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Listen to your weeds!

Native plants can tell you a great deal about your land ... if you know how to read their signs

When looking for property for a garden or farm, it is important to be able to recognize good land and know how to spot possible danger signals. Even after selecting a location, you need to know when warning signals appear and what to do about them.

If pressured or narrowed into a choice that is less than ideal, will you be stuck with situations as you find them, or will you be able to improve conditions? One way of sizing up soil conditions when assessing a particular piece of property is to pay close attention to the naturally occurring plants. They can be a good indication of soil structure and composition.

Land covered with short grasses may be very good if there is plenty of irrigation water. But under drought conditions, this is the worst kind of soil. Lack of tall, deeply-rooted grasses results when moisture is absorbed by short species before it reaches the deeper soil; there is just not enough water for both types of plants.

If the short grass cover is very thick, the land is better than in sparsely covered areas. Soil which supports bunch-type grass will produce better than any other in bone-dry years, but it will be the most useless in wet years. Light soils are better where water is in short supply because the water runs through it more easily, forcing roots to go deeper.

If, as an organic grower, you expect to build up the soil's water retention capacity, a short water supply should not be of too much concern. Even if dryland farming is your goal, it's possible to cover crop steadily until the soil holds enough water for a crop every year.

Pure sand dunes can become good soil. Witness Golden Gate Park in San Francisco where the local children helped plant grasses to hold the soil until it could be built up with manure and compost. Even if not suitable for cultivation, such land could become grazing land, though you would have to be careful about overgrazing. An indication of overgrazed land is the appearance of Guterrezia and Artemisia species and Aristida.

There are fewer clay-sized particles on the crests of windswept sand hills than in the hollows and on the slopes. In these instances you might see buckwheat, wormwood, aristid and rescue grasses trying to bind the soil on the hilltops, but not as many of these grasses in the dips and hollows where there would be twice as many clay-sized particles. In any event, great quantities of organic matter would be needed to create a real soil.

When trying to determine the agricultural potential of sandy land, be sure there is sufficient sweet groundwater. Reed grass, giant reeds, wild rye, even licorice, are indications of a dense clay or sandy loam someplace, because fresh water would be accumulating at the interface between the sand and impervious clay. Willows, Russian olive (Eleagnus) and maybe even a wild rose in low spots are signs that water is available only a few feet down, probably lying atop an impermeable clay. The Eleagnus also supplies nitrogen to the soil, which would be a reason to suspect poor fertility.

Where sand is loamy and bullrushes, rushes and Eleocharis are growing, there could be springs as well as fresh water from lakes available three to nine feet below the surface. The presence of salt-loving plants like grasswort (Salicornia), Spartina, Frankenia and Distichlis would indicate that the water may be brackish and the soil needs to be drained before it can be reclaimed.

Swampy land, on the other hand, would be filled with plants which can grow in oxygen-starved soil because of special air-containing tissue: Equisetum (horsetail or scouring rush), rushes (Juncus), Scirpus, Alisma, Sagittaria and Sparganium. Bog plants that do not have special tissue but that still do well might also be seen: Kalmia (mountain laurel), Vaccinum (huckleberry group), Ledum or Andromeda (both heather family).

Certain plant groups have a few similar physiological requirements, regardless of whether the individual species grow in the north, south, east or west. This allows you to judge soil conditions at least in a general way. In the east, for instance, New Jersey tea (Ceanothus ovatus) indicates sandy territory. And in the west, sand cherry (Prunus besseyi) would tell you the soil is a loose sand. Both in the east and west wild plums (such as chokecherry--Prunus or wolfberry--Symphoricarpos) grow on southeast slopes where they depend on snowdrifts for moisture. Monkey flowers, present mostly in the west, indicate the ground is not only shaded but probably is excessively wet if the growth is lush. What might look like grassland at alpine heights would be good for nothing but grazing, even though it appeared to be a lush meadow.

In dry country the same would be true for desert scrub or chaparral (unless irrigated) for the same reasons to a certain extent--too shallow soil and no year-round source of water. On the west coast, dryland farming in such a location would be possible for winter crops only, as that is when rain can be expected.

In the central plains, sagebrush is a reliable indicator that land can be used for agriculture. Saline land that would be indicated by salt-lovers like Atriplex confertifolia (of the saltbush group) or Kochia would support crops only in very rainy years. This is too chancy for most people unless there is an economical source of organic matter available to counteract the salts.

Think twice about draining a bog having large-stemmed sedges, reeds, arum lilies (Calla palutris), bog iris, mosses (even sphagnum) and no trees. Draining swamps upsets a long train of ecological factors.

