Humus fraction lightest in colour is

Humus fraction lightest in colour is Fulvic acid. Fulvic acids are a mixture of weak aliphatic and aromatic organic acids which are soluble in water at all pH conditions. It is lowest in molecular weight, lightest in colour, soluble in both acid and alkali and most susceptible to microbial attack.
A fulvic acid is a group of chemicals formed when plants and animals break down. It is found in the humus part of soil and peat and is also found in streams and lakes.