| Summary: | 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.
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