The Story So Far: Harpenden Field Site

Find out how our project has been progressing at one of our field sites, Harpenden in Hertfordshire.

A tractor for spreading receiving crushed basalt from a front-loader at our field site in Harpenden

Harpenden is one of our three field sites, central to Work Package 1: Field Sites. As we’re part of the UK Greenhouse Gas Removal (GGR) Demonstrator programme, our field sites are a key part of the ‘demonstrating’ aspect. They demonstrate and test the impacts of basalt application on GGR, crop yield and soil fertility health. Harpenden is a Rothamsted Research facility site that replicates typical intensive arable crop farming.

Since our Demonstrator programme has now been running since May 2021, we’re here to share the story so far on each of our research strands, beginning with our field sites. (See also our progress update on our North Wyke field site here.) We’ll then seek to provide regular updates on what’s happening in each of these areas. Let’s see what’s been happening at Harpenden …

June 2021

The proposed field experiment design has passed through internal scrutiny and we’ve prepared our draft plan. We’re co-ordinating our order of crushed basalt with the site at North Wyke to ensure that we use the same material from the same source (the Breedon Group’s Middleton Quarry). Baseline soil sampling is planned for September, followed by spreading, incorporation and sowing of the N2-fixing bean (Vicia faba) crop in mid-October (this fixes nitrogen from the air in symbiosis with bacteria in its root nodules).

Soil water samplers and greenhouse gas collection boxes have been ordered and will be installed after sowing the first crop. Our team met with colleagues at Southampton to discuss sample numbers, preparation, storage and transfer across sites.

July 2021

Steve McGrath and Ian Shield from Rothamsted Research discussed borehole installation with Steve Banwart and Jared West at Leeds, who visited the Harpenden field site visit on 22nd July.

We have ordered our basalt from the quarry, which is an existing byproduct (<2 mm grade) that needs no further grinding – plus, we also know the mineralogy of it –  and this should be delivered in September. At this stage in our research on the field sites, we are choosing materials for the best experimental design rather than upscaling.

The team has made some decisions too: our preferred application is shallow tillage, with tines (prongs) to penetrate around 15cm into the soil. We introduce the basalt into the soil with as little tillage as possible.

Like the other field sites, we’ve also been doing some stakeholder engagement. Steve McGrath presented on our background and plans to the Net Zero Science Advisory Panel on 20th July.

September 2021

We’re marking out and sampling soil in preparation for applying basalt. Our bean crops are due to be sown in October. Our soil solution samplers are ready but our greenhouse gases (GHG) chambers are not quite.

We are planning to drill a second borehole at Harpenden, with our Leeds colleagues. We have one existing borehole, another to be drilled and we need one more. Leeds are in touch with the Environment Agency to see if we can identify nearby existing boreholes that can be used as our third.

October 2021

Our crushed basalt has been delivered and applied at the rate of 40 tonnes per hectare! We took soil samples before the application. We’ve also installed our soil solution lysimeters at around 1m depth and a 60° angle, below the topsoil. We have some great photos of the spreading.

A photo showing a tractor spreading basalt on our field site at Harpenden

We then planted winter beans at the standard rate of 25 seeds m2. Sampling boxes for measuring GHG have been installed on all plots. 

November 2021

Our bean crop has emerged, and crop establishment is excellent!

A photo showing bean crop establishment at our Harpenden field site

We have uncovered a third existing borehole. Our Leeds colleagues are advising on this and there has been a site visit by the proposed drilling company. We’re discussing how best to proceed and when to drill, as the field conditions are very wet.

January 2022

The new borehole was drilled under guidance from Leeds University colleagues. This will allow monitoring of aquifer water levels and quality. We’ve held some discussions with our Leeds colleagues regarding borehole and river sampling and analyses.

In terms of cross-programme engagement, Ian Shield acts as Co-Investigator and Agronomist for both our project and the Perennial Biomass Crops GGR Demonstrator (PBC4GGR), which allows him to look for synergies to both projects.

April 2022

A good crop of winter beans has established over winter, and we will increase the height by adding a second segment to the sampling chamber at each of the sampling points as the crop grows taller in April. Weekly gas sampling will continue until May before less frequent sampling during the driest months. We are receiving data back from North Wyke on a regular basis on the main greenhouse gases (GHGs): CO2, CH4 (methane) and N2O.

A photo showing our bean crop at Harpenden growing taller in April 2022

In terms of pore waters, good amounts of sample have been obtained recently due to the soil reaching its water-holding capacity in March and April. We’re carrying out alkalinity measurements on the pore waters using a piece of equipment called an auto titrator. Then our filtered and acidified samples are being sent to Southampton once a month.

