Simple Summary: Ammonia gases are produced from urea in the urine and slurry due to the action of urease. Odorous compounds are produced by microbial fermentation that utilizes an undigested nutrient in pig slurry. Various additives have been researched to mitigate ammonia emissions or odorous compound production. These studies involve inducing acidic conditions in urine or slurry, enhancing nitrogen utilization, inhibiting urease, or absorbing odorous compounds in pig slurry. However, information about the comparison of these additives is lacking. In this study, we evaluate and compare the effects of each additive on nitrogen utilization, urinary pH, slurry pH, and odorous compound concentrations. The five experimental diets were (1) a control diet, (2) the control diet with 1% benzoic acid, (3) the control diet with 1% adipic acid, (4) the control diet with 1% bromelain, and (5) the control diet with 1% humic substances. Fecal dry matter and nitrogen output were greatest in pigs fed the humic substances diet. Daily retained nitrogen and the nitrogen retention rate tended to be lowest in pigs fed an adipic acid diet. Urinary pH was lowest in pigs fed the adipic acid diet. In addition, slurry pH did not affect odorous compound concentrations, and some odorous compounds tended to be lowest in pigs fed the humic substances diet. The objective was to evaluate the effects of benzoic acid, bromelain, adipic acid, and humic substance supplementation on nitrogen balance, urinary pH, slurry pH, and manure odorous compounds in pigs. Fifteen castrated male pigs with an initial body weight of 37.9 kg (standard deviation = 4.1) were individually housed in metabolism crates. The animals were allocated to a triplicated 5 × 2 incomplete Latin square design with 15 animals, 5 experimental diets, and 2 periods. The basal diet mainly consisted of corn, soybean meal, and rapeseed meal. Four experimental diets were prepared by supplementing each additive at a concentration of 10 g/kg at the expense of corn starch to the basal diet. Each period consisted of a 4-day adaptation period, a 24 h collection period for slurry sampling, and a 4-day collection period for feces and urine. The feces and urine collected for 24 h on day 5 were mixed at a ratio of fecal weight and urine weight to obtain slurry samples. The apparent total tract digestibility N in pigs fed the humic substance diet was the least (p < 0.05) compared to the other groups. The daily retained N and N retention as % ingested tended (p < 0.10) to be the lowest in the adipic acid group among the treatments. The urinary pH in pigs fed the adipic acid diet was less (p < 0.05) than that in other groups except the benzoic acid group. The slurry pH tended to differ among the treatment groups (p = 0.074) with the lowest value in the pigs fed the adipic acid diet. The concentrations of indole in slurry (p = 0.084) and isovalerate in feces (p = 0.062) tended to differ among the groups with the lowest values in the pigs fed the humic substance diet. In conclusion, adipic acid supplementation in pig diets can decrease urinary pH and slurry pH. Although benzoic acid and adipic acid have limited effects in reducing odorous compounds, humic substances have the potential to reduce some odorous compounds. [ABSTRACT FROM AUTHOR]