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I've made a load of compost which has now fully matured and mixed it with a fair amount of leaf mold and soft sand (I believe sand that is encapsulated in clay). And with this combo made a potting mix for my strawberries. Well, it hasn't worked out very well to say the least.

The thing is I've realized that almost all the "greens" in the compost were from fruit and fruit scraps. Because the piles that I made went to 140/150 degrees Fahrenheit I'm presuming that there was plenty of nitrogen in the mix. The reason that I ask is just that fruit is notoriously low in protein compared to leafy greens (think spinach or nettles) and nitrogen is the building block of amino acids which in turn are the building blocks of protein - one could be forgiven for predicting a nitrogen deficient potting mix.

I just want to rule this out as I have further, specific questions on the deficiencies/toxicities that my strawberries have developed (in part from a lack of minerals associated with "leafy" greens.

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If your compost was hot, there was enough biological activity to release that heat. I would take that as a sign that your input 'greens' had sufficient nitrogen in them for your compost's needs.

Nitrogen is every living cell, including fruits. I found research measuring the nitrogen content of common compost constituents, and it shows fruit waste generally contains significant nitrogen. Table of Nitrogen, Nitrogen/Carbon ratio, and Water Content of corn stalks, rice straw, wheat straw, sugarcane bagasse, fruit waste, vegetable wastes, grass clippings, and leaves.  Fruit waste has lower but still significant Nitrogen content than grass clippings and vegetable wastes.

Shah, Prof. Dr. Zahir & Mohd. Jani, Yaakob & Khan, Farmanullah. (2014). Evaluation of Organic Wastes for Composting. Communications in Soil Science and Plant Analysis. 45. 10.1080/00103624.2013.861909.

However, I wouldn't take that to mean there is plenty of nitrogen available to your plants because of the compost. Compost contributes humus and nutrients to the soil, but its NPK value can still be low and variable. There are also nuances of nutrient availability and loss over time that I am not able to speak to. I would not rule out poor soil fertilization for your strawberries if they are showing signs of it unless I had a soil test done.

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To answer the question in the title of your post, no, it is not safe to assume there's plenty of nitrogen. Usually, finished home made compost is not high in nutrients, including nitrogen, it's considered more of a soil conditioner rather than a potting medium; the usual NPK is often 1-1-1. If you want to use it as a potting medium, you'd need to add fertiliser - as it's fruit you're growing choose one suitable for fruiting plants.

The other question is, what did your compost look like? When it's finished decomposing and is fit for use, it should resemble a reasonably friable soil, often dark in colour, though not necessarily. How you built your compost and what materials you used would have some impact on the end result; this would not matter if you were just using it as soil conditioner, but regardless of its construction, it would need fertiliser in order to grow plants in it.

This link https://ag.umass.edu/vegetable/fact-sheets/compost-use-soil-fertility#:~:text=In%20most%20cases%2C%20finished%20compost,nitrogen%2C%20potassium%2C%20and%20phosphorus. may be helpful to you in respect of compost generally.

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It's important to understand the nitrogen cycle and how nitrogen becomes available to plants. https://youtu.be/A8qTRBc8Bws?t=45

Adding too much carbon to your compost in the form of straw or wood shavings can lengthen the time it takes for compost to mature. Compost that is actively breaking down high carbon materials will experience nitrogen immobilization where the microorganisms take up the nitrogen instead of leaving it in the soil for plants to use. When the microorganisms die they will re-mineralize that nitrogen back into the soil eventually but they hog it until then. You said you used a lot of leaf mold which is heavier on the carbon side so make sure that your compost is mature (decomposition is complete) before using it. It is common to use large screens to sift out pieces that haven't finished composting before using composted soil.

Plants can't absorb nutrients that are still bound to carbon. Mineralization is when microbes decompose organic Nitrogen to ammonium. 68-95°F is the ideal temperature range for mineralization so you may not have had enough time in the ideal temperature range with enough oxygen and moisture for mineralization to be complete. It could be as simple as letting your compost rest for another week before using it.

After mineralization the ammonium has to go through nitrification where it is converted to nitrate. Nitrate is very susceptible to loss through leaching and the nitrification process is halted above 120°F.

If you aren't turning your compost regularly anaerobic composting could be causing denitrification which will cause the nitrogen to off gas from the soil. Nitrate (NO₂) is being broken down to feed the microbes oxygen. As the oxygen is pulled from nitrate by microbes you get nitric oxide gas (NO), nitrous oxide gas (N₂O), and dinitrogen gas (N₂).

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  • Thanks very much for this - I found it both interesting and educational, but I can't accept it as an answer as my question was about the specifics of a "fruit" compost. You do have a point though about the leaf mould which I was planning to raise in another question. The compost itself is very mature a Jun 21, 2023 at 19:57
  • @Carmin Lafond. All the leaves in the compost itself were really well decomposed but the 2 or 3 year old leaf mulch I added as a soil conditioner maybe wasn't. . Jun 21, 2023 at 20:24
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How does the compost pile smell? If during most of it's lifetime it did not smell rancid, then the nitrogen was captured. If at any point it smelled like something is rotting then some nitrogen was lost. My favourite cure for that is wood ash sprinkled over the compost pile. Bud don't overdo it.

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