Trees, too, can provide clues to soil type and structure. Alders are hardly indicators because they grow practically in stream beds, but one particular species, Alnus glutinosa, indicates a lively underground stream. Good pine timber might grow on raised bogs, but where pines look downtrodden, there may be stagnant conditions. Beech trees like dry, well-drained, often chalky land. The trembling aspen (Populus tremuloides) prefers and conditions, but the ordinary poplar grows in moist spots or in streams.

Another consideration is the lasting or transitory nature of the state of the land. Can you be sure that swampy land will still be swampy after a year or two of drought? Rushes and marsh marigolds (Caltha palustris) promise lasting percolation, or at least that the water table will stay near the surface. Certain heath and lichens are pretty good indications that the land will stay dry all the time. Slow growing ferns prove that the damp area has been that way a long time.

How about predilections of plants for certain nutrients or trace elements? Where rock phosphate, for example, lies close to the soil surface, legumes will find a place. Caragana is a good example. Believe it or not, in some dry areas where tar and asphalt are evidence of a petroleum company's interest, some plants not only grow lushly, but have two vegetative-flowering stages--one in spring and one in fall.

The presence of certain plants known to be desirous of certain elements, does not always mean that element is present in the soil. The plant may merely be tolerant, or it may be lush because there is no competition. If the element the plant has an affinity for is a toxic one, the plant may also have chelating compounds which form complexes with the heavy (toxic) metal and render it innocuous. The accumulation of such a mineral by a plant may be under genetic control. This is the case with copper and the grass, Agrostis tenuis. Generally, a plant is tolerant of only one such mineral. (In corn, calcium and sulfur are absorbed by roots in the presence of chelates.)

Some plants are considered indicative of acid or alkaline soil. A study of Scabiosa columbaria showed it refused to grow in acid soil because it finds aluminum ions toxic. When the growing medium was treated with lime, the plant improved. It was not responding to the calcium itself, but to the new state in which it no longer was disturbed by the aluminum. (Aluminum toxicity is not really well understood. Aluminum phosphate is precipitated in root tissue, resulting in a phosphorus deficiency. Not only this metal, but others with trivalent cations have a direct effect on cell division.)

There can be vital differences even between closely related plants. If you see Agrostis on soil high in calcium, you can say bentgrass likes (or accumulates or tolerates) calcium. A research effort with this genus found that A. stolonifera grown in a high-calcium solution would take all the calcium it could. The more it took, the better it grew. However, A. setacea growing in low calcium soil did not respond in that way at all.

The whole calcium picture is a tricky one, so we should not be too quick to make determinations. At any rate, the plant is more dependent on the status of calcium in the soil during seed germination and the establishment of the seedling, than it is during late regrowth stages. White clover (Trifolium repens) has a number of types which vary in nutritional needs and which are adapted to soils with differing calcium content. Various bedstraws, a common indicator weed, have different requirements, too. G saxatile grows on low-calcium soils, but G. pumilum prefers chalky soils.

The perennial fescue, Festuca ovina, indicates lead in the soil; it merely tolerates it. The perennial, Silene inflata, tolerates both zinc and copper, but the tolerance ranges through a whole range of types due to the effect of many genes on this one characteristic.

No scientist really likes to be absolutely dogmatic about soil-indicating plants, because everything seems to have some effect on accumulation: light, temperature, moisture, etc. In rice fields, for example, there is a much greater uptake of cadmium (toxic heavy metal) when the field is not flooded. It also accumulates much more in leaf tissue than in the grains, and so would vary in time. Ferrous iron and (divalent) manganese ions pile up in the soil solution when the rice fields are flooded. The plant picks them up and concentrates them in the leaves just because they are available. Zinc, too, will be in greater concentration in rice leaves under flooding conditions, but the amount of copper in the leaf remains the same regardless.

Another aspect to consider is that even if the plant absorbs a mineral, it may not really use it. So, how can it in effect, be an indicator plant? Even if a root can absorb iron, for instance, from chalky soil, perhaps it cannot make the iron complexing compounds. Therefore, the iron that enters the roots cannot be mobilized to get up to the shoots in a form which can be used in the physiology of the leaf and stem.

On the other hand, while selenium is accumulated by asters, Machaeranthera, Atriplex, Oonopsis and some species of Astragalus (milkvetch), there are some plants which are obligated to accumulate it -- they need it: Stanleya (Princesplume), Xylorrhiza (woody aster) and Astragalus mollissimun (locoweed).

(Organic growers will note that all these species act as converters of the inorganic form of selenium to soluble organic forms. These can be taken up in non-toxic levels by plants which are not selenium accumulaters, such as grasses and grains. This means that there is apparently no harm to neighboring plants for the locoweed syndrome.)


