Abstract:
Topsoil properties were determined in forest gardens
established about 20 years ago on eroded grassland soils
(abandoned tea lands) in the wet zone of the Sri Lankan
highlands. They were compared with adjacent, eroded grasslands
(abandoned tea lands) on strongly weathered soils vs
soils at earlier stages of pedogenic development in a two-way
analysis of variance. Soil pHin forest gardenswas, on average,
6.1, nearly one unit higher than in the adjacent grasslands. In
the garden soils, the cation exchange capacity (CEC measured
at pH 4.8) was nearly double, exchangeable calcium concentrations
five times and exchangeable magnesium three times as
high as in the grasslands soils. Total soil N content was found
to be nearly 40%higher in the gardens. Topsoil gravel contents
in the gardens were less than half as high as in the grasslands.
The increases in exchangeable bases and N in gardens, relative
to grasslands, were attributed to increased nutrient retention and acquisition. Higher retention was partly due to the higher
CECpH4.8, and probably to reduced erosion and increased,
continuous fine root density in the garden topsoils. Higher
field CEC in gardens was likely to result from generally
higher C contents and from the reversal of acidification,
presumably caused by base accumulation and decomposition
processes. Our results suggest that forest garden establishment
on degraded grasslands can lead to accumulation of
mobile nutrients in the topsoil, probably due to increased
nutrient retention, subsoil uptake and litter input exceeding
nutrient uptake by the standing biomass.
