by Charlie Howe
Inhale a long deep breath through your nose. What do you smell?
It’s difficult to break apart the bouquet of molecules that fill our homes, schools, or offices. Describing the qualities of these smells with any precision is just as challenging. Sometimes we simply don’t notice subtle odors. Yet, few stimuli shape our impressions of our environments as quickly or as deeply as smell and only in recent decades have we begun to understand why.
For years humans and other primates have been regarded as primarily optical animals with highly developed powers of vision, but with a relatively undeveloped sense of smell. However, in comparison with our visual and auditory systems, the olfactory system has many more connections to the parts of the brain where emotions are regulated. Smell is hard-wired to feelings of pleasure, memories, and our mood. Neuroimagery now shows that these connections are active even when we are not aware of smells.  
The design of our environments, is predominately a visual endeavor, however it has recently been expanded to include components of smell. Hotel lobbies and airline cabins now release signature scents aiming to instill a sense of well-being. Due to the unique composition of these scents, we come to associate them with the company’s brand. Brand Sense Agency CEO, Simon Harrop describes his company’s latest scent for Singapore Airlines as “… a light sweet scent like pure steam from fresh rice.” Visitors at any Double Tree hotel are greeted with the familiar smell of chocolate chip cookies. If these smells are enticing, it’s no surprise, researchers have found that even the mere suggestion of aromas affect our mood.
Although outdoor environments are more difficult to control than, a hotel lobby, the palate of scents is diverse and central to the experience of being outdoors. Landscapers already exploit our love of flowers. The popularity of the sweet smell of rose gardens, fragrant night-blooming flowers of moon gardens, and spring cherry and lilac festivals may owe more to the powers of smell than we know. Yet, especially in small urban parks the smells that we associate with nature, the earthy aroma of soil, the seasonal smell of decomposing leaves, are often suspiciously absent. Luckily, a more complete understanding of the soil food webs is revolutionizing turf management and may provide a method to recreate the aroma of nature in urban parks.
A 2009, soil restoration project on Harvard Yard in Boston explains the lack of smell in traditionally managed landscapes. The study found turf with shallow roots and low soil nitrogen. The Harvard Yard Soil Restoration Project rebuilt a healthy soil ecosystem by adding abundant organic matter, aerating, and adjusting the equilibrium of soil microbes by adding compost tea formulations rich in live protozoa. After one summer, the root zone depth had doubled, soil nitrogen was higher, and soil microbes flourished, once again producing the molecules that give soil its characteristic smell. Harvard has since decided to rebuild their soil, but small landscape management decisions can also have a big impact on the urban smellscape.
A decade before Harvard began their study, managers at New York’s Central Park made a decision to enhance woodland soil, which in turn, also enriched New Yorkers’ sensory experience in the park. The Central Park Conservancy decided not to remove fallen leaves from the park’s North Woods. The leaves now serve as an organic input to the soil food web and also release the musty fall odor that we associate with deciduous woodlands. 
While smells affect each of us differently, the smell of soil may belong to a group of scents that evoke similar responses across cultures and species. For instance, Camels, use the smell of aromatic soil molecules (geosmin and 2-methylisoborneol) to locate fertile conditions up to 50 miles away in the desert. We often sense the same molecules, for example the atmosphere of fertility that fills the air after rain.
Especially in dense urban environments, we need to create parks that contain healthy ecosystems both above and below ground, at scales ranging from the soil food chain to the urban woodlot. The smells released by these landscapes must attract us from blocks away and, perhaps without us recognizing why, inspire a sense of well-being.
Next time you’re outside, take a deep breath and experience the powers of scent.
 Zald, David H., Pardo, Jose V. “Emotion, olfaction, and the human amygdala: Amygdala activation during aversive olfactory stimulation“. Proceedings of the National Academy of Sciences of the US.
 Berridge, Kent C., Kringelbach, Morten L. “Affective neuroscience of pleasure: reward in humans and animals“. Psychopharmacology.
 Sauer, Leslie Jones. “Soil as a Living System.” Arnoldia, Summer, 1999, page 35-43.