soil crust

Cracking pour overs

cracks in a wheat field
Cracks in the soil in a field after a dry spell. But there are many connections between coffee and soil.

Summer this year has so far been quite hot and dry. Perhaps you have seen the grass dying back. Or maybe you have noticed the cracks forming in the soil in your local parks and fields. Such cracking is the result of the very dry weather and hopefully you won’t find it in your coffee, but there is another effect concerning soil compaction that connects to brewing a morning coffee as well as farming it.

It’s about the rain. As each raindrop falls to the Earth, it makes an impact with the soil underneath. While a light drizzle is not going to have that much of an impact, a larger raindrop of diameter say, 5mm, is going to hit the earth at about 9m/s – and that could cause quite a stir. Each impact will shake off smaller sized particles of soil which dislodge and get stuck in the pores between the larger soil particles. So the smaller particles start to ‘clog’ the pores between the soil particles and reduce the ability of water to penetrate into the soil. And although it seems a small effect, the result of this clogging of the pores by the smaller soil particles is to reduce the water permeability of the soil by 200-2000 times*: a soil crust is formed.

lilies on water, rain on a pond, droplets
The impact of a drop? Each rain drop can have a significant effect on the soil surface

This crust not only reduces the amount of water that gets through to the roots (by reducing the soil’s permeability), it also acts as a barrier for seedlings coming up: while many seeds can get through quite strong layers, even Sorghum struggles to push through if it needs pressures of 13-18 Bar to break through this crust*. So even without any farming machinery causing further soil compaction, just the rain is going to affect how additional water goes through the soil and how plants can grow out of it.

We are getting to the coffee bit.

The crust strength is influenced by the number of small (clay-type) particles in the soil. Clay particles are less than 2 microns in diameter which is smaller than the grind size you would find in even a Turkish coffee grind. But if we were to grind very brittle coffee beans (that shattered into many smaller particles as well as the grind size we want), or we were to use a blade grinder leading to a large distribution of grind size in our freshly ground coffee, we may expect to see some effects like this while brewing.

optical microscope image in water
Two coffee grinds compared under a microscope. How does the uniformity of particle size in a grind affect the clogging of a pour over? Magnfied 5x

If we think about a pour over brew (as opposed to an espresso or an immersion type), the initial pouring of water over the grind bed will dislodge any smaller particles in the grind and clog the grind in the same way as the rain on the soil. So if we were grinding way too fine, or using a blade grinder, or had a preference for darkly roasted (more brittle) coffee beans, it is possible that our pour-overs would tend to ‘clog’ more than if we were using a uniform medium grind of more lightly roasted beans. Has anyone experimented with this?

But the second soil connection we may notice as we prepare our pour over is that after our initial pour, as we let the coffee ‘bloom’ and the CO2 bubbles out, we receive a lovely aroma. A wonderful coffee smell as the grind bed continues to out-gas. This may remind us of petrichor, which is that great, and distinctive, smell of rain. And it turns out that petrichor is formed by the rain hitting the soil surface and producing air bubbles as it falls. The air bubbles then burst releasing aerosols from the soil which are so familiar to us as the scent of rain. A similar process to the blooming of the coffee grind. But just as with the coffee grind, as the water continues to fall and particularly if the pour over clogs to leave us with a water layer on the surface of the coffee (or soil), the aroma will reduce (or at least change) as the mechanism producing the smell changes.

bloom on a v60
Blooming petrichor, or should it be coffichor?

On a farm or in a garden, the effect of this soil compaction can be reduced by practises such as mulching. In addition to reducing the impact of individual rain drops on the soil surface, the mulch reduces evaporation of the water from the surface and changes the albedo of the soil. All things that may help coffee farmers to grow healthier coffee plants. In our pour overs, it is probably not a good idea to add any form of mulch! But this does mean that we can experiment more with the grind!

There are many more connections between your coffee and the earth around us, what will you notice?

*Soil Physics, WA Jury and R Horton, Wiley and Sons Publishing, 2004