Adsorption of chlorsulfuron {2-chloro-N-[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl) amino] carbonyl] benzenesulfonamide} on four soils having different pH values (6.2 to 8.1) was low. Log/log Freundlich isotherms for all soils at 8 and 30 C were linear, the extent of adsorption being greater at 8 than at 30 C. There was an inverse relationship between pH and degradation rate with a half-life of 88.5 days at pH 6.2 and 144 days at pH 8.1 at 20 C. Rapid disappearance was noted during the first 15 days after application. With decreasing moisture and temperature, chlorsulfuron degraded more slowly. The half-life ranged from 229 days at 10 C and moisture content of 75% field capacity to 62.5 days at 40 C and 75% field capacity at pH 7.7.

The impact of two weed management systems on the weed seed reserves of the soil, on the yearly weed problem, and on corn (Zea maysL.) production was assessed where corn was grown under furrow irrigation for 6 consecutive years. In one system, 2.2 kg/ha of atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] was applied annually to the same plots as a preemergence treatment. In the other system, a mixture of 1.7 kg/ha of atrazine plus 2.2 kg/ha of alachlor [2-chloro-2′,6′-diethyl-N-(methoxymethyl)acetanilide] was applied preemergence, followed by a postemergence application of 0.6 kg/ha of the alkanolamine salts of 2,4-D [(2,4-dichlorophenoxy)acetic acid]. The response of weeds and corn is presented only where atrazine was applied annually because the results were similar between both weed management systems. Weed seeds from eight annual species were identified, with redroot pigweed (Amaranthus retroflexusL. ♯ AMARE) and common lambsquarters (Chenopodium album♯ CHEAL) comprising 82 and 12%, respectively, of the initial 1.3 billion weed seeds/ha that were present in the upper 25 cm of the soil profile. After the sixth cropping year, the overall decline in the total number of redroot pigweed and common lambsquarters seeds was 99 and 94%, respectively. Very few weeds produced seeds during the first 5 yr, and no weed seeds were produced during the sixth year where atrazine was applied annually. When the use of atrazine was discontinued on one-half of each plot at the beginning of the fourth year, the weed seed reserve in soil began to increase due to an increase in the weed population. After 3 yr of not using atrazine, the weed seed reserve in soil had built up to over 648 million seeds/ha, and was then within 50% of the initial weed seed population. In the fifth and sixth years, grain yields were reduced 39 and 14%, respectively, where atrazine had been discontinued after 3 yr.

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The persistence of alachlor [2-chloro-2′,6′-diethyl-N-(methoxymethyl) acetanilide], metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide], and propachlor (2-chloro-N-isopropylacetanilide) in soil was examined under laboratory and field conditions using sorghum (Sorghum vulgarePers. ‘NB280S′) or annual ryegrass (Lolium multiflorumLam.) as bioassay species. In laboratory studies, degradation rate of alachlor and propachlor was greater at 50 and 80% than at 20% field capacity at 20 C. Degradation of metolachlor was greater at 80 than at 20% field capacity. Degradation rate of alachlor and metolachlor at 50% field capacity was greater at higher temperatures. Propachlor degradation rate varied with temperature. Under irrigated, cropped field conditions, the order of persistence was metolachlor > alachlor > > propachlor.

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