Since many weeds are "specialists" they are likely to be particularly useful as indicators. Different weeds are adapted to different ranges of environmental variables and are able to grow only where their particular needs are met. For example, certain species, such as knawel, are confined to acid soils, while others are limited to basic soils.

The use of weeds as soil indicators is not a new concept. In 50 AD, the great Roman scholar, Pliny the Elder observed that land supporting wild plum, elder, oak, and thimbleberry was also favourable for wheat production. Many North American immigrants chose land for their farms according to the vegetation it supported. They quickly recognized that white pine--Norway pine-- jack pine communities were characteristic of sandy soils of little agricultural value, whereas forests of birch, beech, maple, or hemlock indicated more fertile soils.

It has been shown that the tall-grass prairies are suitable for orchards, cereals, hay, and fodder crops, while bunch grass regions are more suitable for wheat and grass production (Shantz 1911, Sampson 1939). Wire grass areas are less productive and short grass communities least productive. Highly alkaline soils are unsuitable for arable use and are characterized by tussock grass, salt grass, and greasewood (Hilgard 1906).

The information on weed indicator species is poorly documented, much of it residing only in the minds of observant farmers and gardeners. In preparing the list of weeds in Table 1, numerous sources, some reliable and some undoubtedly less reliable, were consulted. Consequently, the information contained in it should be used as a basis for further observation and research rather than as a guarantee of what to expect from a soil. Before using such a table, there are several things to consider:

1. Some weeds have "ecotypes".

   Populations of a particular weed growing in different locations may differ slightly from each other in their appearance and requirements; they are referred to as "ecotypes". Thus, the ecotype of a particular weed in one area may be more tolerant of acid soil conditions than the ecotype of the same species in another area

2. Limits of tolerance to environmental factors vary.

   Plants, including weeds, differ enormously in their degree of tolerance to changes in soil pH, moisture content, etc.; and some have a narrow tolerance for one variable but a wide tolerance for others. The best indicators are those with narrow tolerances because they would only be found associated with specific conditions.

3. Plants may be sensitive to several environmental factors.

   When we look at Table 1 we notice that many of the plants are listed in more than one category of environmental factors. For instance, perennial sow thistle and docks are both indicators of wet areas; however, the thistle has a preference for more acid soils whereas docks are found in soils with a high lime content. Thus, when interpreting the presence of a weed we need to know all the factors to which it is responding.

4. Perennial weeds often make better indicators than annuals.

   Perennial weeds, having been able to tolerate the conditions in a particular locality for more than one year, are often more reliable indicators than annuals, which may survive only one season.

5. Weed communities are better indicators than single species.

   The presence of a group of weeds that are associated with one another because of similar requirements for certain soil conditions provides a more reliable indicator in contrast to a single weed species, which may only indicate chance establishment.

6. Growth characteristics of a weed may be as revealing as its presence.

   The growth characteristics of weeds and the colour of their leaves and flowers may be as important as their presence in revealing information about the soil. A vigorous growth of leguminous weeds usually indicates a soil lacking in nitrogen; as does the presence of stunted nonleguminous weeds with pale green leaves. Cornflowers make particularly useful indicators as their flowers are blue when found on soils with a high lime content but are pink when they are growing on acid soils.

The observant farmer and gardener will notice subtle changes in the weed populations on his land in response to his agricultural practices. As his soil improves he may find that chickweed, chickory, common groundsel, common horehound, and lambsquarter become the dominant weeds. However, if he finds that the daisy, wild carrot, mugwort, common mullein, wild parsnip, wild radish, and biennial wormwood become dominant, he should review his practices as these weeds thrive on soils of low fertility. The addition of well-balanced compost, organic manures, and other fertilizers together with certain tillage and drainage practices may be required to bring the soil back into production.

We are not advocating that all weeds be encouraged indiscriminately, for even "beneficial" weeds poorly managed, will reduce yield. What we are suggesting is that by being able to identify the weeds on our land and know what their presence indicates, we will be in a better position to manage our soils wisely.

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