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Effect of Picloram, Diuron, Ametryne, and Prometryne on Nitrification in Some Tropical Soils¹

ABSTRACT

Effect of 4-amino-3,5,6-trichloropicolinic acid (picloram), 3-(3,4-dichlorophenyl)-1,1-dimethylurea (diuron), 2-ethylamino-4-isopropylamino-6-methylthio-s-triazine (ametryne), and 2,4-bis(isopropylamino)-6-methylthio-s-triazine (prometryne) at rates from 2 to 100 ppm on nitrification of 100 ppm applied NH₄-N was studied in several soils of Puerto Rico incubated at 24C for 8 weeks.

The effect was greatly dependent on the nitrifying capacity of the soils. Inhibition of nitrification by herbicides was less in soils of high nitrifying capacity than in soils of low nitrifying capacity. Picloram at 2 ppm caused a weak inhibition and at 20 to 100 ppm complete inhibition except in the soil of high nitrifying capacity. Diuron inhibited nitrification at 20 ppm and above in soils of low to intermediate nitrifying capacity. It did not affect nitrification in the soil of high nitrifying capacity even at 100 ppm. Ametryne and prometryne at 100 ppm inhibited Nitrobacter bacteria, but not NH₄^- oxidizers resulting in high NO₂^- accumulation. At certain times 60 to 100% of the applied NH₄-H appeared as NO₂^- when prometryne and ametryne were applied at 100 ppm, respectively. Nitrite accumulated, however, only at soil pH of 6.8 and above.

¹ Contribution from the Department of Agronomy & Soils, Agr. Exp. Sta., University of Puerto Rico, Mayaguez Campus, Rio Piedras, Puerto Rico. Presented before Div. S-3 Soil Sci. Soc. Amer., Washington, D.C., Nov. 6, 1967. This work was supported in part by the Southern Regional Research Project S-62, and USDA. Hatch Funds. The author wishes to express his thanks to Miss Rita L. Rodriguez for help in the laboratory work.

² Associate Soil Microbiologist.

Received for publication March 25, 1969. Accepted for publication August 15, 1969.