Laboratory experiments were conducted to determine the effects of eight fumigants on the nitrification of (NH4)2SO4 and NH4OH (aqua ammonia) in 200-g. samples of sugar cane field soils. All samples were incubated unamended and amended with 500 parts nitrogen as (NH4)2-SO4 or NH4OH per million parts soil. Unamended and nitrogen-amended soils were treated with the different fumigants. The soil samples were maintained at 50% moisture-holding capacity and, at intervals, extracted for ammonium and nitrate nitrogen with 2 parts of a 0.3 N solution of K2SO4 to 1 part of soil. Ammonium nitrogen was determined by nesslerization and nitrate nitrogen by the phenoldisulphonic acid method; in both cases the Klett Summerson photoelectric colorimeter was used for comparisons.
Results indicate that, under the conditions of the experiments, DD at 40 gallons per acre, Telone at 40 gallons per acre, Dowfume W-85 at 8 gallons EDB per acre, S-1283 at 8 gallons per acre, and Vapam at 50 gallons per acre markedly inhibited nitrification for 4 to 8 weeks. The rate of nitrification increased appreciably shortly thereafter and decreased again as the supply of added ammoniacal nitrogen was depleted.
Allyl alcohol at 20 gallons per acre, Nemagon or DBCP at 3 gallons per acre, and PRD G-10 at 40 pounds active per acre were less toxic to the nitrifying bacteria.
An aromatic oil used as a diluent for some of the fumigants did not cause a significant reduction in the rate of nitrification in soil.
Lysimeter studies with DD substantiated results obtained with the fumigant in other experiments. For a period of 8 weeks, very little nitrate nitrogen was leached through soils supplemented with NH4OH and fumigated with DD at the rate of 40 gallons per acre. In the soils which were not fumigated with DD, after the very slow initial rate during the first 2 weeks, nitrification proceeded uninhibited.