Potassium and phosphorus availability due to fertilization of potassium and organic matter for rice in rainfed rice fields

Serdang Village, Beringin District, Deli Serdang Regency, North Sumatra Province, Indonesia, was chosen as the research location held in May-August 2016. This study aims to determine the relationship between potassium and phosphorus availability, nutrient absorption, and grain yield in rainfed rice fields due to planting time, fertilization, and rice varieties. A split-split plot design with two replications was used in the study. The main plot (a planting time): T1: the 10^th day of May; T2: the 20^th day of May; and T3: the 30^th day of May. Subplot (fertilization): K0: control; K1: 50 kg ha⁻¹ KCl; K2: 5 tons ha⁻¹ straw compost; K3: 2.5 tons ha⁻¹ manure + 2.5 tons ha⁻¹ straw compost; K4: 5 tons ha⁻¹ straw compost + 50 kg ha⁻¹ KCl; and K5: 2.5 tons ha⁻¹ manure + 2.5 tons ha⁻¹ straw compost + 50 kg ha⁻¹ KCl. Sub-subplot (rice varieties): V1: Situ Bagendit; V2: Towuti; V3: Batutegi; V4: Inpago-8; V5: Inpago 9; V6: Inpago 10; V7: Ciherang; and V8: Inpari 10. The data were analyzed using regression and correlation tests. Available potassium and phosphorus, and potassium and phosphorus uptake, were positively correlated with grain yields in all rice varieties grown in rainfed rice fields.