1. Introduction
Peat lands cover an estimated area of 400million hectares or equivalent to 3% of Earth's land surface. Tropical peat land is in the range of 30-45million ha which is 10-12% of the global peat land resource [1]. Wise use of tropical peat land had been emphasized taking into considerations the tradeoffs between development and conservation [2]. While perennial crop cultivation such as oil palm on peat land can be seen as a solution for rural development, peat land conservation has also attracted global attention especially from a climate change perspective. The 2013 Supplement to the 2006 IPCC (Intergovernmental Panel on Climate Change) Guidelines for National Greenhouse Gas Inventories: Wetlands - Methodological Guidance on Lands with Wet and Drained Soils, and Constructed Wetlands for Wastewater Treatment - (Wetlands Supplement) provides new and supplementary guidance on estimating and reporting greenhouse gas emissions and removals from lands with organic soils and with wet mineral soils in Wetlands and other land-use categories with these soil types that are subject to human activities [3]. The yield viability and economic feasibility of peat land cultivation for agricultural crop had been questioned such as the Mega Rice Project in Central Kalimantan where one million hectares of peat land converted for rice cultivation had failed [4]. There are also views that Best Management Practices (BMP) should be used to improve yield for crop cultivation such as oil palm on peat land [5].
Currently, the yield economics of oil palm cultivation on peat land is commonly related to two major issues. Firstly land selection taking peat depth and drainability into consideration and secondly Best Management Practices (BMP) which are implemented after initial land development. Depth is related to whether the soils are shallow (0-100 cm), moderately deep(>100 cm -300 cm) and deep (>than 300 cm) [6]. Drainability refers to sustainable drainage conditions where long term sustainable drainage in relation to the depth of underlying mineral subsoil level rather than the present ground surface to the river water levels. Tie and Melling [7] identified four classes (good, moderate, poor and very poor) of drainage differentiating gravity drainage as against the tidal cycle of nearby rivers even after all the surface peat had subsided.
In the past, the land selection criteria used for the development of tropical peats for agricultural development were its depth and drainability [7], [8] and [9]. As such, many important tropical peat soil characteristics such as peat maturity, the presence/absence of wood, nature of wood (decomposed/undecomposed) and the nature of the underlying mineral substratum were not used in their mapping and classification and also in their land suitability evaluation [10] and [11]. Consequently current soil mapping and classification systems such as the Keys to Soil Taxonomy [12] and the World Reference Base for Soil Resources 2006 [13] which were developed for temperate peats also do not recognise the presence of wood and hence cannot be fully used in the tropics [10] and [14]. This is due to the fact that temperate and boreal peats are often dominated by bryophytes and shrub whereas tropical peat land in contrast have various tree species with root penetration to several metres. Rate of biomass production and decomposition is high resulting from decaying roots and root exudates.
Veloo et al., [10] proposed using the Malaysian classification system which takes into consideration depth, morphology, nature of wood and underlying mineral substratum to compliment international classification schemes.
Many of the tropical peat characteristics such as their depth, presence and nature of wood and subsurface tier characteristics therefore need to be mapped as these may impact strongly the suitability and management of tropical peat lands for crops such as oil palm [15]. These characteristics may seriously impact the economics of cultivation and management of oil palms on peat particularly from the yield perspective [6]. For example, being close to coastal areas, the underlying subsurface tier are usually marine clay (often sulphidic), riverine alluvium or sand [16]. Tahir et al.,[17] indicated that both riverine alluvium and sand can be mined to construct roads by building up foundation with spoils (dug below peat layers) from adjacent drains. Good road networks are critical in determining crop evacuation, transportation and crop quality which enhances the yield of oil palm. Peat characteristics are also expected to influence other factors such as GHG emissions and performance.
The objective of this paper is to explore the impact of physical properties of tropical peat on oil palm yield. This study tests the hypothesis that physical soil properties such as peat maturity, presence of wood and nature of underlying substratum and depth affects oil palm yield.