TPC is a 16 000 hectare estate located in Moshi, Tanzania and is currently planted under 8 800 hectares of sugarcane and produces over 60 000 tons of sugar per annum. The influence of volcanic parent material and volcanic ash over TPC, together with the alluvial nature of many of the soils, has imparted a unique combination of soil mineralogical, physical and chemical properties. Furthermore, irrigation with poor quality water has led to sodicity problems on the estate. Understanding the mineralogy and sodicity effects on soil hydraulic properties across the estate can lead to better irrigation management where it is important to prevent the build-up of salts due to over-irrigation. In response to this need, a study was carried out with the aim of characterising the mineralogical, physical and chemical properties in the five management areas of the estate (North, East, West, South and Kahe), in order to determine the relationships between various measured parameters. A total of 70 fields across TPC, as well as four sites outside the estate and two ash layers, were chosen for sampling. Undisturbed soil cores and bulk samples were collected from the A and B horizons from 45 of these fields and the four sites outside. Selected fields were sampled at more than one site to assess field variability, and where cane growth was patchy selected fields were sampled in a patch of poorly growing cane and an adjacent patch of better cane growth. Bulk soil samples were collected from the remaining fields and the two ash layers. Double ring infiltration measurements were carried out on 25 of the selected fields. X-ray diffraction, transmission electron microscopy and aluminium, iron and silica extractions were carried out to determine the mineralogy. Physical and chemical measurements included water retentivity, saturated hydraulic conductivity, bulk density, particle size distribution, organic carbon, pH (H2O), electrical conductivity, water soluble and exchangeable cations (Ca, Mg, K and Na), cation exchange capacity and clay specific surface area. The particle size distribution showed that the soils were mainly loams and sandy loams. Organic carbon values were generally greater in the A horizon compared to the B horizon and varied between 0.4 and 2.5 % in the topsoil and 0.3 and 2.1 % (with the exception of field 11 which had an organic carbon of 4.0 %) in the subsoil. X-ray diffraction patterns of sand and silt fractions were dominated by sanidine while clay patterns were weak and had high backgrounds and very broad peaks, suggesting the presence of poorly ordered material in the clay fraction. The Al and Fe extraction methods and electron micrographs indicated that this poorly ordered material was allophane. However, the dominant clay mineral across the estate was halloysite, in both tubular and spheroidal form, as well as very small (<< 0.5 μm) kaolinite particles. There was also gibbsite in some of the samples analysed. The combination of allophane, halloysite, kaolinite and gibbsite indicated that the primary volcanic minerals have weathered to various degrees across the estate. This is reflected in the alluvial nature of the soils where less weathered material has been periodically deposited onto older, more weathered material over some parts of the estate. The south and west areas had a slightly higher Alo + ½ Feo ratio than the other areas in both the topsoil (1.07 and 0.95, respectively) and the subsoil (1.16 and 1.06, respectively), a possible consequence of less weathered alluvial material that was deposited in these areas. Although the concentration of allophane was low (< 5 %), even in the south and west areas, its presence greatly increased the clay specific surface area (up to 145.94 m2 g-1) and consequently had a significant influence on the soil physical and chemical properties. Water retention across TPC was high, particularly at the lower matric potentials (between 0.13 and 0.45, and 0.09 and 0.24 m3 m-3 at -33 kPa and -1500 kPa, respectively). The high water retention is a result of allophane which gives the soils a high adsorption capacity and a porosity that is dominated by micro-pores. Generally, the south area had the highest water retention at the various measured matric potentials which corresponds to the higher allophane content. Variability in water retentivity across areas and within fields limited further interpretation and correlation with the mineralogical results. Infiltration rate was lowest in the south (60.85 mm hr-1) and highest in the Kahe area (171.20 mm hr-1). The main factor influencing the final infiltration rate was the concentration of sodium in the soil, with higher concentrations causing soil dispersion and blockage of soil pores. Clay dispersion has led to the development of calcareous surface crusts and reduced porosity, thus reducing the infiltration rate. Sodium concentration in the soil is likely to have had a dominating effect over the mineralogical composition of the soil. Poor cane growth in the south and west areas corresponded to higher pH (up to 10.32), electrical conductivity (up to 614 mS m-1), sodium absorption ratio (up to 20.63) and water soluble and exchangeable sodium (up to 53.20 mmolc l-1 and 14.87 cmolc kg-1 soil, respectively) in these areas. The soils are thus more dispersive and the combination of sodicity and allophane has resulted in “fluffy” soils with small particles clogging soil pores and thus surface crusts have formed easily. The combined effect of mineralogy and sodicity in the south is further complicated by the presence of perched water tables. High adsorption capacities and the dominance of micro-pores allow the occurrence of significant capillary rise which brings salts to the soil surface, further exacerbating the sodicity problem. Therefore, over-irrigation should be avoided where soils are prone to sodicity from a combination of irrigation with poor quality water, perched water tables and strong capillary rise action. Fields which are currently experiencing the negative effects of high sodicity, require irrigation with good quality water and adequate sub-surface drainage to ensure the leaching of salts. Further studies with specific focus on the south and west areas would be beneficial in accounting for the variability and in drawing correlations between the mineralogy and sodium content of the soils with the other measured properties. Fields which are prone to increased sodicity through over-irrigation with poor quality water, have strong capillary rise from perched water tables and which require remediation through sub-surface drains can thus be distinguished and the factors influencing sugarcane growth can be more clearly understood. Growth depends on the combination of these soil’s unique mineralogy and sodium content and the influence they have on the infiltration rate, adsorption capacity, micro-porosity and capillary rise from the water table. For future work, water movement modelling to predict saturated and unsaturated flow, as well as in situ measures of unsaturated flow, will lead to further understanding of the soil hydraulic properties and aid in improved irrigation management. / Thesis (Ph.D.)-University of KwaZulu-Natal, Pietermaritzburg, 2013.

Bibliography: leaves 105-129.

As is the case in many other regions in the world, sugarcane production in southern Africa is affected by a wide range of climatic conditions, which can vary considerably from location to location and from year to year. As a result, the season length and growth cycles of sugarcane in southern Africa differ greatly. Such conditions include the hot and dry regions of northern KwaZulu-Natal, Swaziland and Mpumalanga, where sugarcane is mostly irrigated, to the humid sub-tropical coastal belt extending from the far north coast of KwaZulu-Natal to areas in the Eastern Cape, as well as the cool frost prone midlands regions of KwaZulu-Natal. Owing to the wide range of climatic conditions in which sugarcane is grown in southern Africa, there are many different external factors that affect sugarcane production, including a range of pests and diseases, frost occurrences and variations in soil water. The objective of this research was to (1) identify a number of important variables that affect cane production in southern Africa, (2) employ suitable models to reflect these variables, and (3) simulate and map the extent and severity of these variables at a high spatial resolution over southern Africa. Such variables include the Eldana saccharina and Chilo sacchariphagus stalk borers, sugarcane rust fungus, heat units with selected base temperatures, frost, soil water content, soil compaction, irrigation water demand, conducive and non-conducive growing conditions, flowering proficiencies for sugarcane, sugarcane yields and yield increments per unit of irrigation. The distribution patterns of the above-mentioned variables relied greatly upon the various models employed to represent them, as well as the accuracy of the temperature and rainfall databases to which the various models were applied. Although not definitive, the models used to reflect the variables which had been identified were considered to be generally satisfactory. The resolution at which the variables which had been identified in this study were mapped, was also found to be adequate. / Thesis (M.Sc.)-University of KwaZulu-Natal, Pietermaritzburg, 2008.