Vetiver Grass Terraces Slash Fertilizer Runoff in Sri Lanka’s Steep Tea Lands

On the misty slopes of Sri Lanka’s central highlands, tea bushes cling to hillsides so steep that every heavy monsoon shower threatens to wash away the very nutrients farmers pay dearly to supply. A new field experiment conducted at Kenilworth Estate in Ginigathhena has now put hard numbers on that hidden drain, and the findings carry a strikingly practical message: a simple combination of stone terraces and vetiver grass can cut the loss of dissolved nitrogen by roughly 78 percent, dissolved phosphorus by 80 percent, and dissolved potassium by 86 percent compared with tea grown without any conservation measures at all.

The study, carried out by L. A. S. P. Jayasinghe of the Tea Research Institute of Sri Lanka and Simplicio M. Medina of the University of the Philippines Los Baños, was published in the journal Discover Soil. It addresses a gap that has long frustrated both agronomists and environmental scientists. While soil loss from tea lands has been quantified repeatedly, far less is known about how much dissolved nitrogen, phosphorus, and potassium slip away in runoff water itself, and whether the physical barriers farmers already build can meaningfully stem that flow.

The experimental site sits at mid elevation, between 580 and 600 meters above sea level, in the wet zone mid country agro-ecological region where annual rainfall exceeds 3,200 millimeters with a 75 percent probability. The terrain is unforgiving: slopes in the experimental area ranged from 60 to 72 percent. The soil belongs to the Maskeliya series of the Red Yellow Podzolic group, a strongly acidic, sandy clay loam derived from feldspar-rich rock. Initial sampling revealed conditions already below optimal for tea, with soil organic carbon at just 0.7 percent against a recommended 2.2 to 3.0 percent, total nitrogen at 0.15 percent, and every exchangeable cation measured, including potassium at 62.1 micrograms per gram, falling short of favorable levels. In other words, the field was already nutrient-poor before a single drop of runoff was collected, which makes every kilogram of lost fertilizer all the more consequential.

To measure the losses directly, the researchers established twelve erosion plots, each 10 meters long and 3 meters wide, laid out along the contours and arranged in a randomized complete block design with three replicates. Aluminum sheets buried along plot boundaries prevented water from entering or escaping sideways, and 250-liter plastic barrels at the lower end of each plot captured every liter of runoff. Four treatments were compared: a stone terrace built level with the soil surface, the same stone terrace planted with vetiver grass along its inner edge at 15-centimeter spacing, a raised terrace wall 30 centimeters high, and a control plot of tea with no conservation structure whatsoever. The tea cultivar was TRI 2023, fertilized twice a year with VP/UM 910 at 200 kilograms per hectare, following the estate’s standard schedule.

The monitoring window, from October 2020 to November 2021, captured a punishing 5,970 millimeters of rainfall across 26 sampling events. Runoff followed rainfall closely, spiking during continuous high-rainfall periods when the soil became saturated. Over the full study period, the untreated control plots shed 1,681.8 cubic meters of water per hectare per year. Stone terraces alone cut that volume by 55 percent, terrace walls by 68 percent, and the stone terrace with vetiver grass by a remarkable 80 percent, leaving just 332.9 cubic meters per hectare to escape. The differences among the three conservation measures were not statistically significant, but each one dramatically outperformed the unprotected control.

When the researchers analyzed the chemistry of that runoff, an important nuance emerged. The concentrations of ammonium-nitrogen and nitrate-nitrogen in the water, ranging from 3.2 to 3.9 and 2.1 to 2.3 milligrams per liter respectively, did not differ significantly among treatments, and dissolved phosphorus concentrations, between 0.88 and 1.26 milligrams per liter, were likewise statistically indistinguishable. What mattered was volume. Because the conservation structures slashed the sheer quantity of water leaving the plots, the total mass of nutrients exported fell sharply even where concentrations stayed similar. Dissolved ammonium-nitrogen losses in the vetiver treatment dropped to 1.3 kilograms per hectare per year and nitrate-nitrogen to 0.8 kilograms, both significantly lower than the control. For context, tea fields in China under 240 kilograms of nitrogen per hectare have shown runoff nitrogen concentrations of 4.4 to 17.4 milligrams per liter, and Japanese tea catchments have recorded nitrate levels near 50 milligrams per liter in rivers and reservoirs, underscoring how much worse unmanaged systems can become.

