Objectives Studying the effects of long-term fertilization on the dynamics of the topsoil pH and predicting soil pH trend are important to provide optimal nutrient management for soil fertility improvement and stable high yield.

Methods Two different experiments involving application of chemical fertilizer or combined application of chemical and organic fertilizers were carried out in latosolic red soil of southeast Fujian for 16 years and 14 years, respectively, and annual soil pH were tested. Soil pH grey prediction model was set up to explore the difference in soil pH and dynamics among treatments.

Results The main factor responsible for soil acidification was chemical nitrogen fertilizer (urea) application in latosolic red soil. The tested data for sixteen years showed that topsoil pH in CK (no fertilizer) and RF−N (without N in the recommended fertilizer) treatments increased to 6.24±0.18 and 6.21±0.17 from 5.90 of basal soil, respectively, with an annual average increment of 0.021 and 0.019, respectively. Soil acidification was observed in FP (farmer’s practice), RF (recommended fertilizer), without P in RF (RF−P) and without K in RF (RF−K). Even though, there was no significant difference in acidification level. Chemical fertilizers combined application with organic fertilizers inhibited soil acidification. The soil pH in RF+PM (recommended fertilizers and pig manure combination experiment for fourteen years) increased from 5.73 of basal soil to 5.80±0.36, which was significantly higher than that of RF and RF+COM (recommended fertilizers combined with commercial manure combination experiment) with an annual increase of 0.005. However, there was no significant difference between RF+PM and RF+S (recommended fertilizers combined with rice straw). RF+COM had a limited influence on inhibiting soil acidification, as soil pH declined at rate of 0.019 unit/a, which was not significantly different from RF treatment. The grey prediction simulation models of soil pH showed the fitting errors ranged from 0.997% to 4.321% among fertilization treatments; the soil pH change had obvious periodical characteristics, and the inflection point appeared in the third or fourth years of the experiment; soil pH trend was significantly affected by fertilization. Soil pH in CK and RF−N was going up in the first 3 years, and then stabilized at about 6.20 after that. Soil pH declined in the treatments of FP, RF, RF−P, RF+COM, RF+PM, and RF+S, respectively in the third and fourth year. Soil pH in the treatments of FP, RF, RF−P and RF+COM stabilized at about 5.50 in the first 3 or 4 years, but soil pH in RF+PM and RF+S was going up, and reached 6.26 and 5.91 in the 14th year, respectively, which were 0.53 and 0.18 units higher than that of the basal soil, respectively.

Conclusion The main reason that led to soil acidification was long-term application of recommended nitrogen or higher nitrogen fertilizer (urea) in the latosolic red soil. The soil pH change had obvious periodical characteristics, and the inflection point appeared at the third the fourth years after the long-term experiments. The treatments in which1/3 of the N input was provided by decomposed pig manure or rice straw on the basis of recommended fertilization significantly increased soil pH after the inflection point, the effect was as follows: decomposed pig manure > rice straw.