellowing gradually spreads up the plant to the top. Found in very sandy soils or soils low in organic material, also excessively wet or leached soil. Nitrogen regulates vital chemical reactions, needed in stem and leaf growth and induces rapid green growth.
In the early stages of phosphorus deficiency the plants look almost too healthy. Growth is normal but undersized. Plants become dark green frequently changing to purple, especially the undersides of leaves. Sometimes stems also take on this color. Leaves then yellow in the final stages. The plant has poor flowering and fruiting habits. Most common in cold, wet or very acidic (below pH5) soils and very alkaline soils (above pH 7.3). Phosphorus in needed for root formation, flowering, fruiting and ripening.
In potassium deficiency, the older leaves become mottled or spotted, edges become dry and scorched. Dead spots begin to appear, the stems are weak, root systems poor, and fruit ripens unevenly. Potassium deficiency causes a reduction in disease resistance and makes the plant less storable. It is more common in sandy or acidic soils, also where there are excess calcium or magnesium levels in the soil. Potassium is important for the formation of flowers, fruit, leaves and growing tip. Potassium helps with photosynthesis at low light level and in internal water regulation. Potassium improves flavor, fruit, vegetable and flower color. It also provides protection from insect damage, disease, and frost.
Sulfur deficiency closely resembles nitrogen deficiency. The plants turn pale green, the effects show up first in young growth. Leaves turn yellow but they don’t dry out, and stems are weak. Legumes are most affected. Sandy or very wet soils, and soils containing excessive amounts of nitrogen are the most common soil types with a potassium deficiency. Together with nitrogen it makes protoplasm for plant cells.
A zinc deficiency can be similar to a nitrogen deficiency with rolled leaf margins. Chlorosis shows up first in young leaves, which are also reduced in size. Leaves are closely spaced, forming rosettes, and may be deformed. There is poor nitrogen formation in legumes. Soils that are sandy and acidic or alkaline and rich in humus, or excessively high in phosphates, nitrogen, calcium, or aluminum will most often be zinc deficient. Zinc aids in the formation of growth hormones, protein synthesis, seed and grain production, and plant maturation.
The observant farmer and gardener will notice subtle changes in the weed populations as the soil changes. As the soil improves, chickweed, chicory, common groundsel, common horehound, and lambs quarter become the dominant weeds. However, if the daisy, wild carrot, mugwort, common mullein, wild parsnip, wild radish, and biennial wormwood become dominant, that’s a sign of the soil’s low fertility and can be corrected. The addition of well-balanced compost, organic manures, and other fertilizers together with certain tillage and drainage practices may be required to return the soil back to a healthy, well balanced, and productive soil.
There will always be a battle between man and weed, but knowing that weeds can be used not only as a soil indicator, but also the flower of many weeds provide essential nectar and pollen, the foods needed by beneficial insects to complete their life cycle. Most insect pests would actually prefer to dine on weeds rather than your deliberately planted crops, if given the choice, so weeds can be good companion plants, and last, but not least, they’re edible. I can never defeat the weed, so I must live with the weed. Now that I’m using weeds as soil indicators, I have an excuse for not spending hours pulling out the weeds in the garden. Now I can spend time wandering in my blended garden enjoying both plants and weeds. They are rebels, just like me.