Phosphorus told a different story. Losses of dissolved phosphorus ranged from 0.42 to 2.03 kilograms per hectare per year across treatments, but the differences were not statistically significant. Against an applied phosphorus rate of 35 kilograms per hectare per year, the conservation treatments lost only 1.1 to 2.7 percent of the input, while the control lost 6.7 percent. The explanation lies in phosphorus chemistry: the element binds strongly to soil minerals and organic matter in acidic tea soils, so most of what is applied stays in the field, either fixed in place or taken up by the bushes. A study in Taiwan similarly found that less than 5 percent of applied phosphorus left the field in storm runoff. The environmental stakes remain real, since phosphorus in waterways drives eutrophication, but on these slopes the primary lever for protecting waterways is keeping soil particles in place rather than managing dissolved losses.

Potassium, often overlooked because it is not directly linked to water quality impairment in the way nitrogen and phosphorus are, proved to be the biggest casualty of unchecked runoff. Dissolved potassium removal ranged from 1.27 to 9.5 kilograms per hectare per year, and the differences among treatments were highly significant. The vetiver-and-terrace combination lost just 1.27 kilograms per hectare per year, compared with 9.09 kilograms from the unprotected control, with the stone terrace alone at 3.37 and the terrace wall at 2.09 kilograms. The result echoes findings from India, where vetiver planted on bunds significantly improved nutrient conservation, and from Brazilian cropping trials where contour planting reduced potassium losses from 3.2 to 0.5 kilograms per hectare relative to downslope planting.

The mechanism behind the vetiver system’s success is a marriage of engineering and biology. The stone terrace acts as a physical barrier that slows the flow of water down the slope, giving it time to infiltrate, while the dense vetiver hedges act as living filters that trap fine soil particles suspended in the runoff. Neither measure alone performed as well as the two together, and although the vetiver treatment was numerically the best performer, the authors note that its advantage over the other conservation structures was not statistically significant. What was unambiguous was the contrast with doing nothing: every conservation measure substantially reduced both water and nutrient export, and nutrient removal broadly tracked runoff volume across the board.

For an industry in which fertilizer is among the most expensive management inputs, the economics are as compelling as the ecology. Every kilogram of nitrogen or potassium washed downhill must be replaced at cost, and the lost nutrients also degrade streams and reservoirs downstream of the plantations. With climate change expected to intensify rainfall extremes across tea-growing regions, the pressure on steep, thinly covered soils will only grow. The study’s message is that farmers on mid-elevation slopes do not need exotic technology to defend their soil; a well-built stone terrace lined with a strip of vetiver grass, a plant already familiar across the tropical world, can retain the majority of the nutrients that rain would otherwise carry away, protecting both the productivity of the land and the health of the waters below it.

Subject of Research: Effects of soil conservation measures on dissolved nutrient loss through surface runoff in mid-elevation tea lands of Sri Lanka

Article Title: Effects of soil conservation practices on nutrient loss through surface runoff in mid elevation tea lands of Sri Lanka

Article References: Jayasinghe, L. A. S. P., & Medina, S. M. (2026). Effects of soil conservation practices on nutrient loss through surface runoff in mid elevation tea lands of Sri Lanka. Discover Soil, 3(1), Article 122. https://doi.org/10.1007/s44378-026-00280-3

Image Credits: AI Generated

DOI: 10.1007/s44378-026-00280-3

Keywords: tea cultivation, soil conservation, surface runoff, nutrient loss, vetiver grass, stone terraces, soil erosion, Sri Lanka, dissolved nitrogen, dissolved potassium, dissolved phosphorus, sustainable agriculture