The Effect of Organic and Biological Fertilizers on Persistence of Trifluralin Herbicide in Soil Using Gas Chromatography Method

Herbicides that persist in soil are of benefit to farmers seeking to control late emerging weeds in cultivated crops, and to managers looking for long-term vegetation control on rights-of-ways and industrial sites. Herbicides that persist in soil can also create problems for farmers who want to diversify their rotation into subsequent crops which may be sensitive to certain herbicide residues. Several factors mainly soil factors, climatic conditions, and herbicide properties determine the herbicides persistence in soil. Trifluralin is one of the important soil applied selective, pre-sowing or pre-emergence herbicide used to control many annual grasses and broadleaf weeds in a large variety of crops. It is a generally regarded as a moderately persistence herbicide with a half-life time of 1.5 to 6.5 months. Some studies, however, have reported higher half-life times varying from 7.5 to longer than 12 months. These results have been generally obtained at sites with very dry or very cold climates in which soil is frozen over winter. Normally Residual herbicides extend the period of weed control, increasing the efficiency of weed management practices. However, they may persist longer than desired and injure or kill subsequent rotational crops. Thus, most herbicide labels include crop rotation guidelines, but rotational restrictions are often not listed for many crops. This experiment was carried out to determine trifluralin soil persistence affected by its dose and application some organic and biofertilizers.

In order to study the effect of some organic and biofertilizers on trifluralin herbicide persistence in soil, an experiment was conducted as a factorial arrangement in completely randomized block design with three replications in a cotton field at Ferdowsi University of Mashhad. Experimental factors included trifluralin (EC 48 %) dose at two levels (480 and 960 g a.i. ha-1), application of organic fertilizers at two levels (Humic acid (85 %) and folic acid (12 %) and biofertilizers application at two levels (Nitroxin and fertile phosphate2). To determine the trifluralin soil residue, soil samples were taken from 0 to 10 cm soil depth during 3, 7, 15, 30, 60, 90 and 120 days after herbicide application and were kept in a freezer (-18 oC). For trifluralin soil residue, gas chromatography technique was used. The trifluralin value recovered from soil at different time intervals for each treatment was fitted in the first order kinetic equation according to:C = C0e–kt                                                                                                                                                     (1)
Where C denotes the amount of trifluralin recovered from soil at time t, C0 is the amount of trifluralin recovered at t = 0 interval; λ represents the degradation constant, and t is time in days. The DT50 (half-life) and DT90 (the time required to disappear 90 % of herbicide) were calculated as follows:                                                                                                                      (2)
                                                                                                                    (3)

Results showed that application of organic and biofertilizers significantly reduced trifluraline persistence in soil. The lowest trifluralin half-life time was observed for organic and bio-fertilizer application equal to 55.26 and 41.26 days, respectively, at the dose of 480 g a.i ha-1 of trifluralin. The highest half-life times equal to 106.64 and 78.77 days were observed in control treatment without application of fertilizers for the trifluralin dose of 960 and 480 g. a.i ha-1, respectively. Application of organic and biofertilizers plays important role in reducing of trifluralin soil residue and persistence. Therefore, the mentioned fertilizers have positive effects in fields where there is a risk of damage of trifluralin residue for rotational crops.