Soil sampling and legacy soil data

4.2.2.1. Legacy soil data and information

This study is focused on obtaining representative soil data of the natural landscape to reconstruct the original soil pattern prior to urban development and growth of Kampala. Observations revealed that the current undisturbed sites are predominantly under native vegetation, which provides us with confidence that the soils have not been significantly altered more than for agricultural production. This is in contrast to developed and sealed urban scapes

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under which soils have been manipulated, disturbed or transported by human activities. These disturbances invariably have altered the natural soil profiles in terms of their physical, chemical and biological soil properties. Therefore only soil class data at undisturbed sites, sourced from legacy soil survey report and a fresh field survey, were used for this study.

The legacy soil profile data were obtained from the 1956 Uganda national soil survey report (Radwanski, 1960). The report provides data on soil profile development, soil texture, soil depth, colour and drainage conditions, soil pH/ base saturation, development of the A-horizon. These data were relevant to our study because until 1970s Kampala was largely under native vegetation. In 1956, Kampala as an urban centre covered only about 12.95 km2 _{(UBOS, 2002).} Unlike the cities in Europe, most urban centres in SSA were the creation of the colonial powers. As such Kampala started as a small administrative centre gazetted in 1902 on about 0.7 km2 area (Handbook, 1904). This implies that the Uganda soil survey of 1956 was conducted at the time when the city was very small and a large proportion of the present GKMA was largely under native vegetation.

Other soil data were sourced from the international soil reference and information centre (ISRIC) under the Africa soil profile database (http://www.isric.org/content/data) and (http://africasoils.net/). These data were obtained from pre-existing local survey reports and local soil studies in the study area.

Additional information was obtained from soil local studies. For instance, the clay mineralogical, geochemical and sedimentological assessment by Nyakairu et al. (2002) and clay mineralogy and chemical composition (Nyakairu and Koeberl, 2001) in GKMA provided useful

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soil type data for a number of sites. The comprehensive soil profile classification and clay mineralogy information from the major land resource areas in Uganda by Yost and Eswaran (1990) were some of the key local studies that were useful in identifying soil classification at some sites. The legacy data provided 238 soil profile points. The legacy soil data and profile description were used to classify the soils based on the FAO World Reference Base (WRB) soil classification system (FAO-WRB, 2014). These were added to the soil classification data at the undisturbed sites described above. Based on the soil profile data description and analysis, the dominant soil types in the study area following FAO classification system are Acrisols (ACR), Plinthosols (PLI), Nitisols (NIT), Gleysols (GLY), Fluvisols (FLU), Arenosols (ARE), and Ferralsols (FRA).

4.2.2.2. Additional soil sampling

The legacy soil data obtained initially were inadequate for this study as they were collected at a national scale and thus proving few points within the study area. This required additional soil survey to complement the legacy data. As such 1000 points were randomly sampled using the utility add random points in ERDAS Imagine prior to field visit for profile characterisation. However, due to the urban nature of the study area which has led to anthropogenic alteration of soils, a large proportion of these randomly generated points were located at disturbed sites. During field soil sampling (the field soil data collection was carried out between July and December 2015), whenever a point was found to be at a disturbed location, where it was possible a sample was taken from the nearest undisturbed location, otherwise, the point was left out. Thus the soil sampling design was adjusted to ensure the soil samples were taken from mainly natural landscapes and agricultural lands. The rest of the randomly generated points

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which were located in the changed landscape were not used for this study. Fig. 4.2 shows the locations of the relatively representative sites selected for soil survey and sampling to predict the original soils before urban development.

About 163 points were located at or near undisturbed sites, these points were described, characterized, and classified using field soil observations. From the 163 points classified in the field using soil auger 0 – 120cm or water table depending what was reached first. These points were described in the field following FAO_WRB soil classification guidelines in the field which focuses on field observation of the soil depth, colour using Munsell colour chart, soil firmness, stickiness, texture, stone line depth, root distribution with each horizon, gravel percentage, organic matter percentage, and development of soil horizons within a profile, profile position slope gradient and orientation, vegetation type.

Apart from the 163 field observations described above, additional total of 88 soil profile samples were collected. Two types of samples were taken namely; undisturbed core samples and the bulk samples from each of the horizon starting from top up to the start of parent material. Generally, soil profile descripting and soil sampling was between 0 – 100 cm. Core samples were collected using soil sample core ring measuring 4.5 in diameter and height of 5 cm which bulk samples were collected using a knife and a shovel. The sample were then wrapped, properly labelled including the profile location details like coordinates, area name and sample code and then packed in secured bags. These samples were then taken to the soil laboratory for routine and specialised soil analyses to aid in final soil type classification (Appendix 4.1).

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Figure 4.2 Landsat image acquired on 27th February 2015, showing the locations of undisturbed sites where soil profile samples were taken

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