An Assessment of Soil Variability along a Toposequence in the Tropical Moist Semi-Deciduous Forest of Ghana

Understanding the variability of physico-chemical properties of soil along a toposequence is essential for smallholder farming communities. However, these resource constraint farmers in Ghana’s Moist Semi-Deciduous Forest (MSDF) zone poorly understand how slope positions affect soil properties. Therefore, soil variability assessment along a toposequence was carried out on Bekwai-Nzima/Oda compound association. From the summit to valley bottom slope positions, soil samples were taken at two depths (0 - 20 and 20 - 50 cm). As shown by the coefficient of variation, topsoil (0 - 20 cm) had the highest variation compared to the subsoil (20 - 50 cm). The variations observed in most soil attributes (clay, silt, pH, CEC, SOC and TN) for the 0 to 20 and 20 to 50 cm depths were between eroded (summit and upper slopes) and depositional (lower slope and valley bottom) zones. The highly variable soil attributes were silt, TN, Av. P, and Av. K. However, bulk density and sand were the least variable irrespective of soil depth or toposequence. Pearson correlation analysis indicated a significant correlation (p < 0.05) between most soil attributes at the 0 - 20 and 20 - 50 cm depths at different slope positions. Principal component (PC) analysis indicated that the first four PCs explained more than 80% and 70% of the total variation for the 0 - 20 and 20 - 50 cm soil depths, respectively. Statistically, our results revealed a significant effect of slope position on soil properties (p < 0.05) and topography influenced soil characteristics and development. Soil pH, sand, silt and clay contents were less affected by slope gradient, which confirms the inherent nature of these highly weathered tropical soils. The findings of this study can serve as a reference for the formulation of soil management strategies for smallholder farm communities.

Variations in Soil Physico-Chemical Properties as Influenced by Landuse in a Toposequence

Soil, water and air work interchangeably for optimum growth and productivity. Soil as a resource provides crops with essential nutrients necessary for growth and human consumption. With the insurgence of high population growth leading to pressure on lands, poor land management etc., this tendency of the soil as a nutrient pool is seen to be dwindling. It is therefore prudent to investigate and understand the dynamism of soil properties to inform proper management. Understanding topography effects and assessing soil properties on different slope positions is a first-hand step in ensuring proper soil management practices. This paper considers the assessment of physico-chemical properties of soils located on different slope gradients and land use types on the Wamaso research site in Ghana made principally of cape coast granite. In all, eighteen samples were collected from identified horizons of five distinctive subsections of the slope were delineated as follows, summit (PP1), shoulder (PP2), middle (PP3), foot slope (PP4) and toe slope (PP5) respectively. Remarkable variations were observed within PP1 to PP3 and then PP5. Pearson’s correlation index revealed that the associations between most of the soil physico-chemical were not consistent across the identified slope positions. The biplot of the first two principal components together is 68.3% of the total variation and might be interpreted as change in Cation exchange capacity, sodium, sand and clay. On the first axis highlights the influence of sand and clay on soil fertility, also the relationship between pH and Na. Organic carbon derived from organic matter contributes to the availability of the basic soil cations and therefore enhances ECEC of the soils and hence the relationship obtained from the biplot. In sum, this research achieved the objective which sought to substantiate indeed the impact topography has on soil physico-chemical properties thereby serving as a reference for the classification of the soils identified.

Mineralogical Horizon(A)Evaluation of a Toposequence of Soils Derived from Basalt by Thermal Analysis

Thermogravimetry (TG) is a technique where the mass change of a substance is evaluated in function of temperature or time at a constant temperature while submitted to a controlled program in a specific environment. The minerals obtained from a clay fraction are highly susceptible to significant changes in composition as a response to thermal processes in function of temperature or dynamic as a function of time at a constant temperature. Thermogravimetric curves (TG) show mass changes preferentially during the heating processes. The DTG curve derived from the TG curve, shows changes in the inclination that are not exposed in the TG curve. DTG curve thus can show higher values of mass loss which are associated to an elevated degree of weathering for those reactions which involve mass changes, such as desorption, dehydration and reactions of dehydroxilation, observed in clay fraction without iron oxide-hydroxide, indicating kaolinite as a dominant mineral, with higher value 70.97% of the fraction to soil class LVdf.

