Allelopathy
Allelopathy is the secretion by plants of a toxic substance into the soil in order to prevent other plants from growing near them. This trait is most commonly exhibited in desert plants, where water is scarce and competition from other plants for it is a matter of survival.
Establishing allelopathy in plants
When a plant suspected of being allelopathic is tested, one method is to take portions of the plant as an extract or slurry and to use various concentrations of thematerial to see if, when added to soil, it will inhibit the growth of plants or seeds started in that soil. Since organic debris such as stems and leaves are known to leach nitrogen out of soil, and decreased nitrogen itself will reduce plant growth, nitrogen fertilizers are often added to the soil in order to counteract that general effect.
Phenolic content of plants
Invasive plants
The phrase "one man's weed is another man's wild flower" is an accurate summation of the subjective nature of desireable plants. However, when agricultural crops are grown and yields are reduced by overgrowth of "volunteer" plants, or non-native plants crowd out endangered species in their last remaining native habitat, the undesireable nature of the offending plant is not simply subjective, and is usually attributed to its invasiveness, that is, its ability to establish colonies in areas of land that are already inhabited by growing plants. One modality that weeds, non-native plants and other plants known to be invasive use in crowding out established growth is allelopathy.
Examples of allopathy in invasive plants
Verbesina encelioides (A. Gray, Asteraceae) is a perennial weed common in uncultivated fields in semi-arid India that often invades crops in cultivated lands.
Centaurea maculosa Lam. (European spotted knapweed, Asteraceae) is an invasive plant in North America.
References
Ridenour, W.M., and Callaway, R.M. 2001. The relative importance of allelopathy in interference: the effects of an invasive weed on a native bunchgrass. Oecologia, 126: 444-450. doi: 10. 1007/s004420000533.