INFLUENCE OF SOIL DEPTH ON ASULAM ADSORPTION AND DEGRADATION A. G. T. BABIKER and H. J. DUNCAN Department of Chemistry. University of Glasgow, Glasgow Cl2 8QQ. Scotland (Accepted 13 Sqrember
Summary--Asulam ~methyl(4-aminobenzenesulphonyl)carbamate) is not adsorbed to any marked extent by the soils examined above the pH range 4.5-6.0. The adsorption which takes place is inversely correlated with pH. Comparatively higher amounts of asulam are retained by topsoil samples than by their respective subsoil samples. Asulam degradation is rapid in topsoils and slow in subsoils. The addition of yeast extract to the latter enhances degradation. The sensitivity of the analytical procedure is improved by extracting the Bratton-Marshall colour into n-butanol.
INTRODUCTION Conflicting reports have appeared in the literature on the use of asulam as a pre-emergence herbicide (Ball et al., 1965; Brock, 1972; Menges et al., 1972). Experience with pre-emergence herbicides has shown that the minimum concentration required for effective weed control differs from soil to soil (Grover, 1966). When the same concentration is used regardless of soil type there is a risk of crop injury in some soils and insuficient herbicide for weed control in others (Harris and Sheets, 1965). Such behaviour is attributed to differences in availability of the herbicide at the zone of uptake of both the weed and the crop in question (Bryce, 1972; Upchurch, 1966; Holly, 1961). Of the factors affecting availability, adsorption and degradation are generally considered to be the most important (Bailey and White, 1970; Holly, 1961; Sheets et LII.. 1968). In the present in~festigation a comparative study of the adsorption and degradation of asulam was made using soil samples collected from two profiles. The effect of sand, yeast extract and other materials on asulam disappearance was included in the study in an attempt to elucidate the factors involved. The results are used to discuss some of the observed behaviour of as&am in the field. MATERIALS AND METHODS Asulam (methyl(4-aminobenzenesulphonyl)carbamate) technical ingredient 99.5:~; pure was purchased from the National Physical Laboratory (U.K.). Asulam was determined by the Bratton-Marshall reaction as described by Brockelshy and Muggleton (1973). The soil samples chosen for this study represent major soil types prevailing in hill farming areas in West-Central Scotland (Darleith Association) (Mitchell and Jarvis. 1956). The samples were taken from different depths down the two selected profiles in order to facilitate the collection of samples of varying composition. Soil descriptions are as follows: (i) Darleith series: This soil is developed on till derived from calcareous sandstone lavas and is an example of a shallow, freely drained, brown forest
soil. Samples were collected at regular intervals (5 cm segments) down the profile to a depth of 25 cm, which is well below the B layer (Soils l-5). (ii) Dunlop series: The soil is developed on a base rich material which is inherently fertile and is an example of a fine textured (well aggregated). imperfectly drained, brown forest soil (Soils 68). Samples were collected from the A,, Bzg and Bsy horizons. Adsorption
The soils were wet sieved through a 2mm sieve and finally air-dried at 30°C in a for~edraught oven. Sand, silt, clay and c.e.c. were estimated by standard soil analytical procedures (Jackson, 1962) and the organic matter was estimated by the WalkleyBlack method (Walkley, 1947). The relevant analytical data is given in Table 1. Adsorption was investigated under controlled pH conditions. The following buffers were used to maintain the slurry pH (range 2.0-5.63: KCl-HCI. Na acetate-acetic acid and Na2HP0,-NaH2P04. The buffers were prepared from 0.2 M stock solutions (Gomeri, 1955). Soil samples, 2 g air-dried soil, were shaken with 10 ml of 5.0 pg’rnl- ’ asulam solution at 18°C for 16 h on an end-over-end shaker (preliminary investigatioIls showed no appreciable change in adsorption after 16 h). The pH of the slurry was determined after shaking. The slurry was then filtered through Whatman No. 42 filter paper. The concentration of asulam in the filtrate was determined as mentioned above. All measurements were carried out at least in triplicate. Appropriate blanks were included and the filtrate was checked by t.l.c. (Fishbe~n, t967). Adsorption was expressed as the distribution coefficient Kd, (Bryce, 1972) where Kd = Amount) Concentration
