Black walnut trees secrete Juglone, an allelotoxin which can prevent establishment of other plants for a diameter of up to 75 feet.
We are always talking of what plants uptake from the soil; nutrients, minerals, water, but what about the things plants deposit in the soil? Plants actually secrete compounds into the soil through their roots, this process is call rhizodeposition. Proteins; sloughed off plant cells and sugars secreted into the rhizosphere (narrow region of soil affected by root secretions) are fed on by bacteria. The rhizoshpere also contains more protozoa and nematodes than the bulk soil region. Rhizodeposition is an ecologically important process as it represents a loss of reduced carbon from the plant, but is an influx into the soil’s organic carbon pool and it fuels the microflora communities present in the soil. Soil microflora, such as the previously mentioned bacteria and protozoa, is integral to the majority of biological processes actively happening within soils including nutrient and pollutant cycling and the dynamics of soil-borne pathogens.
Garlic mustard is known to produce a chemical which discourages the development of the symbiotic relationship between plants and mycorrizhal fungi.
The secretions from plant roots serve various functions in the soil community, they will also produce different reactions from other members of their ecosystem. One hormone secreted by roots called strigalactones are detected by the mycorrhizal fungi associated with plant roots, they trigger sporulation and aid in colonization. Symbiotic nitrogen-fixing bacteria detect flavonoids put out by roots of legumes and will put out their own chemical signal alerting plants to their presence and result in the formation root nodules. Root nodules contain bacteria which assist the plant in fixing nitrogen necessary for photosynthesis. Other plants secrete allelotoxins which are chemical compounds released by plants to inhibit the growth or kill competitor species near by.
