- 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
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