Agriculture Reference
In-Depth Information
A PPLICATIONS OF THE S UGAR B EET B IOASSAY
Effect of soil properties on herbicide bioactivity. Soil characteristics, particularly soil
pH, organic carbon and clay content have considerable effect on bioavailability of herbicides
[7, 13, 15]. Bioassays are ideal for assessment of the effect of soil properties on herbicide
bioactivity because they detect the herbicide portion in soil that is actually available to plants.
Multiple studies of soils of varying properties have demonstrated that herbicide bioactivity
can be related to either one soil factor or a combination of different soil factors [4, 6, 15, 16].
Effect of soil properties on herbicide bioactivity can also be assessed by artificially altering
one soil characteristic such as soil pH or organic carbon content. This allows for the removal
of the influence of other soil properties and for the measurement of the effect of only the
altered characteristic on herbicide bioactivity as shown in the examples below.
Natural pH 6.9 of a soil was altered through acidification with aqueous HCl to 6.0 and
through alkalization with CaCO 3 suspension in water to 7.5 [17]. After equilibrating the soils
for one week, bioactivity of pyroxsulam in the range from 0 to 3.45 μg/kg was evaluated with
the 4-day sugar beet root length bioassay. As can be seen in Figure 3, sugar beet root length
inhibition was greater when soil pH increased and was reduced when soil pH decreased,
demonstrating that weed control with pyroxsulam varies with soil pH and that it is more
efficacious in soils of higher pH. As pH increases, the organic matter and soil colloids acquire
negative charges which repel herbicide anions that are formed through dissociation of weakly
acidic herbicides, such as pyroxsulam, resulting in reduced herbicide adsorption to soil
colloids and consequently in increased herbicide availability to plants [7, 14, 18].
80
60
40
20
pH 7.5 (alkalized)
pH 6.9 (natural)
pH 6.0 (acidified)
0
0
1
2
3
4
Pyroxsulam ( g/kg)
Figure 3. Effect of soil pH on pyroxsulam bioactivity determined by the 4-day sugar beet root length
bioassay.
Organic carbon content of a soil that had 1.3 % natural organic carbon content was
increased by addition of a wheat straw biochar. Typically biochars have high organic carbon
content, high surface area and porous structure, and may be considered to have characteristics
that resemble soil organic matter. Biochar was added in the range from 0 to 2% and
pyroxasulfone bioactivity was assessed from 0 to 276 μg/kg with the 7-day sugar beet shoot
length bioassay. Sugar beet shoot length inhibition decreased with the increase of biochar
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