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The ‘dark earth horizon’ and onions: Manaus Day 4 a team forged by the heat of the sun

Wake up call at 6.00 am and everyone has tasks to do before we hit the road. The idea is to beat the Manaus rush hour at 7.00am – it makes sense to work early when the heat is not that oppressive, but it makes for very sleepy people trying to remember what they need to prepare. I have to do the smelly chemistry and aliquot out the ‘mustard oil’ (see below for what it is and why we use it). I don’t mind this morning as the alternative is cutting up 8 kg of onions and liquidising them – yes we are trialling a more natural earthworm extractant. I must say that there were many tears shed. It took a little longer to get ready to leave than anticipated so I jumped in the EMBRAPA 4×4 with Alex (our local EMBRAPA contact) and a soil microbiologist and went to get the Machetes sharpened at one of the EMBRAPA field stations that was only 30 mins away from the first site.

Sharpening_at_EMBRAPA EMBRAPA_Field_Station



This was a smart choice as the field station had a kitchen which served coffee while we waited for the kit to be sharpened. Now the coffee came with pre-added sugar (they like sweet stuff here) so I am right at home. This meant I managed a shot of caffeine and glucose which is just what was in order after fighting the Manaus morning rush hour. Alex then showed us a soil profile he had created to show the ‘dark earth Horizon’. Basically this is a 5 foot pit cut so you can see the transition between the dark earth (tera preta) and the sub-soil on which it sits.  Apart from the striking colour difference driven by the charcoal, the other feature that jumps out is that the complete dark earth profile is packed full of pottery shards – this is actually the definition of tera preta when pottery can be seen. Although I am not a archaeologist I am pretty sure that the depth of the profiling means that people must have been living on the site for hundreds if not thousands of years. There just seems to be so much we Pottery_dark_soildon’t know about pre-Colombian civilisations in the Amazon.


With razor sharp Machetes in hand (yes I still have all fingers) we drove on to the site.  The people power had arrived 5 minutes earlier and we all started to get down to our individual jobs – the first of which was to carry all our equipment 1 km to the nexus point which was a tarpaulin hung the previous day from a tree. For me this represented 80 litres of water – not bad for a old man who spends most of his time behind a computer, although it made me wish I had kept those appointments to do some pre-expedition training with Luis.

What_we_came_for Some_field_hazards

Here’s the science bit !! Our aim was to sample from two 60 M squares, one in the dark earth maise field and one in the secondary forest at the back of the site, still on dark earth. For each of these squares we marked nine points, each corner, half way along each side and in the centre of the square.  For every point we dug a 25 cm square 30cm deep hole splitting the soils into top-soil, 10-20 cm and 20 -30 cm.  These went to the guys sitting under the tarpaulin for hand sorting.  At each of the 9 points in the forest we treated the soils with three types of earthworm extractant – Mustard oil, Formalin and onion extract (a natural alternative we were trying). For the corners and centre spot we use a soil drill to extend the holes down to the ‘dark earth’ horizon – oh yes we also put in pit fall transects and do termite surveys. This generates a massive amount of samples.  For each sample we have printed two labels; one paper, to go on the outside of the bags, then one printed on tracing paper which goes inside the sample in the ethanol or preservative.  The label includes a human readable code and a QR code (those square barcodes). Every label had been sorted before we started and stuck on and in the appropriate bags – an effort coordinated by Elodie, Georges right hand postdoc. Yeh it was a very busy day.

Telmare_soil_extract Slices_of_Dark_earth

I will say more about the technology we are using to catch the metadata – but that’s for another day.

The heat was fierce and the quantity of work challenging.  I was drinking 1/2 litre of water an hour and I was still thirsty. The only guys not phased by the sun was the two students from EMBRAPA – their response was “you should be here in August it’s a lot worse” – it was 38oC in the shade today, hotter does bear thinking about!!

But samples were collected and my now traditional Portuguese lesson kept everyone in the van laughing back to Manaus. I have the world’s most amusing heat rash which make my chest look like I have had some medieval torture done to me – but after fish soup that was amazing, and a rapid trip to the supermarket – for guess what – yes more Onions (!!) it’s time to turn in.

Lastly, had the strangest experience when the only non-earthwormer in the hotel came up and introduced himself as an entomologist from the National Museum of Wales, Cardiff – he thought I wouldn’t know where Cardiff was (!!)  It just proves that it’s a very small world.