Re: how does human hair affect the growth of plants?

About the only benefit that human hair could have on plant growth would be as a 
fertilizer supplying nitrogen and sulfur. Hair is a protein, called keratin. 
Proteins contain about 15% nitrogen. Keratin also contains some sulfur. 
However, protein nitrogen and sulfur is slowly available. Microbes must break 
down the protein before they are available for plant uptake. Keratin is not 
easily decomposed so is a slow release fertilizer. 

Human hair is not often sold as fertilizer. However, feather meal is another 
keratin source often used as a nitrogen fertilizer. Unflavored gelatin is made 
from another animal structural protein, collagen. Unflavored gelatin is 
sometimes labeled and sold as a houseplant fertilizer.


References


Re: what is the chemistry of human hair?


Re: Can we use jello as a fertilizer? If you can, why?


Gelatin Composition


Feather Meal Fertilizer


Feather Meal Composition