Samples have also been collected from all 3 boreholes and the River Ver (up and downstream). We will do this on three further occasions spread out during the year so we can account for seasonal variations. Alkalinity and pH have been measured on all samples. In our rainwater sampling, we’re measuring total soluble nitrogen, anions, cations, and pH, and we’re sending rainwater samples to Southampton for the analysis of silicon in them.

Water sampling taken from a borehole in the ground at our field site in Harpenden

July 2022

Our bean crop is still going strong, with pods on some beans, and more growth expected before harvest. Harvest could be early in the season or we could leave beans until they look brown/black (the beans don’t fall off). Sometimes farmers leave harvest till later if their farms have lots of experimental crops: our field site manager for Harpenden, Ian Shield, will advise us on this. We made mesh bags and filled with them crushed basalt and these are ready in crates. They will go into the ground in between the two crops, when the beans are harvested.

A photo showing our bean crops growing at our Harpenden field site in July 2022

We’ve also discussed the plan for the next crop. Originally we planned winter beans followed by winter wheat, followed by oilseed rape. However, we’re concerned about a flea-beetle problem with oilseed rape, which could affect the yield and skew our results. It would be better to sow oilseed rape ASAP to avoid infestation. Winter wheat is sown later and harvested later than other crops such as winter barley. In order to avoid yield losses, it might be better to go with winter barley after the beans, which can establish early, and then oilseed rape can go in early. We could possibly sow winter wheat in a potential fourth year of this project. Ultimately, we’re replicating whatever best practice farmers would use, so there doesn’t seem to be any problem with rotating the crops this way in order to break the pest cycle.

Also, we’re discussing fertilisers – no nitrogen fertilisers went on the winter beans this year, but next year, we need to put nitrogen on. Our options are ammonium nitrate, added ammonium sulphate, urea, or calcium ammonium nitrate. It’s important to note any preference from our geochemists regarding potential acidification. They’ve said no to calcium salt as calcium is a key parameter we measure in fluids. Anything that induces acidity might end up with nitric acid weathering, which will lower the greenhouse gas removal potential. Our nitrogen fertiliser will have ammonium sulphate in it to provide sufficient sulphur. Our geochemists agree that ammonium nitrate or sulphate is best as it’s widely used in traditional agricultural practices, which we’re trying to replicate. Whichever fertiliser we apply will be withheld for sample analysis.

August 2022

Greenhouse gas sampling has carried on weekly through the year. We’ve done another one in the week commencing 8th August and again during the week commencing 15th August, before removing the chambers ready for harvest. Hand harvest for analytical samples was taken on 8th August, prior to the main harvest for plot yields. We’re receiving GHG data back from the North Wyke field site on a regular basis but it is too early to identify trends yet.

There have been no pore water samples since May, because of the incredibly dry spring and summer. But we did collect 4 sets of pore waters during February, twice in March and again in May. The sample tubes will be buried 20cm deep in the next couple of weeks, to avoid shallow tillage (which would risk them being accidentally dug up by the planting or harvesting processes) to 15cm before the next crop. We’re working on pore water data and waiting for more results to come through from the Analytical Unit. Subsamples have been sent to our colleagues at Southampton – for more analyses to add to our data sets.

Boreholes and river samples were taken in the March and June sampling set, and are now being analysed. We’re waiting for a decision on sending subsamples to Southampton for further analyses. The next samples to be taken will be in September and December. However, we are considering changing the frequency of our river sampling from four to eight times a year, given the rapid pH change we’ve seen. Sampling of boreholes will remain at four times a year. Our most recent samples are awaiting analysis. Rainwater sample data is coming through now and being compiled. Subsamples of a few selected samples have been sent to Southampton for further analysis (this will be around 4 times a year). In discussion with our colleague Rachael James at Southampton, we have concluded that alkalinity measurements on rainwater are not needed.

In terms of foreseeable issues, we are wondering if we might encounter problems with the burying of soil water sampling pipes and our mesh bags containing rock dust after harvest, as the ground is too hard if this year’s weather trends continue. After our winter bean harvest, we will water the areas we need to dig, if necessary, with collected rainwater. 

September 2022

Everything is going well. We harvested the beans, and the yield was reasonable given the year and little rainfall: 3.8 tonnes per hectare of very dry beans. There is 200 kilos difference in yield between rock dust and no rock dust (in favour of rock dust) but it is unlikely to be a statistically significant result. We’ve also buried the mesh bags full of basalt. Soil samples have been collected. A new consignment of crushed basalt has been delivered and is due to be spread soon.

Great news from Harpenden! We can't wait to see what happens next. Catch up with our Harpenden updates again in a few months as the experiments progress.

With many thanks to Prof Steve McGrath for help in co-ordinating this blog

- Authored by Victoria Giordano-Bibby

Research Centre Manager