Weeds as Indicators of Soil Health

by: Stuart B. Hill and Jennifer Ramsay

Confronted with a weedy field or garden, one's instinctive reaction is to rush out and destroy the weeds before they take over. Perhaps we imagine them choking out our plants, or, at least, stealing the fertilizer applied for our crop. This attitude towards weeds has predominated throughout history. In 110 AD Plutarch wrote "The richest soil if uncultivated produces the rankest weeds" (Lives: Coriolanus); and more recently Oscar Wilde wrote "The vilest deeds like poison weeds Bloom well in poison air" (The Ballad of Reading Gaol).

Only Lowell and Emerson have injected a ray of hope for the weed. Lowell I suggested that "A weed is no more than a flower in disguise" (A Fable for Critics); and Emerson asked, "What is a weed? A Plant whose virtues have not yet been discovered" (Fortunes of the Republic). Could weeds really have some virtues, a beneficial side to their character? It seems unlikely. Well, yes, actually weeds do have some points, in their favour. For example:

1. Many weeds protect our topsoil from the eroding forces of rain, wind, and sun, especially when the crop cover is poor.
   
2. By providing a cover vegetation, weeds enable beneficial soil animals to be active at the surface, depositing their nutrient-rich faeces and/or acting as biological control agents against various insect pests.
   
3. Many weeds, particularly perennials, possess extensive root systems that penetrate deep into the subsoil, breaking it up and enabling the less vigorous roots of some of our crop plants to penetrate further into the soil. Some roots, such as the leafy spurge, grow to depths of four to eight feet (1.2 to 2.3 meters), whereas Canada thistle roots may penetrate to depths of 20 feet (6.1 meters).
   
4. Breaking up the subsoil also improves drainage and creation.
   
5. Deep penetration by their roots often enables weeds to accumulate various elements from the subsoil, particularly trace elements, and transport them to the soil surface. Through the weed's subsequent death and decomposition, these elements become available to crop plants with less extensive root systems. Different "accumulator" plants concentrate different elements.
   Interestingly, the accumulated elements are often those in which the particular soil is deficient. Some farmers have utilized this property of certain weeds by employing them as green manure. For example, Rogers et a/. (1939) found that a local case of Floridian disease in corn, called white bud, was associated with zinc deficiency and could be prevented by allowing zinc accumulator weeds to develop during fallow years.
   
6. Weeds that accumulate different elements have also been used by prospectors. By analyzing different parts of the plants for high concentration of certain minerals, they have been able to determine the location of mineral deposits such as copper and selenium (Brooks, 1 9721.
   
7. Weeds have also been used as indicators of the presence and quality of ground water (Chikishev, 1965).
   
8. In the past, weeds have often been used both as food and as pharmaceutical products. Interest in these uses and in their development as resources for various industrial products is currently growing in the "developed world".
   
9. However, the primary value of weeds under consideration in this article is their ability to reveal information about the properties of our soils, particularly their nutritional status, pH, and presence of a hardpan. Frederick Clements (1920), the eminent U.S. botanist explained this property when he stated: "Each plant is an indicator. This is an inevitable conclusion from the fact that each plant is the product of the conditions under which it grows, and is thereby a measure of these conditions. As a consequence, any response made by a plant furnishes a clue to the factors at work upon it".

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