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I've seen these little balls in more than one brand of potting mix / universal compost. I just sifted a large amount of compost and found many.

I've seen them in the following colors: white, green, orange and yellow.

At first, I thought this would be slow release fertilizer, but I'm pretty sure they are not.

They literally explode when you press them with your fingers. There some liquid inside. There were no bugs/insects in the soil. I don't think these are insect eggs, also.

I'll try to photograph them under the microscope and will add it here later.

What are those?

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UPDATE: Images with microscope

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    I think your first thought was correct, slow release fertilizers. – benn Nov 2 '20 at 15:41
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    @benn Hey benn, thanks. I really don't think they are fertilizers. Otherwise there would be many more of them. I must have found like only 50 of them in like 8 bags of compost. See the images with microscope I've just added. – cbdeveloper Nov 2 '20 at 15:56
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    maybe this helps: link – benn Nov 2 '20 at 16:14
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    Fertilizer 'beads' are crunchy or hard on the outside with either gooey liquid or tiny pellets inside; slug and snail eggs (which these might be though they look a little large) are soft all through, including the exterior. – Bamboo Nov 2 '20 at 16:15
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As the comments indicate this is slow release fertilizer mixed in with the soil less mix at the plant. The biodegradable resin coated shell allows for a slow release of the fertilizer.

A common trade name is Osmocote. More details are here It was first made by Archer Daniels in the 1960's but has gone through a few owners since then. Details here. The picture below is from their site. I have seen other colour resin shells including green and blue. The reason you don't see a lot of them is that after the fertilizer is released the shells eventually disintegrate.

The reasoning behind their use is that soil less mixes based on peat or bark or coir do not provide sufficient nutrients for most plants. By adding the fertilizer in slow release form to a soil less mix a commercial grower can have the best of both worlds:

  • sterile growing medium
  • consistent growing medium
  • no need to apply additional fertilizer unless the crop is picky like poinsettias

Osmocote

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  • Thank you for the detailed explanation. – cbdeveloper Nov 3 '20 at 7:07

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