Tea Time for Research: gaining a better under­standing of decom­po­si­tion

The major chal­lenge with organic manure appli­ca­tions is timing, as nutri­ents need to be released in sync with plant growth. However, it’s hard to know how quickly nitrogen from slurry and muck becomes avail­able to plants in the soil. Surpris­ingly, tea bags can help to shed light on soil decom­po­si­tion processes. This is no joke! In 2013, ecol­o­gist Joost Keuskamp and his team devel­oped the tea bag method using Lipton brand green and rooibos tea bags. The method enables anyone, not just scien­tists, to assess soil activity in an inter­na­tion­ally stan­dard­ised way. Most recently, in 2024, an inter­na­tional citizen science project saw volun­teers bury unused tea bags in the soil of the Rhön in Germany, only to dig them up again weeks later to gain new insights into the soil’s biolog­ical activity.

What the tea bags reveal about the soil

If carried out correctly, the tea bag method can be a cost-effec­tive and prac­tical way to measure the decom­po­si­tion rate of organic matter. This is how it works: Tea bags of both vari­eties are first weighed to three decimal places on a preci­sion scale, both empty and filled. The filled tea bag is then buried 8cm deep in the soil with the label still visible. This makes it easier to find the bags when they are dug up again 90 days later.

Once dug out, they must be left to dry completely in a warm place before being weighed again. The loss in weight indi­cates how much organic mate­rial soil-dwelling organ­isms have broken down in that time. Farmers are unlikely to be surprised that green tea made from camellia leaves (which has a carbon: nitrogen ratio of 12.8:1) decom­poses faster than rooibos tea, which is made from the branches of the red bush (C:N=60.6:1) . The different speeds at which the tea vari­eties decom­pose enable the Tea Bag Index to be calcu­lated: This combines the decom­po­si­tion rate and the litter stabil­i­sa­tion factor.

Decom­po­si­tion or perma­nent humus

The litter stabil­i­sa­tion factor shows how much organic matter in the teas remains unde­com­posed, even over a longer period of time. This stable frac­tion repre­sents the carbon that can even­tu­ally form perma­nent humus. The litter stabil­i­sa­tion factor increases with the carbon seques­tra­tion poten­tial of the soil and is there­fore higher in peat­lands, for example, than in trop­ical rain­forests. Envi­ron­mental condi­tions can play a key role; high soil temper­a­tures and suffi­cient precip­i­ta­tion promote biolog­ical decom­po­si­tion: The decom­po­si­tion rate increases and the stable frac­tion (litter stabil­i­sa­tion factor) decreases. For example, a 2020 citizen science study found the decom­po­si­tion rate in cooler Sweden was 0.008 to 0.012g/day, while in warmer Austria it was between 0.012 and 0.015g/day.

At the same time, Swedish soils had a litter stabil­i­sa­tion factor up to four times higher than Austrian soils.
Thanks to the stan­dard­ised content of the tea bags, the results of the Tea Bag Index can be compared glob­ally. This approach makes it possible to test the envi­ron­mental condi­tions that affect soil decom­po­si­tion activity, regard­less of the quality of the mate­rial being decom­posed.

Appli­ca­tion of the tea bag method in agri­cul­ture

In 2020, Olav Holst from the Eber­swalde Univer­sity for Sustain­able Devel­op­ment in Germany utilised the method in his bachelor’s thesis, to study the decom­po­si­tion rate of grass-clover silage in sandy soil in Bran­den­burg. He filled the emptied tea bags with the silage and repeated the remaining steps. The grass-clover silage had a C/N ratio of 22.5:1, placing it between the two refer­ence mate­rials. Indeed, the teabag method enabled well-founded conclu­sions about the decom­pos­ability of grass-clover silage to be drawn quickly and inex­pen­sively. Around 40% of its organic mass was broken down within 14 days, and 70% after 40 days. “This rapid decom­po­si­tion indi­cates a swift avail­ability of nitrogen,” says Olav.

About half of the nitrogen contained in the silage was released within 90 days – despite the period being char­ac­terised by drought – with a soil mois­ture levels just above the perma­nent wilting point. Farmers can use the tea bag method them­selves to better esti­mate or compare the decom­po­si­tion rate of organic manures and thus the avail­ability of nutri­ents. Anyone wishing to do so can enter the collected data on the project’s inter­na­tional website. This also stores Tea Bag Index data from other sites world­wide for compar­ison.

A global project for climate change

With the help of the exten­sive data gener­ated, it will be possible to better under­stand and assess the carbon cycle and its effects at local, regional, and global levels. This is impor­tant because the biolog­ical decom­po­si­tion of organic mate­rials in the soil produces green­house gases like CO2. The carbon storage poten­tial of the soil there­fore has an impact on global warming.

At the Swiss Univer­sity of Zurich, the tea bag method is being taken to the next level with the help of under­wear: In the 2021 “Proof by Under­pants” project led by Marcel van der Heijden, the same new, white, cotton briefs were buried to test whether under­pants are also suit­able as a stan­dard­ised method for measuring soil activity. Devel­oped in collab­o­ra­tion with Agro­scope, this spin on the Tea Bag Index proved successful, as by 2024 the Swiss were once again burying cotton briefs for science under the motto “Unter­hosen runter!” (under­pants down). Why? Well, what would you enjoy more: Burying a tea bag or a pair of under­pants?