What is the smell of spring?

Spring smells of worms. Or rather their re­mains. And of rot­ten stems, leaves, parts of in­sects and oth­er de­com­pos­ing or­gan­ic mat­ter. It’s all ter­ri­bly ro­man­tic, isn’t it?

What we call the smell of spring is ev­ery­thing that reach­es our ol­fac­to­ry re­cep­tors from thaw­ing wa­ter evap­o­rat­ing from dif­fer­ent sur­faces. In ru­ral ar­eas and sub­urbs, this is main­ly the smell of the thaw­ing earth with all its or­gan­ic splen­dor.

Melt­ing wa­ter

The de­com­po­si­tion of or­gan­ic mat­ter is the tran­si­tion of chem­i­cal el­e­ments from com­plex or­gan­ic com­pounds to sim­ple min­er­al ones. This process is called min­er­al­iza­tion. As day­time tem­per­a­tures in­crease, the min­er­al­iza­tion ac­cel­er­ates of fall­en leaves, one-year plants, dead in­sects and worms that were frozen un­der the snow. Or­gan­ic acids and their salts en­ter the soil, and also organomin­er­al com­plex­es and min­er­al com­pounds of ni­tro­gen, phos­pho­rous and sul­fur, formed from prod­ucts of de­cay. And in the air there are volatile com­pounds of sul­fur and phos­pho­rous. Some of them are also quite odor­ous. Like amines – or­gan­ic com­pounds that are de­riv­a­tives of am­mo­nia. For ex­am­ple, ca­dav­er­ine and pu­trescine. They are con­tained in prod­ucts of the pu­trid de­cay of pro­teins and have a strong smell of rot­ting flesh.

De­cay of or­gan­ic mat­ter

This cock­tail of aro­mas is sig­nif­i­cant­ly aug­ment­ed by the sub­stance geosmin. It is pro­duced by cyanobac­te­ria and ac­tri­nobac­te­ria liv­ing on the sur­face of the earth. And wet earth smells of geosmin.

The hu­man sense of smell is ex­treme­ly sen­si­tive to geosmin. We are ca­pa­ble of de­tect­ing this smell at con­cen­tra­tions of five mol­e­cules in a tril­lion. In­ci­den­tal­ly, cat­fish and carp smell so strong­ly of mud be­cause of geosmin in com­bi­na­tion with 2-methylisobor­ne­ol. They ac­cu­mu­late in fat and mus­cle tis­sue. Geosmin breaks down in an acidic medi­um, and so when pre­par­ing fresh wa­ter fish we of­ten use vine­gar and lemon.

The main rea­son that the “smell of spring” ap­pears in the city is the ris­ing day­time tem­per­a­tures and the evap­o­ra­tion of thawed mois­ture. But the chem­i­cal com­po­si­tion of ro­man­tic spring here is sig­nif­i­cant­ly sup­ple­ment­ed by mi­cro­scop­ic par­ti­cles of road sur­faces, build­ing ma­te­ri­al and dust. They are main­ly car­bon­ates and cal­ci­um sil­i­cates from wet con­crete, soaked in at­mo­spher­ic mois­ture, and also par­ti­cles of soot (amor­phous car­bon, a prod­uct of in­com­plete com­bus­tion) from fac­to­ry pipes and car ex­haust.

Snow­drops

An­oth­er char­ac­ter­is­tic spring smell comes from the re­vival of trees af­ter win­ter. In their trunks and bark, the sap melts. Its chem­i­cal com­po­si­tion is dif­fer­ent for dif­fer­ent trees. Tan­nins are found in the bark of some trees, for ex­am­ple. This group of phe­nol com­pounds con­tains a large num­ber of —OH groups. Tan­nin has an acrid smell, and in the food in­dus­try it is used to give drinks an as­trin­gent taste. Tan­nin is also used for the tan­ning of leather and fur.

Trees are vir­tu­al­ly the only re­spectable source of spring aro­mas. But as strange as our de­light at the smells of wet dust and de­com­pos­ing worms may seem, these are still the smells of spring. And spring means that the cold weath­er is over, and that now ev­ery­thing is prob­a­bly go­ing to be all right.