Page 180 - Soil Degradation, Conservation and Remediation

6.2 Sources of Soil Pollutants 169

be tied up and therefore unavailable. The literature indicates that there is much
variation in the effect that cations and nutrients can have on herbicide activity and
breakdown, depending on soil composition, nutrient type and concentration, and
chemistry of the herbicide.
Soil microorganisms are partially responsible for the breakdown of many
herbicides. The types of microorganisms and their relative amounts determine
how quickly decomposition occurs. Soil microbes require certain environmental
conditions for optimal growth and utilization of any pesticide. Factors that affect
microbial activity are temperature, pH, oxygen, and mineral nutrient supply.
Usually, a warm, well-aerated, fertile soil with a medium soil pH is most favorable
for pesticide degradation. The persistence of some pesticides in soil is shown below:

Persistence Common herbicides
1 month 2,4-D, glufosinate, glyphosate, MPCA
1–3 months Acetochlor, alachlor, bentazon, butylate, DCPA, dimethenamid, EPTC,

flumetsulam, foramsulfuron, halosulfuron, lactofen, linuron, mesotrione,
metolachlor, metribuzin, naptalam, siduron

3–12 months Atrazine, benefin, bensulide, bromoxynil, clomazone, diuron, ethalfl uralin,
homesafen, hexazinone, imazaquin, imazethapyr, isoxafl utole, oryzalin,
pendimethalin, primisulfuron, prodiamine, pronamide, prosulfuron,
simazine, sulfentrazone, terbacil, topramezone, trifl uralin
>12 months Bromacil, chlorsulfuron, imazapyr, picloram, prometon, sulfometuron,
tebuthiuron

6.2.6.5 Processes of Accumulation of Pesticide Residues in Soil

Pesticides applied on crops and pests undergo several transformations, including
volatilization to the atmosphere, microbial assimilation, biochemical degradation,
photochemical degradation, diffusion, erosion and runoff, absorption by plants,
leaching to the groundwater, and accumulation in soil (Fig. 6.14 ). These processes
are responsible for the movement of pesticides and their residues within the
environmental components.
The principal process of pesticide accumulation in soil is adsorption, which may
be chemical in nature (as with electrostatic interactions) or purely physical (as with
van der Waals forces). Adsorption takes place between charged pesticide molecules
(sorbate) and soil particles (adsorbent), including clay minerals, sesquioxides, and
humus. Positively charged pesticide molecules can bind to negatively charged
particles of clay and organic matter. The extent of adsorption depends on the proper-
ties of soil and the compound, which include size, shape, confi guration, molecular
structure, chemical functions, solubility, polarity, polarizability and charge distribution
of interacting species, and the acid–base nature of the pesticide molecule (Senesi
1992 ; Pignatello and Xing 1996 ). Soil pH, or the acid/base balance of the soil solution,
affects the chemical’s reactivity and certain soil functions such as microbial metabolism.
Weber et al. ( 1969 ) showed that maximum adsorption of basic compounds occurs