Response of Anam Rice Variety (Oryza sativa L.) to Different Levels of Phosphorus and Potassium Fertilizers

Introduction
Phosphorus and potassium are two essential macronutrients that play significant roles in the growth, development, and yield of rice (Oryza sativa L.). Phosphorus is vital for energy transfer, root development, nucleic acid synthesis, and overall plant vigor. Potassium is involved in enzyme activation, osmoregulation, photosynthesis, and the translocation of carbohydrates and nutrients. While nitrogen has traditionally been the primary focus in rice fertilization programs, optimizing phosphorus and potassium application has gained importance, especially for newly developed rice varieties with potentially different nutrient requirements. Rice is a staple food crop in Iran, with the northern provinces of Guilan and Mazandaran as the major rice-growing areas. The introduction of the Anam rice variety has created opportunities to enhance rice productivity per unit area due to its adaptability and high yield potential. However, limited comprehensive research exists on the optimal phosphorus and potassium fertilizer rates for this variety under local environmental conditions. Therefore, identifying the appropriate phosphorus and potassium needs to maximize growth, yield components, and grain yield of the Anam variety in northern Iran’s rice-growing regions is essential.
 
Materials and Methods
A two-year field experiment was conducted during the 2020 and 2021 growing seasons at the Rice Research Institute of Iran’s experimental stations in Rasht and Tonekabon. These sites represent typical agro-ecological conditions of northern Iran with clay and loam soil textures, respectively, showing low to moderate fertility, especially regarding phosphorus and potassium. The experiment was arranged in a factorial design within a randomized complete block design (RCBD) with three replications. Treatments included four phosphorus fertilizer levels (0, 30, 45, and 60 kg phosphorus pentoxide ha⁻¹) and four potassium fertilizer levels (0, 50, 100, and 150 kg potassium oxide ha⁻¹) applied in all possible combinations. Phosphorus and potassium fertilizers were applied as basal fertilizers before transplanting seedlings during land preparation. Nitrogen fertilizer was applied at 75 kg N ha⁻¹ as urea, split into two applications: two-thirds basal and one-third at maximum tillering stage. Additionally, 20 kg ha⁻¹ zinc sulfate was broadcast evenly and incorporated into the soil before transplanting in all plots. Standard crop management practices, including irrigation, weed control, pest, and disease management, were carried out throughout the growing seasons. Morphological traits such as plant height, total and fertile tiller numbers, number of panicles per square meter, and filled/unfilled grain numbers were measured according to the International Rice Research Institute bulletin. For measurements, ten hills were randomly sampled from each plot after border plants removal, and mean values were calculated. Thousand-grain weight was determined by weighing 1000 randomly selected grains with a balance accurate to 0.01 g. At maturity, harvesting was conducted on 4 m² per plot after border plant removal, and grain yield were recorded. Data were analyzed using SAS software (version 9.4) with combined analysis of variance, and means were compared using LSD at 5% significance (P ≤ 0.05).
 
Results and Discussion
Analysis of variance revealed significant effects of phosphorus and potassium fertilizer applications on agronomic traits of the Anam rice variety, including total and fertile tiller numbers, panicles per square meter, filled grains per panicle, and 1000-grain weight at the 1% significance level (P ≤ 0.01). The combined application of 60 kg P₂O₅ and 150 kg K₂O ha⁻¹ increased total tiller number, fertile tiller number, panicles per square meter, filled grains per panicle, and 1000-grain weight by 45%, 44.4%, 64.5%, 64.7%, and 55.8%, respectively, and decreased the number of unfilled grains per panicle by 33.3% compared with the control treatment. The interaction between fertilizer treatments and location was significant for grain yield at P ≤ 0.01. The highest mean paddy yields were recorded in the Rasht and Tonekabon research fields under the combined application of 60 kg P₂O₅ and 150 kg K₂O ha⁻¹, reaching 5061.8 and 4909.0 kg ha⁻¹, respectively. These yields correspond to increases of 75.2% and 47.7% compared with the control treatment, and increases of 4.4% and 2.12% relative to the recommended chemical fertilizers treatment in the Rasht and Tonekabon, respectively. These results indicate that optimal phosphorus and potassium fertilization positively influences growth and yield across different environmental conditions and soil types. The improvements in yield components and grain yield are attributed to the synergistic roles of phosphorus and potassium in plant physiology. Phosphorus promotes root growth and energy metabolism, increasing tillering and panicle formation, while potassium activates enzymes and regulates osmotic potential, enhancing photosynthesis and assimilate translocation, which supports grain filling and increases grain weight. Therefore, the combined application of 60 kg P₂O₅ and 150 kg K₂O ha⁻¹ is recommended to maximize the productivity of the Anam rice variety.
 
Conclusions
This two-year field study demonstrated that phosphorus and potassium fertilization had significant and synergistic effects on the agronomic performance and grain yield of the Anam rice variety under northern Iran’s agro-ecological conditions. The combined application of 60 kg P₂O₅ ha⁻¹ and 150 kg K₂O ha⁻¹ consistently improved key yield components including total and fertile tiller numbers, panicle density, filled grain number, and 1000-grain weight. This fertilization regime produced the highest grain yields in both Rasht and Tonekabon, proving its effectiveness across diverse soil types and environments. The observed benefits are due to the complementary functions of phosphorus and potassium in root development, energy metabolism, enzyme activation, and assimilate translocation. Thus, the combined application of 60 kg P₂O₅ ha⁻¹ and 150 kg K₂O ha⁻¹ is recommended as the optimal fertilization strategy to maximize the productivity of the Anam rice variety in northern Iran’s rice-growing regions.