«Tea yield and soil properties as affected by slope position and aspect in Lahijan area, Iran F. Khormalia,*, Sh. Ayoubia, F. Kananro Foomania, A. ...»
Effect of slope positions on the solum thickness, physico-chemical properties of the surface horizons and tea yield Analyses of the variance of the solum thickness and soil physico-chemical properties of the different slope positions are shown in Table 3. The results revealed that slope position had significant effects on properties such as solum thickness, thickness of the epipedons, saturated soil moisture, clay content, total N, calcium carbonate content, and exchangeable Mg at 5% level of confidence. Differences in CEC, pH, available K, and basic cations were nonsignificant mainly due to high leaching conditions. Comparison of the means of different physico-chemical properties and tea yield by Duncan test on summit, shoulder, backslope and toeslope positions are presented in Table 4. There was not any significant difference between the tea yield and slope positions. The thickness of the epipedon increased downward on different slope positions. The investigations made by Ovalles and Collins (1986) confirmed that there was a decrease in the thickness of the surface horizons from summit to backslope with 10% slope gradient. Moreover, Young and Hammer (2000) relate the differences of the soil properties on backslope compared to summit and shoulder to the thinner surface horizon of the backslope. In the present study the solum thickness is highest on toeslope position. Khaier and Khademi (2001) found the thickest surface horizon on the toeslope and footslope positions.
104 F. Khormali et al. / International Journal of Plant Production (2007) 1: 98-111 F. Khormali et al. / International Journal of Plant Production (2007) 1: 98-111 105 106 F. Khormali et al. / International Journal of Plant Production (2007) 1: 98-111 F. Khormali et al. / International Journal of Plant Production (2007) 1: 98-111 107 Table 3. Analyses of variance of some soil properties on different slope positions.
The saturated moisture content of the soils on the toeslope position was lowest comparing the other slope positions. This is mainly attributed to the lower clay and organic matter. The highest saturated moisture was found on the backslope position. The investigations done on the variation of the organic carbon on the different slope positions showed that the toeslope positions are mostly high in organic carbon even compared to the summit position. Wilding et al. (1982) explains the higher organic carbon of the lower slope positions by the finer soil texture and therefore higher moisture reserved in the soil which promotes vegetative growth and reduces the oxidation of organic matter. Another reason for the higher organic carbon content of the lower slope positions is the depositions from upper positions. Malo et al. (1974) also reported an increase in the organic carbon level and the solum thickness from shoulder to summit positions. Brubaker et al. (1993) showed the decrease in the clay, organic carbon, CEC and available K in the lower slope positions. Cheng (1987) however, reports that erosion of the material from upper positions and their subsequent deposition in the lower positions is responsible for the higher organic matter in the lower positions. The high amorphous iron and clay remains in the surface layers of the upper positions. In the present study however, the organic matter in the toeslope position was lower mainly due to the poor drainage conditions, lower vegetative growth and deposition from upper sections. The results of the tea yield in the toeslope position supports the above hypothesis. Jones et al. (1989) related the higher organic matter and yield of the upper slope position to the more dense rooting systems and plant residue in this position. The lower organic carbon of the shoulder position is consistent with the findings of Khaier and Khademi (2001), Khormali et al. (2006), Murali et al. (1978), and Pierson and Mulla (1990).
Total nitrogen is lowest in the toeslope position. This is explained in a same way as for the organic carbon. The near surface water table and the hydromorphic properties reduced the yield and consequently the organic matter which is responsible for the reduction in total N. The highest calcium carbonate was detected in the soils of shoulder position and is significantly different from toeslope position. The higher surface erosion and the subsequent outcropping of the underneath calcium carbonate rich layer is mainly responsible for the higher quantity of lime in the shoulder position. Eghbal and Southard (1993) and Matzek (1955) reported the similar findings. Matzek (1955) believes by increasing the slope gradient, and the higher runoff, downward leaching of the carbonate reduces.
108 F. Khormali et al. / International Journal of Plant Production (2007) 1: 98-111 F. Khormali et al. / International Journal of Plant Production (2007) 1: 98-111 109
Slope aspect and the soil properties and yield variations
The slope aspect indirectly affects the surface runoff and erosion. Slopes of the same gradient but with different aspects are not under the same risk of soil erosion. The main effect of the slope aspect on the surface runoff and erosion is through differences in the microclimate. The solar radiation received by a sloping landform is highly related to the aspect. The role of slope aspect is highly visible in the dryer regions than the humid areas (Zaiden et al., 1982). Daniel et al., (1987), reported that topography causes rapid evapotranspiration on southern aspects by changing the microclimate of the area, and in the meantime increases the rate of soil forming processes in the north facing slopes which results in a thicker solum with higher organic matter and denser vegetation.
As seen in Table 5, slope aspect had no significant effect on the soil properties mainly due to the high rainfall which compensates the radiation differences among aspects. In other words, the humid climate of the area lowers the importance of the radiation difference received on different aspects.
Land suitability evaluation
A summary of physical land suitability evaluation in different land forms in three aspects done by three comparison methods are given in Table 6. According to Sys et al (1991) guideline for physical land evaluation, the area was marginally suitable (S3) and non-suitable (N1 or N2) for tea production. The physical suitability classes determined by simple and number and intensity limitations methods had identical limitation degrees, whereas parametric methods (including storie and square root) showed lower classes for tea production in all landscape positions due to the interaction among ratings of land characteristics. This conclusion is in accordance with the findings of Ayoubi (1996) and Manrique and Vehera (1984). The results of square root method (e.g Khidir procedure) indicated that using root square equation instead of simple multiplication provided realistic results. Mandal et al. (2002) reported that land index calculated by khidir method (square root) was highly correlated with actual cotton yield in Nagpur district in India. The most limiting physical factors are climatic factors especially mean of minimum temperature during the coldest month of the year. Other limiting factors of the lower importance are the soil and land properties which control the suitability class of land forms for tea production.
Shoulder position showed the lowest suitability due to higher slope gradient and lower soil organic carbon and depth. The similar results were also found for backslope position.
The results of this study showed that thickness of the solum and surface horizons and properties such as organic carbon content, total N, clay, carbonate and exchangeable Mg varied significantly on different slope positions. The tea yield however, was not significantly different on different slope positions. Organic carbon and total N were significantly low in the toeslope position. The field observations confirmed the presence of shallow water table and poor drainage conditions which are responsible for the reduction of tea yield. The slope aspect did not significantly affect the soil properties and tea production 110 F. Khormali et al. / International Journal of Plant Production (2007) 1: 98-111 mainly due to the almost uniform and high precipitation in the area. The most physical limitation factors are climatic factors especially mean of minimum temperature during the coldest month of year. Investigations on the soil-landform relationship revealed that there is a strong link between the soil properties and the slope positions. More detailed studies would be helpful for the management of the sloping geomorphic surfaces.
Table 6. Physical Land suitability subclasses of tea production in different slope positions using three comparison methods.
Acknowledgements The authors express their gratitude to the Lahijan Tea Research Center for their support.
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