Site and land use effects on some physical properties and the distribution of total carbon, aluminium, and iron within aggregates of some humic soils in KwaZulu-Natal, South Africa.
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Date
2023
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Abstract
Humic soils occur mainly on old land surfaces, with a warm, misty climate having native forests
and grasslands as the predominant natural vegetation, in KwaZulu-Natal (KZN) and along the
coast of Pondoland and the eastern escarpment of Mpumalanga. The effects of site characteristics
and response to replacement of native vegetation with sugarcane cultivation on these soils,
remain unclear. This study investigated the effects of different site conditions, including native
forest, grassland, and sugarcane production, on (i) general soil characteristics such as pH, clay
mineralogy, and total Al, Fe, and C in bulk soils, (ii) bulk density (BD), total porosity (TP),
saturated hydraulic conductivity (Ks), and moisture content at field capacity (FC) and permanent
wilting point (PWP), (iii) AS and size distribution, and (iv) the distribution of total Al, Fe, and
C within different aggregate size fractions in humic soils.
The humic soils studied were developed on dolerite and were under grassland at Cedara and
Karkloof and on sandstone and under native forest and sugarcane at Eshowe in KZN. Disturbed
samples were collected from soils under each land use at 5 cm intervals from 0-20 cm, 10 cm
intervals from 20-60 cm and 20 cm intervals from 60-100 cm depth to compare (a) forest and
grassland sites and (b) forest and sugarcane sites. These samples were air-dried and a portion
was used for AS measurements while the rest was ground and sieved. The samples for AS were
fractionated into large macro-aggregates (LM; > 2000 μm), small macro-aggregates (SM; 250-
2000 μm), micro-aggregates (M; 63-250 μm) and silt + clay (SC; < 63 μm).
Results showed that Karkloof soils had lower pH but a higher TOC content than those at the
other sites. Cedara and Karkloof soils had higher Al, Fe, clay, and silt content than soils at
Eshowe that had a higher sand fraction. After 30 years of sugarcane cultivation, there was an
increase in BD and Ks on the sandy clay loam humic soil at Eshowe. However, TOC could not
solely explain the water retention characteristics, suggesting that other factors such as texture, especially silt, played a significant role. Soils at Karkloof had higher TOC and MWD, resulting
in lower BD and higher TP, thereby increasing Ks, FC, and PWP compared to soils at Cedara
and Eshowe. Moreover, TOC only explained Ks and water retention characteristics in the top 30
cm depth, suggesting that texture played a more significant role in these properties below 30 cm.
Soils at Karkloof and Cedara had higher MWD than those at Eshowe. Exchangeable acidity (and
thus Al) and silt played a critical role in aggregation at all depths, while TOC was mainly
responsible for the formation of LM and SM in the upper 30 cm. Below 30 cm, clay, total Fe,
and Al predominated in all aggregates. The aggregate size distribution showed that LM and SM
dominated at all sites at the expense of the SC fraction. The M size fraction in soils at Cedara
had higher TOC content, while both LM and SM had a higher concentration in soils at Karkloof
and Eshowe. Total Al and Fe concentrations were higher in the LM and SM than in the M
fractions at Karkloof and Cedara, with the SC fraction at Eshowe having higher concentrations
of these elements. Sugarcane cultivation decreased AS and TOC in both LM and SM, while Al
and Fe increased in all aggregate size fractions. It was also found that total Al and Fe did not
explain the protection of TOC in the aggregates of the studied soils.
The results of the study imply that total Al and Fe contents increase when these soils are
cultivated. Therefore, to ensure sustainable production, agricultural management practices such
as lime application are needed to reduce soil acidity, and improve the availability of
exchangeable bases. Despite these fertility challenges, the high TOC in these soils promotes the
formation of stable aggregates, thereby positively affecting BD, TP, Ks, FC, and PWP, especially
in the topsoil. To improve or maintain their TOC status, it is strongly recommended that practices
such as adding organic matter, reducing tillage, using cover crops, crop rotation, maintaining soil
moisture, and applying lime be used. This is because the stability of aggregates against
disaggregation is of paramount importance in preventing soil degradation.
Description
Doctoral Degree. University of KwaZulu-Natal, Pietermaritzburg.