Farm-scale estimation of total nitrogen content in surface paddy soils by extraction with commercially available hydrogen peroxide

We recently proposed a simple method for estimating total nitrogen content in paddy soil. In this method, soil is extracted with a commercial 3% hydrogen peroxide (H₂O₂) solution at 25 ^◦C for 40 h, and electrical conductivity (EC (H₂O₂)) of the extract is measured. This study aimed to further evaluate the method’s applicability to soil samples collected at the farm scale by using the original and six additional H₂O₂ solutions that are locally and commercially available. The results obtained with the original solution indicated that the determination coefficients between EC (H₂O₂) and total N were statistically significant at all farms examined: Moka, 0.78 (n = 13); Kyoto, 0.50 (n = 16); Kizu, 0.43 (n = 89); and Kawatabi, 0.25 (n = 18). The EC of the tested H₂O₂ solutions varied from less than 0.05 to 1.4 mS cm⁻¹ because of the addition of different stabilizers. EC (H₂O₂) values obtained with the less stabilized H₂O₂ solutions (one from Japan, one from USA, and the analytical grade 6% solution) agreed well with those obtained with the original solution. Thus, the proposed method can be useful for estimating the farm-scale variation in soil total N, provided a H₂O₂ solution with a low EC (<0.2 mS cm⁻¹) is used for the extraction.