By Dr. Markus Heidak
The bedrock upon which the world’s tea plantations rest is a varied and vital source of minerals.
Geochemical formations, the presence of heavy metals, and the natural depletion of scarce minerals over time are important considerations for growers.
While the tea plantations in Sri Lanka and India that were developed during colonial times look pretty much the same then as today these plantations are neither antique or outdated pieces of history. To those walking these plantations for the first time, the initial impressions must cause the feeling of travelling back in time.
However, these gardens are rather modern examples of agricultural areas in which numerous natural processes and innumerable manmade processes collide and interact with the growing tea plants. To ensure constant tea quality and to increase yields it is essential to understand these hidden processes and how they affect the plantation.
Effective management takes into consideration influences on a local scale. These include different types of rocks (e.g. granite, basalt); climatic conditions, various natural aerosols (e.g. Saharan dust, sea spray), human-induced emissions and impacts (e.g. exhaust fumes, pesticides and fertilizers) as well as socioeconomic changes (e.g. expansion of industrial areas). All affect the soils and the growing tea bushes.
In 2014, the scientific consultancy Heidelberg Geo Bio Consult (HGBC) was tasked with collecting and analyzing rocks, soils, and tea leaves from several organic certified and non-organic tea plantations for an independent research study.
The study was designed so that growers can better understand how parent rocks affect the chemical composition of the soils and the cultivated tea. The goal is to develop methods to improve quality and food security.
Several dozen representative samples (rocks, soils, tea leaves) were analyzed and interpreted using ICP-OES (inductively coupled plasma optical emission spectroscopy), and ICP-MS (inductively coupled plasma mass spectrometry) along with detailed analysis of soil pH with a strong focus on the natural chemical element cycles.
The results showed as many as 60 different elements within tea leaves, including macro- and micro-nutrients such as iron (Fe), potassium (K), calcium (Ca), manganese (Mn), and, heavy metals such as lead (Pb), cadmium (Cd), and, mercury (Hg) as well as rare earth elements such as cerium (Ce), neodymium (Nd), and gadolinium (Gd).
Sufficient amounts of these nutrients are crucial for strong and healthy plant growth, but the concentrations of heavy metals should be kept as low as possible due to their detrimental effects on humans and the environment.
The toxicological health effects on humans through the consumption of rare earth elements are not entirely known but major tea import countries have specified thresholds for elements such as lead, cadmium and mercury which may not be exceeded.
An illustration of a tea plantation's underlying geochemistry.
a) Plantation with pristine element cycles between rocks, soils and tea plants.
b) Plantation with high levels of heavy metals within rocks and tea leaves.
c) Plantation with depleted amounts of elements within rocks, soils and leaves.
Sampling demonstrated there are no distinct differences between organic or non-organic plantations. The natural plantations contained sufficient and normal amounts of elements. They can be described in the chart at right (Fig. 3a) as plantations with pristine element cycles between rock, soils, and tea bushes.
Areas with enriched amounts of minerals showed that rocks affected the tea plants by releasing different chemical elements into the soils which were then taken up into the leaves. There was a clear correlation between rocks containing heavy metals (e.g. Pb, Cd, Hg) in polluted areas where leaves showed an uptake of heavy metal (Fig. 3b).
Levels of up to 6.75 ppm of lead (Pb) were found in the leaves when high levels (up to 255 ppm) were found in the analysed rocks.
This can have a significant impact on export. Currently there is no limit for lead but restrictions are under consideration. Exceeding thresholds for consumption would prevent export to the EU. The same correlation was seen in rocks containing rare earth elements and leaves, and was recognizable for increased nutrients as well.
In regions with strongly weathered rocks, where elements have already been released and washed out. Our sampling showed depleted element levels in the soils and in the leaves (Fig. 3c). In these areas adding fertilizer is unavoidable to keep yields constant.
These results cannot be generalized for all tea plantations worldwide since there are also other parameters that steer the availability and the uptake of elements (e.g. climate, soil pH). Therefore, the geological condition of each plantation has to be individually examined.
The research study showed that in terms of quality control and food security, plantations should not only focus on the applied pesticides and fertilizers but also on geological and geochemical information.
The study demonstrates that an analysis of rocks should taken into account in order to cultivate consistent tea quality, to prevent natural contamination (e.g. heavy metals), to reduce the amounts of applied fertilizers and to cultivate tea more cost efficient and sustainable.
Dr. Markus Heidak is a geologist and geochemist with offices in Dubai, UAE.
CORRECTION: A previous version incorrectly reported a 0.2 ppm limit for lead in tea under European Union legislation. For lead in tea currently no limit applies.
The latest amendment of the Contaminants Regulation (EU) 2015/1005 explicitly states: “As consumption of tea and herbal infusions can be an important contributor to dietary exposure, a maximum level for these commodities should be established. However, in absence of data on dry tea leaves and dry parts of other plants for the preparation of herbal infusions allowing the establishment of such a maximum level, occurrence data should be collected in view of the possible establishment of a specific maximum level in the future.”