The soils used in this experiment (Soils 6-8) were sieved as before but were not dried. Asulam was added to the soil in a sealed container and mixed by hand. Incubations were carried out under nonleaching conditions in 100 ml capacity bottles plugged 197
A. G. T. BANKER and H. J. DUNCAN
I. Soil analytical
, ; 4 5
55.1 SO.0 21.9 11.4 .10.6
13.X ‘0.0 ‘X.5 25.2 43.9
6 I x
52.1 41.0 35.4
34.9 73.0 22.2
I6.X 16.6 10.5 7.x
with cotton wool. The moisture content of each sample was maintained by adjusting the sample to constant weight every other day. The recovery factor and uniformity of distribution were determined by subsampling each soil sample and extracting immediately after treatment. The soil samples (10 g) were shaken with 1OOml acetate buffer pH 5.6 (prepared as before) for 3 h. Asulam in l&25 ml aliquots was acidified, diazotized and coupled as described before. The Bratton-Marshall colour was then concentrated by extraction into n-butanol before spectrophotometric determination. The extraction of the Bratton-Marshall colour into n-butanol (Felling and West. 1968) is a modification introduced here to improve the sensitivity of the method. The same procedure was used in residue extraction and cslimation. The recovery factor ranged between 70”,, and lOO”,,. All measurements were carried out at least in duplicate. Appropriate blanks were included and the filtrates were checked by t.1.c. as before.
(0.4”. w/w) + NH,NO, (O.l”,, w;w); (4) glucose (0.4”,, w/w) + NH,NO, (O.l”,, w/w) + yeast extract (0.7”,, wjw). Each soil sample was aerated daily and deionized water added daily to compensate for evaporation. Asulam at 20.6,~g.g~’ was then added and mixed with the soil. The samples were then incubated at 30 C for 7 days.
Soil 8, after being mixed with yeast extract (0.33”,, w/w) and incubated as above, was treated with the following amounts of asulamP5.0, 10.0 and 20.0 pcg’g- ‘. The soil samples were then treated as before.
Sand (acid washed) and soil (Soil 6) were mixed in varying proportions from 100°O soil to loo”,, sand and the moisture content of each sample was adjusted to 6X”,, (w!w) of the value at field capacity. Asulam at 11.o /1g g ’ was added and the soil samples wcrc mixed and then incubated at 20 C for I7 days.
The soils (Soils 6-X) were treated with asulam at 20.1 /Lg.g- ’ and then incubated at 30 C. Asulam residues were assessed 5, 10 and 18 days after treatment. Ir$urnc~
A subsoil sample (Soil 8) was used in this experiment. The soil samples were given a preliminary incubation for 15 days in covered beakers with and without additives. The additives were: (1) glucose (0.1” 0 ww): (3) glucose (0.4”,, w/w).. (2) NH4N0, Table
2. Effect of soil depth.
The adsorption of asulam judged by Kd values, is inversely correlated with pH. The correlation coefficients (5) for 8 soils range between -0.99 and -0.92, all significant at P < 0.01 (Table 2). A plot of Kd against pH (using the method of best fit) can be
017 ~IS~/L~M~ tlr~qrrrrlaiion
and pH on asulam
pH where KC/ = 0
~ 3% pH + 22.55
; 4 5 6 7 8
12.25pH 3.04 pH + 17.64 12.71 - I.54 pH + 8.20 ~ I.24 pH + 5.94
- 0.98 - 0.99 - 0.99 - 0.98 -0.96
5.84 5.80 5.65 5.32 4.79
-3.26 pH + 18.9 ~ I .OOpH + 5.05 -0.97 pH + 4.50
-0.97 - 0.94 - 0.92
* All significant at P < 0.01
and degradation Table 4. Effect of various additives in soil 8. Asulam at 20.6,ng.g-’
on asulam persistence (oven dry basis)
Amount persisting mean (ng.g-‘)
Control (no additive) 0.4 glucose 0.1 NH&NO3 0.4 glucose + 0.1 NH,NO, 0.4 glucose + 0.1 NH‘,NO, + 0.2 yeast extract
20.0 * 1.4
19.3 * 1.9 20.6 k 0.3 20.4 * 0.5
15.1 f 0.5
Adsorption as expressed by /r’ value tends to decrease with increase in soil depth (Table 2). The /j values range between 22.55 and 5.95 for the surface and bottom samples respectively (Soils l--5). The change in p value with soil depth is more pronounced in the case of the discrete soil horizons (Dunlop series). /J’values of 18.90, 5.05 and 4.50 were obtained for the 4”. Blq and B,, horizons respectively (Soils 6, 7 and 8). The pH at which negligible adsorption occurs also decreases with soil depth (Table 2 and Fig. 1 (a) and (b)).
