Honours
Thesis (2001) Ilyas Siddique,
“Changes in soil
fertility associated with the establishment of forest gardens on degraded
grassland soils in the wet zone of the Sri Lankan highlands”
Abstract in English (htm)
Abstract in Sinhalese
(htm, gif images)
Related Informative Resources
(htm)
References (htm)
Publications Relevant to the Thesis:
Siddique I,
Gutjahr C, Seneviratne G, Breckling B, Ranwala SW,
Alexander IJ (2007) Changes in soil chemistry associated with the establishment
of forest gardens on eroded grassland soils in
Siddique I, Seneviratne G, Breckling B,
Alexander IJ (2001) Restoration of soil chemical fertility by forest garden
establishment. Pragņa - IFS
Science Bulletin XIV(3-4):11-15.
Siddique,
I, C Gutjahr, G Seneviratne,
SW Ranwala,
Abstract (English):
On the widespread eroded and acidified soils in the wet zone of the Sri Lankan highlands, abandoned tea plantations remain grasslands for many years because regular fire inhibits succession. Tree-dominated, 'stratified', multispecies agroforests, called forest gardens, have been planted for their socio-economic benefits. In this study, topsoil fertility and vegetation characteristics were determined in forest gardens established about 20 years ago on degraded grasslands (abandoned tea lands) in the wet zone of the Sri Lankan highlands. These were compared with adjacent, degraded grasslands (abandoned tea lands), on strongly weathered soils versus soils at earlier stages of pedogenic development in a 2-way analysis of variance.
In forest garden soils, the cation exchange capacity (CEC, measured at pH 4.8) was on average nearly double and the base saturation was more than four times as high, compared to the adjacent grasslands. While soil pH was about one unit higher, ratios of exchangeable calcium-to-extractable aluminium were on average nearly nine times higher in forest garden soils. The total nitrogen content was found to be on average nearly 40 % higher in the garden soils. In contrast, topsoil gravel contents in the gardens were less than half as high as in the grasslands. The in situ weathering Reddish-Brown Latosolic Soils showed more than twice as high a CEC[pH4.8] and respiration rate, and more than three times as high exchangeable base contents as the more strongly weathered Red-Yellow Podzolic Soils. 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 CEC[pH4.8], 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 carbon contents, and substantially higher pH, presumably caused by base accumulation and decomposition processes. The greater response of Reddish-Brown Latosolic Soils to garden establishment was probably due to higher permanent CEC and rates of base input from weathering, while garden trees might have "pumped" calcium and magnesium from the subsoil, in addition to higher retention.
No conclusions could be drawn about land use effects on soil phosphorus pools, organic matter contents and fractions, respiration rates, porosity and earthworm abundance, presumably because spatial variability independent of land use between adjacent garden and grassland plots meant that more subtle changes would only be detectable with larger sample sizes. In contrast, the results suggest substantial increases in the soil's chemical capacity to retain and supply nutrients to plants about 20 years after forest gardens are established on eroded and acidified soils of both great soil groups examined. Besides selected data from this study, extensive informative resources are compiled in the appendix, including bibliography, databases, seed sources and contacts to organisations, mostly accessible on the internet, on directly related subjects ranging from forest gardens, soil fertility and plant genetic resources, to sustainable natural resource management, indigenous knowledge and rural development.