Selecting the Best Organic Fertilizer

Robert Pavlis

People love to go organic, which is a good thing. Unfortunately, the information about organic fertilizers is not always correct or complete. Here are some real important things you need to know about organic fertilizer.

Limitations of Organic Fertilizer
Limitations of Organic Fertilizer

Why Do We Fertilize?

We don’t feed plants! We replace missing nutrients in soil. The nutrient that is most often missing is nitrogen, so I will use nitrogen as the main example in this post, but most of the contents also apply to the other nutrients.

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Plant Available Nitrogen (PAN)

Gardeners deal with two groups of nitrogen compounds; those available to plants and those that are not available to plants. Scientists use the term “PAN” to describe the amount of Plant Available Nitrogen, which consist mostly of ammonium and nitrate.

Non-available nitrogen is also called “organic nitrogen” since the nitrogen is tied up in large molecules like proteins. These contain nitrogen, but plants can’t absorb them through roots.

Compost Science for Gardeners by Robert Pavlis

Why is this important? Lets say you add some fish fertilizer to plants and the NPK is 5-1-1. You think you added 5% nitrogen, but if half of this is organic nitrogen then you only added 2.5% plant available nitrogen (the PAN is 2.5%). Knowing the PAN value is important so that you add the right amount of nitrogen.

Nitrogen Availability Over Time

Let’s continue with the fish fertilizer example. 2.5% of the nitrogen is available to plants right away, but what about the other 2.5%? What happens to it? Organic compounds continue to decompose and eventually the nitrogen will become ammonium or nitrate and be plant available. Organic fertilizers decompose at different rates and therefore release nitrogen at different rates.

The amount of nitrogen available to your plants depends on the type of organic fertilizer you select and on the environmental conditions. If you select the wrong one, or apply it at the wrong time, your plants will starve.

PAN Over Time

The following chart shows the cumulative PAN over time. The “start PAN” shows the amount of nitrogen that is available to plants as soon as the fertilizer is applied. Synthetic fertilizer is available right away. Fish emulsion and blood meal decompose quickly and are plant available within a couple of months. Compost and vermicompost has almost no nitrogen available at the time of application and they decompose very slowly over many years to release nitrogen.

The above information is assembled from various sources including University of Massachusetts, Ohio State University and University of California. The numbers are approximate and depend on the exact contents of the fertilizer, time of application, temperature and soil characteristics. Plant meals like alfalfa meal and cotton meal behave similar to compost.

Why is some nitrogen never released, even after 5 years? At a certain point the remaining nitrogen is absorbed by microbes as quickly as it is released, where it forms new organic nitrogen. It never becomes plant available.

Is Organic Fertilizer a Slow Feed?

Organic fertilizer is commonly recommended as a better choice because it makes nutrients available to plants slowly over a long period of time. It is clear from the above table that not all organic fertilizers are equally slow in their release pattern. In this regard fish emulsion and blood meal behave more like synthetic fertilizer. They don’t release nutrients quite as fast as synthetic fertilizer but they are much faster than composts.

Composts are better for long term addition of nutrients, but they are a poor choice in situations where you need to add nutrients right away to solve a nutrient deficiency.

Other Nutrients

Other nutrients, with the exception of potassium, behave much like nitrogen since these nutrients are also part of large molecules.

Potassium is different since it does not get incorporated into organic compounds. It remains as free ions that are easily released once cells are disrupted. For this reason a lot of potassium can leach out of compost piles and never reach targeted plants. The amount that remains in organic fertilizer is readily available to plants.

Organic Fertilizers Improve Soil

One of the main benefits of using organic fertilizer is that it helps build better soil. It holds water, increases the amount of air, improves structure through aggregation and increases CEC, the soils ability to hold on to nutrients. But this only happens if the organic fertilizer adds significant amounts of organic matter (OM).  The amount of OM is higher in solid fertilizers than liquid forms. On a pound for pound basis, dry fertilizers will improve soil more that liquid forms.

Consider these examples. Compost tea contains almost no OM – it’s mostly water. It does little to improve soil quality.

In another post I compared the amount of OM added using compost and Neptune fish fertilizer. Compost adds 9.7 kg per sq meter (1.2 sq yard) while fish fertilizer adds 0.014 kg or 700 times less. They are both organic but only one improves soil.

Moisture content is also important. Products like alfalfa meal and cotton meal contain only about 10% water compared to bagged compost which has 50% water and fresh cow manure which is 85% water. Plant meals add more OM on a weight basis, however, these meals tend to be more expensive, so on a cost basis compost adds more OM than plant meals. Homemade compost is even more cost effective.

If you can’t hold the fertilizer in your hand and feel solid material, it contains very little organic matter. Fertilizers, such as fish emulsion, compost tea and blood meal, contain almost no organic matter and have limited impact on soil quality.

Organic Fertilizer Improves Microbe Populations

Microbes need two basic things to prosper: nutrients and a carbon source. Organic fertilizers can provide both. Microbes use the same nutrients as plants so any time you are providing plant nutrients you are also feeding microbes. The organic matter is the carbon source.

Which organic fertilizer is best for building your soil microbe biome? The one that provides both nutrients and OM, in larger quantities.

As discussed above, some organic fertilizer provides nutrients for only a short period of time. This does cause a microbe population explosion, but once the nutrient source is used up, the population crashes which is not great for microbe health. Some organic fertilizer provides nutrients, but almost no OM. Without a carbon source microbes don’t grow well.

The best organic fertilizer for building microbe populations provides nutrients over a long term and provides a high level of OM.

The Best Organic Fertilizer

There are a number of factors to consider. Always use local sources – they are much better for the environment because they don’t have to be trucked as far. Cost is an important factor. Don’t buy expensive specialty products. When it comes to OM, quantity can be more important than quality.

The absolute best option is homemade compost. It is local, cheap and of high quality. Check out “Composting Science for Gardeners” and learn how to make great compost using many different methods.

Other good options include commercial compost, vermicompost and plant meals. The worst options usually come in a liquid form because they tend to have low nutrient levels that are released quickly and they have almost no organic matter.

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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

26 thoughts on “Selecting the Best Organic Fertilizer”

  1. Hi Robert. Great site, my farming bible 😉

    I have a question. How does the plant assimilate minerals after foliar spraying with organic fertilizer? I read that soil bacteria are needed to “transform” organic compounds into minerals. Are there enough bacteria on the plants to convert the organics into minerals for the plant to assimilate it? But is it something else?

    • 1) organic fertilizer contains simple nutrients – your minerals – and it contains larger organic molecules. When you foliar feed, only the very small nutrient molecules enter the plant. For example, iron ions get in, but EDTA iron – which is not that big of a molecule – does not. So most organic molecules will not get in the leaf and are of no benefit for foliar spraying.
      2) The outside of the leaf is covered with microbes – mostly bacteria. If some organic material lands on the leaf, they will decompose it and some of the minerals might make it into the plant. But this would be a very small amount.
      3) Once the nutrients are in the leaf they are “assimilated” in the same way as those arriving from the roots.


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