168
Approximately 19% of the rainfall received in Malawi is lost through surface runoff
(GoM 2008). It is therefore possible that a significant proportion of pesticides currently used in agriculture in Malawi are lost through this pathway. In the event of
increased precipitation and floods, the concentration of pesticides such as acetamiprid and metolachlor would therefore need to be high if these episodes occur
immediately after their application. About 33% of Malawians do not have access to
potable water (WHO and UNICEF 2015). They depend on surface- and groundwater for drinking and household chores (Chidya et al. 2016; Chimphamba and Phiri
2014) and are consequently at greater risk of pesticide exposure.
3.5 Pesticide Sorption
Soil management practices influence sorption – the distribution or partitioning of a
pesticide in an environment. Sorption not only reduces the risk of pesticide leaching
but can also reduce the pesticide degradation rate, as the pesticides are not available
to microorganisms. Dinisio and Rath (2016) reported high sorption of abamectin
occurring in soils rich in organic matter. In another study, metolachlor and atrazine
sorption increased in soils amended with biochar (Deng et al. 2017; Trigo et al.
2016). Biochar has some of the same effects as sugarcane burning after harvest and
increases sorption. Adsorption of atrazine and endosulfan was better in soils covered with rice husks (Rojas et al. 2014). Leaching of MCPA significantly reduced in
Mediterranean agricultural soils amended with olive oil mill wastes (Peña et al.
2015). These results show that efforts to improve soil fertility have a significant
influence on the exposure of pesticides to the environment through the enhancement
of pesticide degradation and sorption.
Crop management is also an important factor in pesticide sorption. In Malawi, as
in many of the sugarcane producing countries, the burning of sugarcane fields prior
to harvesting is practiced. Whilst much ash from burnt sugarcane residue is blown
away from the fields by the wind, some ash remains, significantly influencing pesticide adsorption (the adhesion of pesticide molecules to surfaces of particles).
Effective pesticide adsorption by burned sugarcane residues would lead to reduced
availability of pesticides that typically target soil dwelling pests or those pesticides
that have to be absorbed by plants through the roots (Yang and Sheng 2003). Farmers
practicing residue burning may therefore report reduced efficacy of pesticides such
as chlorpyrifos, dimethoate and clomazone (Xu et al. 2008; Kamm and Montgomery
1990). In addition, the practice negatively affects the population of microbes and
total organic matter (Souza et al. 2012). Microbes are essential components for the
degradation of pesticides. Thus, burning reduces pesticide risk through increased
pesticide sorption. At the same time, it may also increase pesticide exposure risk
due to an increased demand for inputs (fertilizers and herbicides).
Increases in rainfall coupled with intensive farming using nitrogen fertilizers and
the burning of crop residues can result in the acidification of soils. The pH of a soil
and the ionic state of the pesticide influence pesticide fate. For example, at pH 4,
T. K. Donga et al.
Approximately 19% of the rainfall received in Malawi is lost through surface runoff
(GoM 2008). It is therefore possible that a significant proportion of pesticides currently used in agriculture in Malawi are lost through this pathway. In the event of
increased precipitation and floods, the concentration of pesticides such as acetamiprid and metolachlor would therefore need to be high if these episodes occur
immediately after their application. About 33% of Malawians do not have access to
potable water (WHO and UNICEF 2015). They depend on surface- and groundwater for drinking and household chores (Chidya et al. 2016; Chimphamba and Phiri
2014) and are consequently at greater risk of pesticide exposure.
3.5 Pesticide Sorption
Soil management practices influence sorption – the distribution or partitioning of a
pesticide in an environment. Sorption not only reduces the risk of pesticide leaching
but can also reduce the pesticide degradation rate, as the pesticides are not available
to microorganisms. Dinisio and Rath (2016) reported high sorption of abamectin
occurring in soils rich in organic matter. In another study, metolachlor and atrazine
sorption increased in soils amended with biochar (Deng et al. 2017; Trigo et al.
2016). Biochar has some of the same effects as sugarcane burning after harvest and
increases sorption. Adsorption of atrazine and endosulfan was better in soils covered with rice husks (Rojas et al. 2014). Leaching of MCPA significantly reduced in
Mediterranean agricultural soils amended with olive oil mill wastes (Peña et al.
2015). These results show that efforts to improve soil fertility have a significant
influence on the exposure of pesticides to the environment through the enhancement
of pesticide degradation and sorption.
Crop management is also an important factor in pesticide sorption. In Malawi, as
in many of the sugarcane producing countries, the burning of sugarcane fields prior
to harvesting is practiced. Whilst much ash from burnt sugarcane residue is blown
away from the fields by the wind, some ash remains, significantly influencing pesticide adsorption (the adhesion of pesticide molecules to surfaces of particles).
Effective pesticide adsorption by burned sugarcane residues would lead to reduced
availability of pesticides that typically target soil dwelling pests or those pesticides
that have to be absorbed by plants through the roots (Yang and Sheng 2003). Farmers
practicing residue burning may therefore report reduced efficacy of pesticides such
as chlorpyrifos, dimethoate and clomazone (Xu et al. 2008; Kamm and Montgomery
1990). In addition, the practice negatively affects the population of microbes and
total organic matter (Souza et al. 2012). Microbes are essential components for the
degradation of pesticides. Thus, burning reduces pesticide risk through increased
pesticide sorption. At the same time, it may also increase pesticide exposure risk
due to an increased demand for inputs (fertilizers and herbicides).
Increases in rainfall coupled with intensive farming using nitrogen fertilizers and
the burning of crop residues can result in the acidification of soils. The pH of a soil
and the ionic state of the pesticide influence pesticide fate. For example, at pH 4,
T. K. Donga et al.