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PHYSIOCHEMICAL CHARACTERISTICS OF SOIL OF A MIXED SHOREA ROBUSTA FOREST IN RUPANDEHI DISTRICT, NEPAL

Physiochemical characteristics of soil of mixed Shorea robusta forest in Parroha community forest of Rupandehi district (central Nepal) were analyzed. Soil samples were collected from south eastern (SE) and south western (SE) slopes of the forest and analyzed for water holding capacity (WHC), pH, organic matter, nitrogen, phosphorous and potassium. The soil acidic (pH 4.2-6.2) with organic matter 1.87-2.73 %, nitrogen 0.09-0.13 %, phosphorus 4.7-23.3 kg/ha, and potassium 141-191 kg/ha. The SE slope, with high canopy cover and litter content, had relatively better soil characteristics as compared to the SW slope.


INTRODUCTION
Soil is the loose, friable, unconsolidated top layer of earth's crust. It is the medium in which roots grow, anchor the plants and is the reservoir of water and nutrient necessary for the terrestrial plant life. Soil has a distinctive type of fl ora, fauna and microorganisms. The soil surface is often covered with a layer of leaf litter, plant's residue, or other fresh or partially decomposed organic matter (Lekhak and Lekhak 2005). Forest soil infl uences the composition of the forest stand and ground cover, rate of tree growth, vigor of natural reproduction, and other silviculturally important factors (Bhatnagar 1965). Soil posses many characteristic physical and chemical properties that determine its quality and usefulness. Physiochemical characteristics of forest soils vary in space and time due to variations in topography, climate, physical weathering processes, vegetation cover, microbial activities and several other biotic and abiotic variables. Vegetation also plays an important role in soil formation. For example plant tissues (from above ground litter and below ground root detritus) are the main source of the soil organic matter (OM) which is called 'life blood' of soil because it has an effect upon the physical, chemical and biological properties of the soil (Pandey et al. 1996). OM plays a dominant role in supplying plant nutrients especially nitrogen, phosphorus and sulphur. High OM improves infi ltration rates and water holding capacity (WHC). Soil organic matter acts as buffering in soil pH, improves soil texture, increase the activities of soil organisms. The pH value of soil ranges from 3-10 (FORSPA 2002). Soils with a near neutral pH (6.0-7.0) are the most fertile. Soil microorganisms thrive best between pH 5.5 to 7.8. The soil pH determines the kind of plants it can support by infl uencing availability of nutrient and their uptake by plants. Nitrogen (N) is most often the limiting nutrient in plant growth; it is a constituent of chlorophyll, plant proteins and nucleic acids. The amount of N in most natural soil may vary from 0.03 to 0.10% in the surface layer (Etherington 1974). Phosphorous has been called 'the key to life' because it is directly involved in most of the life processes (Pandey et al. 1996). Phosphorous (P) occurs in the soil in both organic and inorganic forms. Natural soil usually contains P between 0.02-0.5% most of which derived from parent materials (Etherington 1974). Similarly, potassium (K) is the third most likely nutrient element to limit plant growth. Natural soil usually contains much more K than P or N and the value ranges between 0.03-2.5 % (Black 1968).
Generally the sal forests in Nepal are in a degraded state because of indiscriminate cutting, recurring forest fi re, and uncontrolled grazing, leading to nutrient impoverishment. In fact, more than half of the tropical soil in the world is highly weathered, leached and impoverished, and therefore mechanisms to conserve nutrient in the ecosystem are important (Sanchez 1976, Jordan 1985. The objective of the present study was to analyze the physiochemical characteristics (water holding capacity, texture, pH, OM, N, P and K) of soil at two slopes (south-east and south-west facing) of Parroha community forest in Rupandehi district where south-east facing slope is dominated by Terminalia alata and south-west by Shorea robusta.

STUDY AREA
Parroha Village Development Committee (VDC) area is located in Rupandehi district which lies between 27°20' to 27°45' N latitude, 83°10' to 83°30' E longitude. It is a part of Terai region of Nepal and covers a total area of 73196.1 ha of which about 73% is agricultural land, urban areas and roads, 23% forest and remaining 4% water resources (Anonymous 2007). Present research was conducted in Parroha community forest of Parroha VDC with an area of 633 ha. The users' group of the forest includes 1267 households (HH) and 8849 members. The landscape is south facing with several micro aspects ranging from south-east to south-west facing slopes. Two sites (SE slope: South-East facing slope and SW slope: South-West facing slope) lying between 250 and 550 m asl were selected for the present study. For management purposes the Parroha Community Forest has been divided into three blocks. Present study was carried out at Rani block in SE slope and Kapase block in SW slope of PCF separated by a Ghamaha Khola, a perennial river. The SE slope is dominated by Terminalia alata whereas SW slope by Shorea robusta with their associated species like Anogeissus latifolia, Lagerstroemia parvifl ora, Buchanania latifolia, etc.
The climate of the study area is typically tropical, dominated by South east monsoon. A hot climate generally prevails throughout the years except in the short winter. Mean temperature ranges from nearly 10°C during winter to >35°C during late summer (April-May).