One should remember that there are gradations of these characteristics. Some plants will grow where the soil is dry and salty, or dry and chalky, or dry with hardpan. Even though acid-preferring plants grow in a spot, this does not mean there is necessarily little calcium there. It could be that the subsoil is quite calcareous but that the upper soil layers have been depleted of humus or denuded of calcium because of wrong cultivation methods or the use of too much chemical fertilizer (which induces acidity). Acidity may also be the result of mere lack of soil aeration, a stagnant condition.

Since these many characteristics overlap, one cannot always trust such criteria as listed here. Some weeds are so vital they will grow practically anywhere. This is why they are still weeds after thousands of years.

In the final analysis, whatever the condition of the soil, two things must be done: adding organic matter to act as a buffer and adding mineral dusts to supply needed elements in inorganic form to be changed by the plant into the organic forms plants prefer for their metabolism.

In particular, let me warn you about the use of common names, which are almost worthless for identification. The same name may apply to half a dozen different plants with quite different environmental habitats and needs. They may even be from different families. If you do not have the correct botanical name, do not try to diagnose the soil condition. Even closely related species may be very different: one a swamp plant, another a hardpan dweller.


Though there are numerous variables involved in using plants as soil indicators, it is nevertheless helpful to be able to make some generalizations, especially for garden or row crops. As common names are confusing and often interchangeable, this list includes the botanical name for accurate identification.
Found in all soils:
 lamb's quarters-Chenopodium album
 chickweed-Stellaria media
 shepherd's purse-Capsella bursa-pastor

Found in alkali areas only:
 saltgrass-Distichlis spicata (coast)
 D. stricta (desert)
 goosefoot-Halogeton glomeratus

Moisture-loving sedges-Cyperus plants suggesting poor drainage:
 dwarf St. John's wort-Hypericum
 silverweed-Potentilla anserina
 creeping buttercup-Ranunculus repens
 sumac-Rhus integrifolia
 curly dock-Rumex crispus
 sorrel-Rumex acetosella
 hedge nettle-Stachys palustris
 mayapple-Podophyllum peltatum
 thyme-leaved speedwell-Veronica
 American hellebore-Veratrum viride
 white avens-Geum album

Plants preferring alkaline soil:
 some mustards
 blueweed-Echium vulgare
 gromwell-Lithospernium officinale
 field peppergrass-Lepidium campestre
 true chamomile-Chamomilla matricaria
 salad burnet-Poterium sanguisorba
 scarlet pimpernel-Anagallis arvensis
 bladder campion-Silene latifolia

Plants preferring acid soil:
 bracken fern-Pteridium aquifolium or aquilinum
 spurrey-Spergula arvensis
 corn marigold-Chrysanthemum segetum
 sow thistle-Sonchus arvensis
 scentless mayweed-Matricaria inodoro
 English daisy-Bellis perennis (slightly acid)
 sorrel-Rumex (slightly acid)
 prostrate knotweed-Polygonum aviculare

(slightly acid)
 lady's thumb-Polygonum persicaria
 knapweed-Centaurea nigra (very acid)
 hawkweed-Hieracium (very acid)
 goose tansy or rough cinquefoil-Potentilla monspeliensis
 silvery cinquefoil-Potentill argentea

(very acid)
 wild strawberry-Fragaria
 rabbit-foot clover-Trifolium arvense

(slightly acid to very acid if swampy type)

Plants which usually do best in heavy soil:
 coltsfoot-Tussilage farfara
 creeping buttercup-Ranunculus repens
 dandelion-Taraxacum officinale
 English daisy-Bellis perennis
 broadlead dock-Rumex obtusifolius

Plants usually found in light or sandy soil:
 spurry-Spergula arvensis
 corn marigold-Chrysanthemum segetum
 sheep's sorrel-Rumex
 cornflower-Centaurea cyanus (esp. when flowers are pink)
 small nettle-Urtica urens
 shepherd's purse-Capsella bursa-pastoris
 white campion-Lychnis alba
 Maltese thistle-Centaurea melitensis
 St. Barnaby's thistle-Centaurea solstitialis

Plants found on hard, crusted soils:
 all the chamomiles
 morning glories
 quack grass-Agropyron repens
 goosefoot-no matter how poor the soil-Chenopodiums

Plants which grow on salty soils:
 Russian thistle-Salsola kali
 seas aster-Aster tripolium
 asparagus-Asparagus officinalis
 shepherd's purse-Capsell bursa-pastoris
COPYRIGHT 1999 Countryside Publications Ltd.
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Copyright 1999 Gale, Cengage Learning. All rights reserved.

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Publication:Countryside & Small Stock Journal
Geographic Code:1USA
Date:Jul 1, 1999
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