One of the claimed benefits of compost in the garden is to provide the soil and plants with enzymes and hormones. Is this true? What would enzymes and hormones do for the garden? Good questions in the quest to understand compost better.
Enzymes – What are They?
You hear a lot about enzymes these days, and not just in the garden. They are being added to all kinds of medicinal products and cleaning products. A number of detergents have been promoting enzymes for years.
An enzyme is a special protein that causes chemical reactions to take place. Sometimes they are like a minister who joins two people together in marriage. Enzymes grab onto 2 or more molecules and join them together. Other enzymes tear large molecules apart into smaller molecules as shown in the picture above. On a molecular level, enzymes are responsible for most of what goes on in any living organism. They make the plant. They are also key to breaking down compost.
Hormones – What are They?
Hormones are chemicals that are produced in all living things to manage their internal chemistry. Hormones are like the police and politicians – they control things and decide what will and will not happen in the organism.
In plants, hormones control plant growth. They determine when a plant starts to grow and how fast it will grows. They are also responsible for bud break and flowering.
A more general term that is catching on for plant hormones is ‘plant growth regulating substances’—I think saying ‘hormones’ is easier!
Does Compost Add Enzymes and Hormones to Soil?
There is no doubt that compost contains both enzymes and hormones. As plants are decomposed these chemicals will be released into the compost juice. The microbes that live in compost are also producing their own enzymes and hormones. In fact bacteria and fungi both produce and excrete enzymes to digest the organic matter that surrounds them.
Compost was tested and contained the following hormones (Ref #2); 42.0 to 248.8 mg kg-1 auxins, 33.1 to 198.3 mg kg-1 cytokinins and 10.1 to 200.2 mg kg-1 gibberellins.
So compost does contain these chemicals, but what happens to them once they are added to soil? Enzymes and hormones are large or semi-large organic molecules that make great food for microbes. They also degrade on their own through chemical reactions. Most of these chemicals don’t last for more than a few minutes or hours in the soil before they are degraded (ref 1).
The reality is that enzymes and hormones from compost don’t last long enough in the soil to have an effect on plants.
Plants Use Enzymes and Hormones
Plants certainly need both enzymes and hormones to grow properly so there is no doubt they are important to plants. However, plants produce the enzymes and hormones that they need. They don’t need to get them from the soil and in the case of enzymes roots are probably not even be able to absorb them from the soil even if they were present.
There are cases where the addition of enzymes to soil in agricultural situations can be beneficial, but these applications are specific for certain crops and are far beyond anything a gardener would want to do.
If compost adds enzymes and hormones to the soil, I doubt that they play a very large direct role in plant growth. Notice the word ‘direct’. What compost does do is feed microbes in the soil, and microbes that live right next to plant roots, the rhizosphere, do produce plant growth regulating substances and these do affect plant growth–A great topic for a future post.
Benefits of Compost
Compost has many benefits, but adding enzymes and hormones to soil is not one of them.
1) Compost Food Web Information; http://theearthproject.org/id64.html
2) Isolation and Identification of Some Plant Growth Promoting Substances in Compost and Co-Compost; http://scialert.net/fulltext/?doi=ijv.2008.30.40
3) Production of Plant Growth Promoting Substances by Rhizosphere Microorganisms; http://www.sciencedirect.com/science/article/pii/S023243938480044X
4) Photo Source; Tim Vickers