Assessment of Dithionite and Oxalate Extractable Iron and Aluminium Oxides on a Landscape on Basement Complex Soil in South-Western Nigeria

The study investigats the morphological, physical and chemical characteristic of a toposequence on basement complex in Ejioku area, south-western Nigeria on latitude 07°28.675'N;and longitude 004°07.219'E;180 m above sea level at the upper slope. It terminates at the valley bottom on latitude 07°28.756'N;and longitude 004°07.229'E;175 m above sea level. The topography of the site is moderately slope (not greater than 10%). The aim is to examine the two forms of sesquioxides (crystalline-dithionite extractible Fe & Al and amorphous-oxalate Fe & Al) and their distribution on the toposequence. The crystalline form of Fe oxide dominates all the positions and ranges from 7.2 g/kg at the valley bottom to 444.3 g/kg at the crest compared to the amorphous forms that range from 2.7 g/kg to 10.9 g/kg. The crystalline and amorphous aluminium oxide contents of the soils are low (2.9 g/kg - 43.3 g/kg and 1.3 g/kg - 8.7 g/kg respectively). There is significant negative relationship between Fed and Fed/Feo (r =-0.15;P n = 16). There is also a correlation between Feo and Ald (r = 0.63;P n = 16), FeO and FeO/Fed (r = 0.44;P n = 16), signifying a high level of weathering. The relatively high amount of Fed virtually at all the horizons indicates that the crystalline and less active forms of the oxides exist more on the landscape and may be responsible for the non-availability of some nutrient like phosphorus that may be sorbed to their crystals lattice. All forms of Al in the soils are low especially, the oxalate extractable forms, when compare with the dithionite extractable forms. However, there is a significant correlation between Ald and Feo/Fed (r = 0.57;P < 0.01). As a result of high accumulation of crystalline form of Fe and Al oxide with increasing depth, most especially, at the crest/upper slope, there is greater amount of concretions, nodules and plinthites which will lead to further deterioration of the soil for agricultural purposes.

Apatite Loss in Pothwar Loess Plain(Pakistan) Fits a Simple Linear Reservoir Model

Apatite is the dominant phosphorus(P) mineral in early stages of soil development, and its redistribution as labile forms under pedogenesis controls terrestrial bioavailability. Quantitative distribution of labile forms of P and apatite-P was examined in Pothwar Loess Plain, Pakistan where the degree of pedogenesis varied with relief. Four soil types, Typic Ustorthents(Rajar), Typic Calciustepts(Missa), Udic Calciustepts(Basal), and Udic Haplustalfs(Guliana), were sampled from three replicated locations at genetic horizon level. With the exception of total P value at surface, the mean total and apatite-P decreased towards the surface in Udic Haplustalfs and Udic Calciustepts where dicalcium and octacalcium phosphate increased toward surface. Iron(oxides and oxyhydroxides) adsorbed and occluded P forms were also in greater quantities in Udic Haplustalfs and increased toward the surface, whereas they were lowest and uniform in the Typic Ustorthents. Aluminum- and organic P correlated with soil organic matter. Apatite-P decreased exponentially with an increase in the sum of iron adsorbed and occluded P fractions, and fitted the equation M(x)= M0 [1- exp(-wx)] with r2= 0.996, where M(x)is the mean apatite-P concentration in solum, M0 is the apatite-P content in the loess sediments, x is the cumulative iron adsorbed and occluded P, and w is an empirical factor indicating the change rate of apatite-P in the loess. From the calculated apatite-P of 740 mg kg-1at the time of deposition, mean apatite-P loss was 60% in Udic Haplustalfs, 33% in Udic Calciustepts, 23% in Typic Calciustepts, and 8% in Typic Ustorthents. The transformation of soil P to labile forms was faster and deeper in level or slight depressions followed by gently sloping areas in wide plains, and was the least in the gullied land.