Soil Sampling and Laboratory Analysis
Soil was sampled during October-November of 2005. In each slope (site) three parallel transects were visually defi ned from 250 to 550 m asl. Each transect was divided into four elevation bands at the interval of 100 m elevation increment. In each elevation band of each transect three quadrats (10 m × 10 m) were sampled. Among these three quadrats one lied along the transect line and two on either sides at a distance of 40 to 100 m. Soil samples (15 cm depth) were collected from the four corners and centre of the 10 m × 10 m quadrat following the method given by Saxena (1989). Soil from three quadrats of each elevation band of each transect were mixed homogeneously to make a single sample and packed in an airtight poly bag until the determination of moisture. Thus, 12 samples were collected from each slope. After moisture estimation the soil was air dried in shade. Water holding capacity was analyzed at Laboratory of Ecology, Central Department of Botany, Tribhuvan University while total nitrogen (N, %), available phosphorus (P) as P 2 O 5 (kg/ha), available potassium (K) as K 2 O (kg/ha), pH and organic matter (OM, %), were determined at Soil Management Directorate of the Department of Agriculture, Hariharbhawan, Lalitpur. Water holding capacity was analyzed following the methods described in Zobel et al. (1987). Organic matter was determined by Walkey and Black rapid titration method, soil pH by using electric pH meter, total nitrogen by modifi ed micro-Kjeldahl method, available phosphorous by Olsen's modifi ed carbonate method and available potassium by fl ame photometer. All these methods of soil chemical analyses have been described in Gupta (2000). Mean of different soil variables at two slopes were compared by Analysis of Variance (ANOVA) using Statistical Package for Social Sciences (SPSS ver. 11.5).
The acidic nature of soil of the study area may be attributed to the high rainfall (average annual rainfall 1391 mm), which is suffi cient to remove basic cat ions from the soil surface. Moreover density and crown cover of trees were lower, which has enhanced the growth of grasses. The grasses tend to use more bases, resulting decrease in soil pH (Barbour et al. 1999). Water holding capacity (WHC) of soil in the studied forest ranged from 39-49 % (Table 1 and 2). The value was similar to the values reported by Poudel and Sah (2003) in tropical Shorea robusta forest in eastern Nepal (49%±6.30%) and Bashyal (2005) in tropical forest of Palpa (36%-48%). The present value was lower than the value reported by Pant (1997) in hill S. robusta forest (70-78 %). According to Bhatnagar (1965), WHC of soil from regenerating areas is higher due to high litter content.  Table 3) and ranged from 1.87-2.73 (Table 1-2). According to soil fertility rating chart of Soil Management Directorate of the Department of Agriculture, Hariharbhawan, Lalitpur (Annex 1), the soil of present study area had low to medium fertility which might be due to rapid decomposition and mineralization of organic matter under relatively high temperature. Brady (1984) mentioned that the lower soil OM occurred more commonly in warmer climate than in cooler. The OM content in the soil of the study area (1.87-2.73 %) lies within the range of values reported by Marasini (2003) for Churia forest (1.33-2.45 %), and Paudel and Sah (2003) for terai Shorea robusta forest in eastern Nepal (1.01-2.42). The soil OM of the present study forest was lower than the value reported by Chhetri (1997) in Chitwan National Park (8.61-10.29 %) and higher than the value reported by Dhungana (1997) in hill Shorea robusta forest of Kavrepalanchowk (0.54-1.88 %).

Table 3. Mean (± SD) of different attributes of soil sample at two sites (N = 12 for all soil attributes for each site). F-value and signifi cance level were obtained from analysis of variance (ANOVA
The soil nitrogen content at SE slope was higher than at SW slope (P<0.01). It might be due to high OM at SE slope than SW slope (Table 1-2). Higher value of N and OM at SE slope (Table  3) might be due to higher canopy cover and litter accumulation. The N content (0.09-0.13 %) in soil of the study area was similar to N content reported by Marasini (2003) in Churia forest of Rupandehi (0.05-0.15 %). The present value of soil N was less than the value reported by Chhetri (1997) in Chitwan National Park (0.18-19 %). According to soil fertility rating chart of Soil Management Directorate of the Department of Agriculture, Hariharbhawan, Lalitpur (Annex 1), the soil of present study area had low to medium fertility. This might be a common feature of Shorea robusta dominant areas (Bhatnagar 1965). The low N content in soil at present study site might have been due to rapid utilization of N by the plant and the continuous losses through leaching, run off, and volatilization process. Nitrogen in soil is quickly exhausted as soil nitrates are highly mobile in water (Larcher 1995).
The two slopes had nearly equal available phosphorus in soil ( The forest soil in the study area contains adequate quantities of all the nutrients, except phosphorous. According to soil fertility rating of Soil Management Directorate of the Department of Agriculture (Hariharbhawan, Lalitpur), organic matter and nitrogen had a low to medium level and potassium a medium level, while phosphorous had a low rating. Overall, soil of SE slope was better in terms of the measured soil characteristics than of SW slope, probably because of high canopy cover and litter production.