Compost is good for the garden, in part, because it adds nutrients for the plants. That sounds like a fertilizer. But almost everything you read says that compost is NOT a fertilizer. Something doesn’t make sense—let’s have a closer look at this myth. Is compost an organic fertilizer?
What is Fertilizer?
Before we look at compost we need to understand the term fertilizer. Here are some definitions from the internet dictionaries:
- A chemical or natural substance added to soil or land to increase its fertility.
- Any substance used to fertilize the soil.
- Any of a large number of natural and synthetic materials, including manure, nitrogen, phosphorus, and potassium compounds, spread on or worked into soil to increase its capacity to support plant growth.
It certainly sounds as if compost fits into this category, but none of the definitions I found actually list compost.
It turns out that the above definitions are definitions in common use. When we ask about the legal definition of fertilizer we get a very different answer. Legally, at least in some parts of North America, fertilizer is “a soil amendment that guarantees the minimum percentages of nutrients (at least the minimum percentage of nitrogen, phosphate, and potash)” (ref 1) . An organic fertilizer is a fertilizer from organic sources.
The key words are “minimum percentages”.
Since compost is made from plant material, and the inputs are very variable, manufacturers have a hard time controlling the amount on nutrients in the final product. Because they can’t or won’t guarantee “minimum percentages” from batch to batch, they can’t legally label the compost as fertilizer.
Interestingly, in Manitoba, Canada, the definition of fertilizer does not mention minimum percentages. So in Manitoba, even under the legal definition, compost is fertilizer.
Fertilizer Numbers in Compost
A lot of information sources refer to compost as a ‘poor source of nutrients’. They say that compost has ‘very low levels of nutrients’. Is this true?
Home made compost contains nitrogen, phosphorus and potassium—the NPK fertilizer numbers—in levels around 3-0.5-1.5. The key nutrient that might be deficient in soil is nitrogen and home made compost has about 3% nitrogen, with a range of 1 to 6%. Commercial sources for compost report levels of 1-1-1. I suspect that home made compost has higher levels of nitrogen because it is less finished.
That is a low level of nitrogen for fertilizer, but consider this. Fish fertilizer is considered to be a great fertilizer, and it has average fertilizer numbers of 5-1-1 with a range of nitrogen between 2 to 5%. It is not all that different from home made compost.
Compared to commercial fertilizer that might have 20% nitrogen, compost does have lower levels of nutrients. But 3% nitrogen is a fairly good fertilizer when compared to other natural organic products. For example it is higher than coffee grounds which are considered to be a “good source of nitrogen”.
It is also important to know how much fertilizer is being added to a garden. Usually you add very small amounts of commercial fertilizer, compared to compost. If commercial fertilizer is 20% and compost is 2%, nitrogen, and you add 10 times as much compost as commercial fertilizer – then both applications are providing the same amount of nitrogen to the soil.
Compost also contains a reasonable amount of micro-nutrients. This makes sense. Since plants need micro-nutrients to grow, and compost is made from plant material, the compost must also contain the micro-nutrients that were absorbed by the plant.
Will compost provide all the nutrients your plant needs? That is a question for another post.
Does Compost Feed Plants?
I think this is a dumb question, but I ask it because many web sites say something like this:”In the simplest terms, fertilizers feed plants. Compost feeds the soil”–clearly implying that compost does NOT feed plants.
That is a dumb statement. First of all soil does not need to be fed–it is not living. Things live in soil, but soil itself is not living. Things, also live in air, but we don’t go around calling air a living thing that needs to be fed! Compost makes nutrients available to living organisms. As we will see in future posts it also improves soil structure so it improves the environment for living things.
There are certainly differences between compost and fertilizers, but the differences have nothing to do with feeding soil. Both provide nutrients for living things, including plants.
Is Compost an Organic Fertilizer?
From a legal perspective, compost is NOT a fertilizer especially if you make it in your back yard. From the point of view of a gardener, compost is a fertilizer. It certainly adds nutrients to the soil, which can then be used by plants.
Most sources of gardening information say that it is not a fertilizer and although legally speaking this is correct, this is one instance were a small white myth is OK. Gardeners would understand their gardens better if they think of compost as a fertilizer.
What Type Fertilizer is Compost?
First of all it is organic. More importantly, compost is also a slow release fertilizer as explained in my post; The Real Value of Organic Fertilizer . Unlike commercial fertilizer, compost adds nutrients to the soil very slowly over several years. In my last post I discussed the fact that so called ‘finished compost’ is still decomposing. This continued decomposition provides a steady slow release of nutrients to the soil.
A new term that is being floated around is ‘biofertilizer’ and this term would seem to be a good fit for compost. Unfortunately, the term biofertilizer has been butchered by many in the organic movement and the term no longer has an accepted definition. Until this mess gets sorted out it is probably best not to use this term to describe anything.
In conclusion, compost is a good organic fertilizer–unless you are a lawyer who lives outside of Manitoba!
1) Legal definition of Fertilizer and Organic Fertilizer: http://www.ext.colostate.edu/mg/gardennotes/232.html
3) Definitions for Biofertilizer: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4108841/
4) Photo Source: grabadonut