Why is it important that we estimate length? When we look at living soil, and the organisms in our sample, we look at the balance between fungal and bacterial biomass. Different plant communities need a different soil communities – annual flowers and vegetables have different requirements compared to orchards and woodlands. In general, the more perennial, the older the ecosystem community, the more the balance shifts from bacterial dominance to fungal dominance.
Estimating lengths using the eye pieces
We estimate the length of filamentous organisms such as fungi in comparison to the width of our field of view.
If we are looking through the eye pieces, having an eye piece micrometer can be useful. This can be tricky; since visual perception is quite subjective, we may under or over-estimate.
So I have started estimating length in a more tangible way: Using a string on screen, in my camera field of view.
Estimating lenghts via the camera view – string method
I cut a double-length of the width of the image on-screen. So if my live-view window is 19.5 cm across, my piece of string will be 39 cm, just so I have a little more flexibility when measuring organisms.
1 width of screen = 1 unit. So I divide my 19.5 cm by 10, and mark my string in 1.95 cm distances with a permanent marker. Voila! Now I can easily estimate my units of the filamentous organisms on screen: Each of these sections is 0.1 unit.
If I am using live view, I will focus through the depth of field to judge where the fungal strand begins and ends. If you wanted to be super specific, you could focus through and follow along with your string. But that seems to be beyond what the purpose of our assessment is.
Essentially, the aim is to get a reasonably accurate estimate to see what the fungal:bacterial biomass ratio is, so we can maintain or shift it in a direction that meets our plants needs. And for that, a string is good enough.
Estimating lenghts via the camera view – using a bit of algebra and a tape measure
If you need to estimate lenghts, say for an assessment where we have no control over the width of the field of view, we can use a flexible sewing tape measure and follow this formula:
If you want to remind yourself where the formula came from, here is a random example for measuring on-screen, let’s assume the image is 25 cm wide (x-axis), and the fungus is 8 cm long.
Digging deep into the remnants of Algebra, we can now link these two true statements and express them as an equation. Next we figure out how to solve it:
And this leads us to the generic formula for estimating lengths of filamentous organisms mentioned above.
Of course this can all be done very efficiently with microscopy measuring software such as toupview. Just not everyone has access to this or wants to.
This is quick enough, for me no slower than eyeballed estimations, and results in a more accurate estimation.
Just be mindful – we can’t mix using an eye piece assessment with a camera view assessment.
What have you found useful for judging length – let us know in the comments! Particularly if you have computer-assisted/AI-based suggestions (which would speed up this process significantly!)
Soil Food Web Student 