Phosphate fertilizers improve phosphorus-depleted agricultural soil for better plant growth; however, overuse may lead to human health issues. Analyzed soil samples were treated with five phosphate fertilizers and Spinacia oleracea to quantify the levels of natural radionuclides 232Th, 226Ra, and 40K. The study aimed to investigate how phosphate fertilizers contribute to the accumulation of radionuclides 232Th, 226Ra, and 40K in Spinacia oleracea using a high-performance germanium detector. Results revealed the mean activity concentrations in agricultural soil mixed with phosphate fertilizer were 232Th (66.66 ± 6.39 Bq/kg), 226Ra (54.78 ± 5.41 Bq/kg), and 40K (837.17 ± 62.89 Bq/kg); Spinacia oleracea were 232Th (5.79 ± 0.83 Bq/kg), 226Ra (40.24 ± 5.38 Bq/kg), and 40K (1268.12 ± 129.95 Bq/kg). The soil-to-plant transfer factors were 232Th (0.244), 226Ra (0.572), and 40K (1.756). The mean annual limits on the intake of radionuclides were 232Th (231.67 ± 33.20 Bq/kg), 226Ra (1609.53 ± 215.33 Bq/kg), and 40K (50724.67 ± 5,198.13 Bq/kg). The mean annual effective dose was 0.849 ± 0.103 mSv/year, and the internal radiation index was 0.504 ± 0.059 Bq/kg. The results indicate that the mean activity concentrations of 232Th, 226Ra and 40K are below standards but exceed those of the control sample, suggesting regular monitoring of phosphate fertilizer quantity applied to the soil for cultivation.