Fig. 1. Effect of pH on asulam adsorption. (a) Darleith series; Soils 1~~0, 2 -0. 3-& 4-A and 5-O. (b) Dunlop series; soils 6~--0, 7-0 and 8-L Standard errors are represented by vertical lines. expressed formula
as a straight
in all 8 cases,
Kci = apH
where a and /I are constants Table
(see Fig. l(a) and (b)).
Asulam degradation irl soils collected down a soil projje. Rapid disappearance of asulam occurs in the topsoil samples (Soil 6) with 21.0, 7.0 and O.O”,, remaining 5, 10 and 18 days after treatment respectively. However, only a very slow disappearance occurs in the subsoil samples (Soils 7 and 8) (Table 3). Effect of various additioes on asularn degradation. No appreciable disappearance of asulam occurs from the soil alone (Soil 8) or from the soil in the presence of glucose and NH,NO, either alone or in combination. However, appreciable disappearance (26.7”,,) occurs when yeast extract is added to the soil containing glucose and NH,NO, in combination (Table 4). Effect of asulam concentration. The disappearance of asulam appears to be affected by asulam concentration in the case of the yeast extract treatment (Soil 8 + 0.33”,; yeast extract) in which 56.0, 47.0 and 26.0::, of the added asulam at concentrations of 5.0. 10.0 and 2O.Opg.g-’ disappear in I5 days after treatment (Table 5).
by soil depth.
at 20.1 ng’g-’
Amount persisting mean (pg.g-‘)
(oven dry basis)
Soil Dunlop series
Incubation time (days)
5 10 18
4.3 + 0.1 4.4 f 0.1 0.0 * 0.0
21.0 7.0 0.0
5 10 18
16.5 + 0.6 16.1 + 0.9 14.5 * 0.9
X2.1 80.1 72.1
5 10 18
18.1 f 1.6 18.9 f 0.6 16.9 f 1.8
90.0 94.0 84.0
A. G. T.
Table 5. Effect of asulam concentration on asulam persistence in Soil 8. Yeast extract (0.33”,, w/w) was mixed with the soil. Assessment was made 7 days after treatment
Asulam concentration (K%.C’)
Amount persisting mean (pg.8 ‘1
“,, Persistence 0
44.0 53.0 74.0
2.2 f 0.3 5.3 f 0.6 14.8 + I.5
No disappearance of as&m occurs from pure sand. However, the addition of fresh topsoil (Soil 6) leads to considerable asulam disappearance I7 days after treatment (Table 6). fion.
The results show that comparatively greater amounts of asulam are adsorbed by topsoils than by subsoils. The adsorption which takes place is negatively correlated with pH. The comparatively larger adsorption values obtained with topsoils is in general agreement with findings for other herbicides (Harris and Sheets, 1965; Yuen and Hilton, 1962) and is generally attributed to the higher organic matter content of the topsoil (Yuen and Hilton, 1962). Correlation analysis between p value, obtained as described above. and the organic matter, c.e.c. and clay content, shows that /r is highly correlated with organic matter (t = 0.92 significant at P < O.Ol), to a lesser extent with C.C.C. (T = 0.67 significant at P < 0.01) and not at all with clay content. The relationship between organic matter and adsorption is best illustrated by plotting the 0o organic matter against the /I value (Fig. 2). An examination of Fig. 2 reveals that the relationship between organic matter and fl value is not perfectly linear. A deviation from linearity can be expected if all the organic matter as determined by the carbon chemical analysis method does not participate in adsorption (Lambert, 1968; Hamaker er crl., 1966) and can be due to the following: (1.) Close packing of the organic matter, which is claimed to cause less adsorption of herbicides in some organic soils by limiting the absorbing surface per unit weight of the organic matter (Hamaker er (II., 1966). (2.) Differences in the stage of decomposition of the organic matter (Walker and Crawford, 1968). Table
Fig. 2. Variation of fi value with soil organic leith series--O; Dunlop series-.
