Adornis D. Nciizah

Particulate organic matter, soil texture and mineralogy relations in some Eastern Cape ecotopes in South Africa

Contrasting reports have been given on the relationships between soil texture, mineralogy and particulate organic matter (POM). The objective of this study was to determine the effects of soil texture and mineralogy on aggregate POM in several ecotopes in the Eastern Cape province of South Africa. Soil from the surface 0–20 cm was collected from 14 ecotopes. Particulate organic matter was fractionated into litter POM, coarse POM and fine POM. Isolation of the POM fraction was done by dispersing the soil in 5 g l-1 sodium hexametaphosphate and passing the dispersed soil samples through a 0.053 mm sieve. Both total soil organic matter (SOM) and the POM in each fraction were determined using the weight loss on ignition procedure. Most ecotopes were either sandy loam or sandy clay loam and primary minerals especially quartz dominated the soil mineralogy. The clay content was significantly related to the total SOM (r = 0.78), hematite (r = 0.83) and quartz (r = -0.74). The results suggest that in soils that are dominated by primary minerals such as quartz, the clay content, rather than the clay plus silt content, is a better indicator of fine POM and total SOM.

Physical indicators of soil erosion, aggregate stability and erodibility

The sustained interest in soil erosion research is an indication of both its importance and the lack of definite solutions that can halt its negative impacts on the environment. This study reviewed the literature on trends, new perspectives, gaps and conflicts in soil erosion studies in the South African context. The suitability of using the relationship between aggregate stability and interrill erodibility as a predictor of the soil susceptibility to erosion was also investigated. This relationship is often used instead of the expensive and time-consuming in situ soil erosion studies and models. There are contradictory reports on its ability to offer quick results on the susceptibility of soil to erosion. However, the reviewed South African and international literature showed that aggregate stability is a widely used physical indicator of soil interrill erodibility. Nevertheless, there is no general agreement on the most suitable aggregate stability indices to use.

Soil sealing and crusting effects on infiltration rate: a critical review of shortfalls in prediction models and solutions

Soil sealing and crusting is a common feature in most arid and semi-arid soils with severe agricultural and environmental implications. One of the main effects of soil sealing and crusting is a marked reduction in hydraulic conductivity and infiltration rate (IR), which triggers run-off and erosion. However, measurement of crust IR in the field is cumbersome, time-consuming and expensive. The extremely dynamic nature of processes involved in crusting and crust characteristics makes it very difficult to measure and predict infiltration. Resultantly, many researches aimed at identifying alternative indices of soil sealing and crusting or at improving the methods used to measure IR on crusted soils directly or indirectly have been conducted. Of particular importance is the use of models developed for estimating IR and hydraulic conductivity, but most of them do not account for the presence of a crust layer and therefore fall short on crusted soils. These methods and in particular models and their suitability in estimating IRs on crusted soils are discussed in this article. Trends and perspectives, gaps and conflicts relating to soil sealing and crusting effects on infiltration are also described.

Integrating Biofertilizers with Conservation Agriculture Can Enhance Its Capacity to Mitigate Climate Change: Examples from Southern Africa

Climate change is now a palpable threat to sustainable agriculture in Africa. Whereas some advances have been made in the agricultural sector on climate change adaptation, the same cannot be said of mitigation. Climate-smart approaches have been experimented on and recommended, but stunning results are yet to be achieved. The objectives of this chapter were to examine the contribution of conservation agriculture (CA) to soil organic carbon (SOC) stock and how SOC management could enhance climate change mitigation under southern Africa conditions. Whereas application of CA increased SOC stock by up to 104% under experimental conditions, it was not sustainable in the field because the organic matter input was limited and unreliable. Increased use of inorganic fertilizers, especially nitrogen, has been recommended to boost biomass production. Unfortunately, inorganic fertilizer manufacture and application e.g. urea is associated with increased greenhouse gas (GHG) emission. Instead, this study proposes increased use of biofertilizers namely biological nitrogen fixation (BNF) bacteria and vesicular–arbuscular mycorrhizal (VAM) fungi. This review concluded that biofertilizers would sustainably boost SOC input without the danger of GHG emission and should be a compulsory aspect of CA.

Physical crust formation and steady-state infiltration rate in soils dominated by primary minerals in some South African ecotopes

Physical crust formation is affected by the inherent soil properties such as texture and mineralogy. However, inconsistencies exist in soil mineralogy effects of soil crusting. In addition, investigations of soils dominated by primary minerals such as quartz, especially in South Africa, are scant despite the abundance of such soils. Effects of texture and aggregate size on crust formation and steady-state infiltration rate (SSIR) in soils dominated by primary minerals were investigated. Soil aggregate sizes of <2, 2–3 and 3–5 mm were exposed to ∼60 mm h−1 simulated rainfall. Aggregate size significantly affected crust strength (P < 0.05). Physical crusts ∼0.2 to ∼0.8 mm thick with a surface layer of loose grains formed over a thin plasmic layer. Crusts with strengths between 0.25 and 3.42 × 10−4 kg m−2 developed on the <2 mm aggregates compared with <2.23 × 10−4 kg m−2 in the >2 mm aggregates. The reverse occurred in the Alice Jozini ecotope, which had a relatively low clay content of 120 g kg−1. The SSIR was between 1.24 and 3.60 mm h−1 in ecotopes dominated by primary minerals and clay content between 12% and 26%. In the Amatola Jozini ecotope, which was dominated by kaolinite and higher clay content, SSIR was 15.23 mm h−1. Consequently, physical crust formation decreased but SSIR increased with increase in aggregate size if there was a concomitant increase in the clay fraction, decrease in primary minerals and presence of kaolinite.

Aggregate breakdown mechanisms as affected by soil texture and organic matter in soils dominated by primary minerals

Soil aggregates breakdown mechanisms depend on soil properties such as texture, clay mineralogy and organic matter content. Little is known about the effect of soil properties on aggregate breakdown mechanisms in South African soils. The objective of this study was to establish the relationship between aggregate breakdown mechanisms and selected soil properties in some South African soils dominated by primary minerals. Soil samples with varying properties were collected from the surface 0–0.2 m from 14 ecotopes in Eastern Cape province. Aggregate stability was determined following the fast wetting (FW), slow wetting (SW) and wet stirring (WSt) methods. Soils with high quartz were the least stable due to its inability to bond with other clay minerals or soil organic matter (SOM). In contrast, soils with high kaolinite were the most stable. Slaking was the dominant aggregate breakdown mechanism in most of the studied soils. Aggregate stability was significantly correlated with particulate organic matter (POM) for FW (r2 = 0.74, P < 0.05) and SW (r2 = 0.64, P < 0.05). It was concluded that increasing POM content in soils through SOM inputs and farming methods that reduce POM mineralisation can reduce slaking during rapid wetting by raindrops.