You simply can’t underestimate how much rock we will have to move if we want to make a substantial contribution to our net-zero ambitions with enhanced weathering on thousands of cropland fields. From the perspective of each farmer this will involve spreading 20-40 tons of basalt per hectare which can be a challenge by itself. How could this work from a practical perspective? At Project Carbdown we have undertaken a few practical experiments over the last 2 years which we want to document in this blog post.

A very important side-aspect: We need to get the farmers on board! They are the ones who will actually spread the basalt on their fields. Our farmers tell us: “We are happy to help, but you can’t put a lot more workload on our shoulders. This has to be quick and easy. And we can’t purchase expensive new equipment for this either.”

In essence: The job is to find a way to spread 40 tons/ha of basalt on many hectares of agricultural land as homogenous as possible in an easy and speedy way with the tools that are already available to the farmers.

To compare these experiments we need to look at the following criteria:

• Speed: How much rock dust can be spread on how much land in what time?

• Effort: What kind of effort, including machinery, is required?

• Mixing: How well is the rock dust mixed/spread on the land?

We tried out a few approaches:

Experiment 1: Spreading 40 t/ha superfine basalt. By hand.

In our first field experiments last year we spread 1,5 tons of superfine basalt dust on our test plots - by hand. In Fürth it took 8 people 4 hours to cover only 400 square meters. Afterwards we mixed the basalt into the top 15 cm using a machine. During mixing we saw that if the soil is a little wet the fine basalt dust tended to form clumps in the soil, which isn’t ideal for weathering.

It was fun and getting our hands dirty helped us to get a deeper understanding of what we were doing. But: This rather scientific method does not scale (and it does not mimic how this works at scale either).

• Speed: slow = 1,5 tons on 400 m² in 4 hours

• Effort: high = 8 people for 4 hours

• Mixing: close to perfect

Experiment 2: Spreading 200 t/ha superfine basalt. By wheel loader.

In this experiment we spread the equivalent of 200 tons of superfine basalt dust per hectare on just 200 square meters using a wheel loader. We used 4 of these big-bags and it took 40 minutes.

As you can see in the video this approach is impractical for several reasons (although it was fun, to be honest):

• The massive load of dust is a health hazard.

• We found that working with rock dust in many big bags is tedious, slow and makes scaling hard.

• Wet rock dust problem 1: You can’t let the dust get wet in the bags. Rock dust plus water create robust clumps. Where would you store/park all those big bags in a dry place if you want to scale this to 40t/ha and several fields?

• Wet rock dust problem 2: We successfully mixed the rock dust into the soil because the soil was very dry (it hadn’t rained for >4 weeks), but from other experiments we know that this much rock dust on a wet soil could have given us huge problems.

It becomes pretty obvious that this approach simply can‘t be scaled up to large fields. Spreading at scale must become easier and can’t involve big bags!

BTW, later we wouldn’t use so much basalt (200 t/ha). With this much basalt quite likely you would create conditions where secondary minerals are formed which reduce the amount of captured CO2 and slow the weathering process down due to coating of the basalt grains. We only used this amount to create a strong “signal spike” for our electronic sensors that were built into this plot.

• Speed: slow = 4 tons on 200 m² in <1 hour

• Effort: medium = 1 person and a wheel loader for <1 hour

• Mixing: good, but only because the soil was rather dry (no rain for 4 weeks)

Experiment 3: Spreading 0,2 t/ha superfine basalt. With Manure.

Piggy-backing the basalt spreading onto a process that’s already happening at the farm sounds like a good idea. For this option we mixed two big bags of superfine basalt (2 tons) into 300 tons of manure, which resulted in an emulsion mixture. This was pumped into a manure spreader and taken to the field (10 ha). While this approach looks easy it limits the amount of basalt that you can spread onto one hectare: How much basalt can you mix into how much manure and not over-load the field with manure?

Using this approach is a nice way to add some basalt, but there is no way how to scale this into massive amounts of basalt that we need for climate relevance (0,2 t/ha of basalt have just a marginal climate effect per hectare).

• Speed: slow = we spread 2 tons on 10 ha in several hours

• Effort: minimal = we added only 30 minutes on top of the usual manure process

• Mixing: great = the basalt was spread very evenly

Experiment 4: Spreading 40 t/ha coarse basalt. By spreader.

Finally we ordered a big pile of coarse basalt (< 2000 microns) delivered on a large truck. Suddenly many things got much easier:

• This pile could be left out in the weather for weeks.

• When it got wet it was even easier to spread and created less of a dust storm.

• We could use standard spreader equipment built for lime and fertilizer.

Spreading took about 30 minutes for one hectare in total, including loading basalt onto the spreader trailer 5 times.

Finally we have found a viable option, but this can’t be done with superfine (<200 microns) basalt, for three reasons: All spreader trailers that we had at hand couldn’t work with such a fine material, the superfine dust would have created a massive, dangerous dust cloud and we could have only transported/stored the superfine rock dust in big bags (or silos), which would have made this process much more complicated and unattractive.

But we also learned that:

• The basalt is never spread as homogeneously as we had assumed in our field and lab experiments. Because you need to drive back and forth on the field a couple of times and because the spreader doesn’t spread evenly (see video) there will always be areas where you have x% more or less basalt than on average on the field. Depending on your MRV method this can make things complicated for the CO2 metering.

• With bulk goods you can have a handling loss of basalt: We couldn’t pick up about one ton of the 30 tons from the field we used to store the pile on.

• We did a surface area measurement of the coarse material and it actually had 30-50% more surface area than the superfine basalt, so we expect the weathering rate to be in the same range as the fine stuff. Keep in mind: surface area seems to be more important than grain size.

If we look at the whole story this method remains our preferred way of scaling CDR with basalt on croplands for now (as we want to scale up the basalt tons as fast as possible).

• Speed: fast = we spread 30 tons on 0,7 ha in 30 minutes

• Effort: ok = we used already existing machinery

• Mixing: ok = with a spreader you will never get a perfect homogenous spreading, which doesn’t affect the weathering much, but makes monitoring/measuring harder

Coming up: Spreading fine basalt converted into pellets

The basalt companies usually have a lot of waste basalt dust under 200-500 microns . But this is, as we have seen, hard to store, handle and spread. Since this is actually waste for the mining companies the cost per ton is often so cheap that there should be enough room for an additional processing step: to convert the dust into pellets. These should be as easy to spread as coarse basalt. Let’s see.