(3.) Unknown anomalous characteristics exhibited by the particular soil in question (Lambert. 1968). The inverse relationship between adsorption and pH is probably due to changes in the net charge of the molecule with pH which could alter its affinity towards the adsorbants (Bailey et rrl., 1968; Abernathy and Wax, 1973). Previous studies (Babiker and Duncan. 1975) have shown that asulam bears a net negative change down to a pH value between 3.0 and 3.9. Therefore in the practical soil pH range low retention of asulam by soils would be expected as is the case with many anionic herbicides (Abernathy and Wax. 1973; Kirkham, 1964). Drgrahf
The rapid disappearance encountered from the topsoil samples and the very slow disappearance from subsoil samples is in agreement with findings with other herbicides (Upchurch, 1966; Hahn et (I/.. 1969). Under field conditions, low temperatures, less 0, and more CO2 (Upchurch, 1966: Sheets. 1959: Linscott c’l (I/.. 1969) can be expected to further reduce the capacity of subsoils to dissipate asulam. The high degradation rate observed under conditions conducive to increased microbial activity, e.g. high organic matter and the inclusion of an additive such as yeast extract (Hill e/ LI/., 1955) would appear to suggest the participation of soil micro-organisms in asulam dissipation. However, it cannot be claimed that the degradation of asulam is solely microbial. The possibility of a non-biological route cannot be dismissed at this stage without further study. It should be noted. however. that the assay for ;ISLIlam used here and also adopted by others (Ball c’t t/l.. 1965) depends on the presence of a primary aro-
6. Effect of sand soil (Soil 6) mixtures on asulam persistence. Asulam at II.Opg.g- ' (ocen dry basis). Assessment was made 17 days after treatment Soil-sand
Amount persisting (/lP’g-‘)
20.0 40.0 80.0 100.0
60.0 20.0 0.0
Moisture content at field capacity (“,, w/w)
46.7 41.6 28.7 21.2
f & k *
0.3 0.4 0.6 0.2
1.0 1.5 3.7 10.7
+ + * _t
0.1 0.1 0.5 0.1
x.2 13.6 27.3 9x.3
Asuiam adsorption and degradation matic amino group. In this study the identity of asulam was checked by t.1.c. and the possibility of interference from other soil constituents was catered for by including t.1.c. and by incorporating suitable blanks. Under these conditions no degradation products were detected. The degradation pathway of asulam in soils should be a matter of interest in view of recent work with aniline-based herbicides (Kaufman, 1975; Bartha, 1975). Several degradation pathways are involved (Kaufman, 1975). e.g. oxidation of the NH2 group to NHOH, NO and NOz groups and their reduction, hydroxylation, acyiation, ring cleavage and condensation reactions. The importance of the individual pathways may be influenced by the soil characteristics and the microbial flora (Plimmer ef al., 1970). Taking the adsorption and degradation data together it would appear that the capacity of a soil to degrade asulam is a determining factor in accounting for the overall behaviour of asulam in soil. Rapid degradation or leaching could account for the short term pre-emergence activity of asulam observed in the field (Brock, 1972). On the other hand as breakdown in mineral soits is slow, as shown with the subsoil samples, the chemical should persist longer under arid and semi arid conditions (Ogle and Warren, 1954). In such situations, if furrow irrigation is adopted, adequate pre-emergence activity could therefore be anticipated in view of the longer persistence and the reduced vertical movement of the chemical (Helling, 1970; Ashton, 1961). Such behaviour coutd account for the promising performance of asulam noted by Menges et al., 1972) on some loamy sand soils under furrow irrigation. Acknowled:yement-One of us (A. G. T. Babiker) is grateful to the Agricultural Research Corporation of the Sudan for financial
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