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Glyphosate residues persist in a sandy soil

P.S. Cornish

NSW Agriculture, Horticultural Research and Advisory Station, PO Box 581, Gosford NSW 2250

Glyphosate is widely regarded as being non-residual in soil. More precisely, however, glyphosate is usually adsorbed quickly and inactivated. Microbial degradation occurs subsequently. Despite the persistence of adsorbed residues, there appear to be no published experiments in which residues of glyphosate have resulted in plant damage under field conditions. However, observations I made in 1990 suggested an effect of glyphosate residues on tomatoes transplanted into a sandy loam, yellow podzolic soil. This paper describes subsequent field and pot experiments which aimed to determine if phytotoxic glyphosate residues persist in this soil type, and under what conditions.


A field experiment tested for residual effects of glyphosate (0, 2, 4 and 8 L/ha) on tomato seedlings (cv. Flora Dade) transplanted 1, 2, 4, 9, 20 and 30 days after applying glyphosate to the soil surface. Soil was high in available P (64 g/g, bicarbonate extract) and plots were weed-free. Glyphosate rate comprised main plots split for time to transplanting. There were three replicates and 10 plants per sub-plot. Five plants were cut and their dryweight measured 16 days after transplanting. Mature fruit yield was measured in the remaining five plants.

A pot experiment examined the effect on phytotoxicity of superphosphate (0,43 kg P/ha) mixed with soil before spraying glyphosate. This soil was low in P (9 g/g) before adding superphosphate. The experiment also examined the effect of soil water content (field capacity versus drying) between spraying and transplanting. Glyphosate was applied at 0 or 4 L/ha and seedlings were transplanted 15 days later. Plant dry weights were recorded 16 days after transplanting.

Results and discussion

In the field experiment there were visual symptoms of glyphosate toxicity at all rates of application when transplanting was one or two days after application of herbicide to soil. Dry weight of young plants and fruit yield were both reduced (P<0.05) when transplanting followed application of herbicide by up to 9 days. Effects on dry weight were recorded at rates of glyphosate as low as 2 L/ha, but effects on yield were found only at 4 or 8 L/ha. In the pot experiment glyphosate reduced mean dry weights from 0.56 to 0.24 g/plant (P<0.001). Reduction in growth was greater where P was applied before herbicide application. The effect of P is consistent with findings that glyphosate sorption is related to the unoccupied P sorption capacity of soil (1). Soil water regime had little effect.

These experiments show that glyphosate residues can persist in soil at phytotoxic concentrations for at least 15 days under specific conditions: a sandy soil (low P-sorption capacity), following substantial P application (further reducing the unoccupied P-sorption capacity), and using tomatoes which are very sensitive to the herbicide. The study serves as a salutary reminder of the care needed with the use of agricultural chemicals, even with products like glyphosate which have a good reputation for environmental safety.


Hance, R.J. 1976. Pesticide Science 7, 363-366.

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