Glyphosate residues in a sandy soil affect tomato transplants

Glyphosate residues in a loamy sand soil were suspected of damaging transplanted tomatoes at Gosford in 1990. Field and glasshouse experiments were conducted to determine whether phytotoxic residues of glyphosate persist in this soil type and, if so, under what conditions. In the glasshouse experiment, visible symptoms of glyphosate toxicity occurred in tomato seedlings transplanted into soil that was sprayed 1, 5 or 15 days earlier with glyphosate (360 g a.i.L) at 4 L productha. Glyphosate also reduced plant dry weight (16 days after transplanting), but only where soil nutrient deficiencies were corrected after transplanting. In this case, seedlings transplanted 15 days after spraying suffered an average reduction in dry weight of 57%. Greater reductions in dry weight occurred where superphosphate (43 kg Pha) was mixed through soil before spraying (75 v, 35% reduction). In the field, glyphosate residues reduced plant dry weight 16 days after transplanting, even when transplanting followed spraying by up to 9 days, and possibly as many as 30. At 9 days, reductions of 50, 74 and 78% were recorded with glyphosate (360 g a.i./L) applied at 2, 4 and 8 L/ha, respectively. Effects of glyphosate on fruit yield were significant (P<0.05), but much smaller than effects on earlier plant dry weights. The phytotoxicity of glyphosate residues in this loamy sand appears to result from a combination of inherently low P sorption capacity and application of superphosphate, leading to low adsorption of glyphosate by soil. This may be exacerbated when dry conditions occur between application and planting. On the present evidence, a plant-back period of 3 weeks could be considered safe when transplanting tomatoes into this sandy soil, provided some mixing of soil occurs at transplanting. It is recommended that farmers perform a simple bioassay to confirm safety.