Explanation:

Allelopathy refers to the beneficial or harmful effects of one plant on another plant, both crop and weed species, from the release of biochemicals, known as allelochemicals.
It is a process by which a plant releases chemicals that can inhibit its competitors.

Soil sickness, a general term for a problem that may well be caused by residues of allelochemicals that persist in the soil after the plant is gone, may make some sites unsuitable for growing other plants.
Allelopathic chemicals can be present in any part of the plant. They can be found in leaves, flowers, roots, fruits, or stems.
They can also be found in the surrounding soil. The toxic chemicals may inhibit shoot/root growth, they may inhibit nutrient uptake, or they may attack a naturally occurring symbiotic relationship thereby destroying the plant's usable source of a nutrient