In my last post Compost – Is it an Organic Fertilizer, I concluded that compost was an organic fertilizer and that it adds nutrients for the garden. I’d now like to have a closer look at the compost fertilizer numbers, the NPK, to better understand how and when the nutrients from compost are made available to plants. This discussion will also uncover some interesting facts about reported fertilizer numbers for organic fertilizers.
Compost Fertilizer Numbers
What are the fertilizer numbers—NPK—of finished compost? For this post I am going to use the numbers from reference #1, provided by the University of Massachusetts. The numbers reported for nitrogen are lower than most references (#2 and #3) which average around 3%, but the differences will not change the concepts in this discussion. Actual numbers depend very much on the ingredients used to make the compost, and the degree of finishing that takes place. Reference #1 gives fertilizer numbers of 1-1-1.
Different Forms of Nitrogen
Compost has 1% nitrogen, but what does that really mean? Can plants use that 1% immediately? If not, how long does it take for that 1% to be made available to plants?
You can think of nitrogen as being available in two major forms. One form is called ‘extractable nitrogen’ or ‘readily available nitrogen’. This form includes nitrate and ammonium, both of which dissolve easily in water and that is why they are called extractable. Labs can wash this out of compost with water and then analyze the values. This is the form of nitrogen that can be used by plants.
A second form of nitrogen consists of large organic molecules such as proteins, which can be called ‘slow release nitrogen’, or ‘organic nitrogen’. Plants can’t use this nitrogen immediately but as these molecules degrade the slow release nitrogen is converted to extractable nitrogen and then plants can use the nitrogen.
Nitrogen in Compost
The 1% nitrogen in compost consists of 0.03% extractable nitrogen, and 0.97% slow release nitrogen. What this means is that although compost contains a significant amount of nitrogen, at the time it is added to the garden, almost none of it is immediately available to plants.
Compost – Slow Release Nitrogen
How quickly do the large molecules degrade so the nitrogen is made available to plants? According to reference 1, only about 10-30% is made available during the first year. At this rate the compost you add today will feed your plants for 4 or 5 years, but at a low rate. Compost is a slow release fertilizer.
Phosphorus in Compost
Compost may contain as much phosphorus as nitrogen and almost all of it is tied up in large molecules so it too is in a slow release form. However, unlike nitrogen, when it is released, it tends to combine strongly with soil making it difficult for plants to get at. Compost will provide very small amounts of phosphorus over many years.
In practical terms, compost added today does not provide phosphorus to your plants in the next couple of years. Since most soil is not phosphorus deficient, this is really not a problem. For more details on how phosphorus reacts with soil have a look at Rock Phosphate Myth.
Potassium in Compost
Potassium does not get incorporated in large molecules and so it is readily available to plants. However it is also very soluble in water, and rain or ambitious hose watering can easily leach potassium out of the compost pile before it is finished. This is one reason some people cover their compost bins.
Compost will make potassium available to your plants as soon as you add it to soil. It is not a long term soil additive—whatever is in compost is available immediately.
Compost vs Other Organic Fertilizer
Every source of organic fertilizer will have the same issue. In all types of organic matter nitrogen and phosphorus are tied up in large molecules and are released slowly over time. Potassium is released quickly, and can be washed out of the soil.
These are limitations with all organic fertilizer that need to be taken into account by the gardener.
Is Compost a Good Choice to Solve Nutrient Deficiencies?
Organic fertilizers are good for the long term feeding of plants and the development of good soil. They are not a good choice for a quick feed. If your garden is deficient of a nutrient other than potassium, than inorganic fertilizer is a much better option. If your tomatoes are not performing due to a lack of nitrogen—add synthetic nitrogen to solve the problem. Compost will not help the problem this year.
If your garden has a significant deficiency of nutrients—synthetic is better than organic—in the short term.
If your goal is to improve soil structure, and provide a long term feeding for plants, compost is an excellent choice.
This is explained in more detail in Organic Fertilizer – What is its Real Value.
1) Compost Use and Soil Fertility: https://ag.umass.edu/vegetable/fact-sheets/compost-use-soil-fertility
2) Guide to Nutrient value of Organic Material: https://s3.amazonaws.com/assets.cce.cornell.edu/attachments/5283/A_Guide_To_The_Nutrient_Value_Of_Organic_Materials.pdf?1418849909
4) Photo Source: Public Domain