Marginal environment to me is that conditions are such that life or habitat for some organisms would be difficult to maintain.  Such as there are plants that can easily live in wetlands, with hydric soils, and some plants there presence is atypical as the habitat is marginal.  Living on Mount Everest is marginal for human life as there are so many factors that suggest even visiting is a human feat.  I think marginality means the habitat is at or near the limit of the range.  If you live on what you grow in a garden, it is marginal if to live it takes nearly all you living effort to survive on that garden.  Too close to a volcano, the air quality may become marginal for some life forms in that they are not healthy, may have issues with reproduction or growth, getting enough oxygen, accumulating potential air toxins, or periods of too hot.

Thanks  william

marginal environments actually are the sub-optimal environments because usually of bad soil characteristics (, low fertility, sodicity, salinity etc..), or very low rain fall, prevailing of high temperature, such environments could be improved but with different cultural and technical practices. that is by growing of tolerant cultivars, soil amendments 

marginal environment is the sub optimal condition which is not fully favorable for any life.

In environmental studies, marginal environment can be seen as areas of low yield and productivity. devoid or in lack of specific conditions for maintaining life. Which as such threatens survival of biota. Think of arid environment; desert environment,  unproductive soil and ecosystem, environment facing threats like severe oil polluted environment, land polluted with radioactive waste and things like that. Marginal land can be defined in the context of the form of catastrophe or lack the environment possess. 

Regards

Charles Anukwonke