Assessing available phosphorus (P) in paddy fields is challenging due to waterlogging-induced reducing conditions. This study tested the applicability of the Truog, Bray 2, and Mehlich 3 extraction methods in both air-dried and incubated soils, as well as the ascorbic-acid-reduced Bray 2 (AR Bray 2), which simulates reducing conditions, for evaluating rice growth under P fertilization. In addition, to investigate the chemical characteristics of the extraction methods, active Al and Fe and P sequential extractions were measured. Soil samples from four representative regions in Yamagata Prefecture were used. Pot cultivation tests using ‘Haenuki’ and ‘Tsuyahime’ cultivars were conducted with varying P fertilizer levels. Variations in P availability across soil types were influenced by levels of active Al and Fe. Sequential extractions identified NaHCO₃-P and NaOH-P fractions as important for P availability. Bray 2 in both soils and AR Bray 2 were the most effective methods, showing a strong saturating exponential correlation with rice growth and P uptake, whereas Mehlich 3 and Truog showed weaker correlations. Bray 2 and AR Bray 2 show potential but require further evaluation for practical application due to the small number of soils. Future efforts should prioritize developing methods that account for P dynamics under reducing conditions, thereby improving P management strategies and supporting sustainable rice production.

Accurate evaluation of plant-available phosphorus (P) in flooded paddy soils requires consideration of redox dynamics and soil-specific properties. This study evaluated five soil P extraction methods, such as Truog, Bray 2, Mehlich 3, Olsen, and ascorbic acid-reduced Bray 2 (AR Bray 2), using soils collected from 20 paddy fields in a cold region of Japan that have received long-term fertilization. All four methods, except AR Bray 2, were conducted under air-dried and flooded incubation conditions. Additionally, we conducted pot experiments with the two rice cultivars to measure P uptake. Bray 2 extracted the highest amount of P (543.6–1045.4 mg P kg⁻¹). Incubation increased extractable P by factors of 2.4–4.9 with the Mehlich 3 and Truog methods, indicating enhanced P solubility under reduced conditions. The Olsen method showed minimal sensitivity to redox changes (−31.4 mg P kg⁻¹). Principal component and cluster analyses suggested three patterns of soil P behavior under changing redox conditions: (1) stable P extractability regardless of redox status; (2) increased P availability after incubation; and (3) P extractability depending on the extraction method used. These patterns were not explained by regional or taxonomic classifications. A comparison of soil extractions and P uptake indicated that no single method consistently predicted shoot P concentrations across all soils, suggesting that conventional P extraction methods may have limited ability in long-term fertilized paddy soils. Our findings demonstrate that soil-specific redox behavior and cultivar-specific P demand critically influence the effectiveness of standard P tests. Therefore, selecting diagnostic methods tailored to soil characteristics and crop requirements is essential for accurate P evaluation and sustainable fertilizer management in rice cultivation.

Transplanting high-density nursery (HDN) seedlings reduces seedling raising costs by increasing the sowing density compared to regular seedlings and shortens the seedling growth period. However, there is a concern that the poor early growth of HDN seedlings hinders rice production in cold regions, such as northern Japan. This study conducted a growth comparison test using a combination of phosphorus (P) and silicon (Si), which stabilizes early rice growth. The results showed that the application of P or Si alone increased the shoot dry weight at the heading stage. The application of 60 mg kg⁻¹ or more P fertilizer increased the HDN shoot dry weight, while 2 g kg⁻¹ Si was effective in increasing the shoot dry weight. However, the joint application of P and Si did not increase plant height or stem number or synergistically increase the shoot dry weight compared to the application of P or Si alone. As the P concentration in the soil increased owing to the P treatment, Si suppressed the excessive uptake of inorganic P and maintained its concentration at an appropriate level. The application of P and Si is important to increase the shoot dry weight of rice. In addition, Si may play a role in regulating the appropriate level of P uptake by HDN. The application of P and Si stabilized the initial growth of the HDN.

Acidic subsoils can restrict root growth and nitrogen (N) uptake and increase the risk of N loss; however, the extent of genotypic variation remains unclear. We evaluated two bread wheat cultivars, Haruyokoi and Harukirari, and one spelt line, KU-1025, under limed and acidic subsoil treatments to clarify whether maintaining root growth in acidic subsoil contributes to greater N capture and lower N loss. After 78 days, we measured shoot dry weight, shoot N uptake, root dry weight, total root length, soil nitrate (NO₃-N) concentration, and cumulative N leaching. The acidic subsoil reduced shoot N uptake, root length in the subsoil, deep-root ratio, and NO₃-N depletion, indicating that it restricted root proliferation and N acquisition. KU-1025 showed the greatest shoot dry weight, shoot N uptake, root dry weight, and total root length under both treatments. It also maintained a high deep-root ratio under acidic subsoil conditions and showed lower soil NO₃-N concentrations and less N leaching than the two wheat cultivars. Across genotypes and treatments, shoot N uptake was positively correlated with root dry weight and total root length, whereas N leaching was negatively correlated with these traits. These results suggest that maintaining a large root system, rather than deep rooting alone, is important for improving N capture and reducing N loss under acidic subsoil conditions, and that KU-1025 may provide useful genetic variation for breeding wheat adapted to acidic subsoil environments.