Graduate Thesis Or Dissertation

 

The effect of soil factors on glyphosate availability in soil Public Deposited

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  • Glyphosate (N-(phosphonomethyl)glycine) is an effective foliar-applied herbicide with broad-spectrum activity. Greenhouse and laboratory experiments were conducted to determine the importance of soil moisture, autoclaved soil, soil type, sphagnum peat, soil pH, added phosphorus, and plant residues on crop establishment and growth when glyphosate was applied before emergence of the crop. Glyphosate was applied preemergence with a track-mounted sprayer in 281 or 374 L/ha spray volume. Counts and fresh weights were taken to determine the effect of the glyphosate application. Radiolabelled glyphosate was used to determine Freundlich adsorption isotherms for Chehalis, Crooked, and Semiahmoo-2 soils at various pH or phosphorus levels. Glyphosate application to sandy soils (Chehalis and Crooked) reduced fresh weight of Italian ryegrass (Lolium multiflorum Lam.), whereas application to finer-textured soils seldom caused plant injury, indicating that soil texture may be an important factor. Also, on Chehalis and Crooked soils, increasing the soil pH caused increased injury to Italian ryegrass, suggesting that more glyphosate was available at higher soil pH. This was confirmed in an adsorption study using radiolabelled glyphosate. Other interactions also should be considered. The incidence of damping-off (Pythium spp.) was higher on plants grown in glyphosate-treated Chehalis soil than plants grown in untreated soil. Barnyardgrass (Echinochloa crusgalli (L.) Beauv.) and Italian ryegrass were injured by a preemergence application of glyphosate to sphagnum peat. However, glyphosate application to muck (organic) soils did not injure bioassay species. These results suggest that nondecomposed organic matter does not render glyphosate unavailable, and therefore, caution is advised when applying glyphosate to media with sphagnum peat or other nondecomposed plant material. Adjusting soil moisture, autoclaving soil, or adding phosphorus did not influence glyphosate availability in soil. Dead or dying perennial ryegrass residues, whether chemically treated or not, reduced fresh weight of Italian ryegrass seedlings. Roots or whole plant residues were more inhibitory than shoots. Increasing the time interval between treating perennial ryegrass and planting Italian ryegrass reduced phytotoxicity to Italian ryegrass. Adding fertilizer to pots to alleviate competition for nutrients did not prevent the phytotoxic effect of perennial ryegrass residues. Experiments exposing ryegrass seedling roots or shoots to various concentrations of glyphosate, in the absence of soil, showed that roots were damaged more than shoots when roots were treated with glyphosate. However, roots and shoots were injured equally when shoots were treated with glyphosate, indicating that glyphosate is readily translocated to the roots. Italian ryegrass and bentgrass (Agrostis tenuis Sibth. 'Highland') were more sensitive to glyphosate applied preemergence than crimson clover (Trifolium incarnatum L.), alfalfa (Medicago sativa L.), and tomato (Lycopersicon esculentum Mill. 'Bonny Best'). Glyphosate activity in soil varied considerably from one study to another, suggesting that a particular factor, or combination of factors has not been identified. But, soil activity did occur, hence, precautions should be taken when applying glyphosate preplant or preemergence to: a) light-textured soils; b) limed soils; c) potting media with sphagnum peat; and d) areas with high weed densities (plant residues). One precaution could be to increase the time interval between treatment and planting to reduce the possibility of glyphosate or phytotoxin injury because microorganisms would have more time to degrade glyphosate or phytotoxins.
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