Cultivation effects on nitrification in potato soils

Abstract

Nitrification rates were found to be less rapid in newly tilled potato soils than in aged cultivated soils. Studies were undertaken to determine what factor or factors were responsible for the slower nitrification rates. Aged cultivated and virgin soils were obtained from various locations and compared in residual mineral nitrogen content, nitrification rates and in bacterial populations. Total residual mineral nitrogen content (ammonium and nitrate) in aged cultivated soils was usually higher than that in virgin soils. To compare nitrification rates, aged cultivated and virgin soils were enriched with 0, 50 and 100 ppm ammonium nitrogen and incubated at 24° C for 0, 10 and 21 days. Nitrification rates were consistently lower in virgin soils than in aged cultivated soils. Also, nitrification was higher in soil samples collected in June than in those collected in December. Determination of soil bacteria (Nitrosomonas and Nitrobacter) was done by the most probable number method. Population of Nitrosomonas in virgin soils ranged from 260 to 460 cells per gram in virgin soils and from 7,200 to 35,000 in cultivated soils. Correspondingly, Nitrobacter in virgin soils ranged from 45 to 78 and in cultivated soils from 4,100 to 35,000. The lower rate of nitrification in virgin soils was attributed to the low bacterial population. The presence of a nitrification inhibitor in virgin soils produced by native vegetation was not considered probable. In one experiment where various amounts of aged cultivated soil were mixed with virgin soil the nitrification rates in the latter increased in proportion to aged cultivated soil added. This may not have occurred if nitrification inhibitors were present.