The Importance of Soil
Soil is an incredibly complex, and often overlooked component of an eco or agricultural system. Containing air, water, organic matter, and mineral matter, soil covers the top several meters of the earth’s surface above the bedrock. Soil is the basis for all eco-systems as it provides the necessary nutrients and structure to allow vascular plants, such as tea plants, to grow. Essentially every component needed to make a plant grow, except sunlight, is contained in the soil.
While most of the flavanols and theaflavins (molecules that contribute to taste) are created during the fermentation process of black tea production, the climate and soil conditions can also contribute to the taste profile of tea. This idea is common in the wine industry, where the “Terroir” of a product creates a unique taste profile as no two locations have the same soil, water, and climate. However I think it's fair to apply the same concept to tea.
There are a number of factors that contribute to the fertility of a soil: the ability to hold water and air, the availability of nutrients - especially potassium (K), nitrogen (N), and phosphorus (P), and the acidity of a soil play important roles in how well different plants can grow in a certain soil. There are a million and one elements that contribute to creating a soil, as a soil is at the intersection between bedrock, vegetation, atmosphere, climate and precipitation - all interplay to create unique regional soil characteristics.
Tea plants of the Darjeeling region are typically planted on previously forested soils on sloping mountainsides of the Himalayan foothills. Soils tend to be fertile, but unsuitable for most large-scale agricultural crops due to the intense topography. A 2006 comparative study published in the International Journal of Soil Science found that tea cultivation actually increased the fertility of soil when compared to forested soils in the same region, which they attributed to higher organic matter and more careful fertilization.
The Soils of Darjeeling
Tea plants can grow in a wide range of conditions, yet the Darjeeling region has a unique combination of environmental factors to make such a delicate and complex tea. Soils in the Darjeeling region tend to be of the loam variety, loam being an ideal soil for growing plants. Darjeeling soils are classified either as sandy loam or as clayey loam depending on the proportion of clay and sand in soil. This influences a soil’s capacity to hold water as water particles will cling to the smaller clay particles, where as water in sandier soils will drain quickly.
Despite the higher water holding capacity the clayey soils in Darjeeling tend to be fairly well drained, particularly those on the higher slopes, simply because gravity acts on incoming water before it can completely infiltrate the ground. Tea plants also prefer slightly acidic soil where soils on most tea plantations hover between 4.5 and 5.5 pH. According to the Food and Agricultural Organization (FAO) the constant weathering of bedrock due to precipitation makes for mineral rich soil, and the previous forestation of the soil explains the high organic matter found in Darjeeling soils. Yet the high volume of precipitation causes some leeching of the soil resulting in poor levels of lime, magnesium, iron oxides, phosphorous and nitrogen. This means that most tea plantations have to add lime, nitrogen and phosphorous back into the soil.
Sadly I could find little formal research on how soil chemistry influences tea plant growth and the resulting tea flavour, however I hope this post has shed some light on the importance and complexity of soil within tea production, as well as some of the soil characteristics that help create Darjeeling tea.