Soil salinity may damage crop production. Besides proper management of irrigation water, salinity reduction can be achieved through soil amendment. The objectives of this study were to evaluate the effects of rice husk biochar and compost amendments on alleviation of salinity and rice growth. Field experiments were conducted at two salt-affected paddy rice fields located in distinct sites for five continuous crops. Treatments, with four replicates, consisted of continuous three rice crops per year (RRR), two rice crops rotated with fallow in spring–summer crop (FRR), FRR plus compost at 3 Mg ha⁻¹ crop⁻¹ (FRR + Comp), and biochar at 10 Mg ha⁻¹ crop⁻¹ (FRR + BC). Salt contents and hydraulic properties of soils, plant biomass, and plant uptake of cations were investigated. Soil bulk density (BD), exchangeable sodium (Na⁺), and exchangeable sodium percentage (ESP) were reduced remarkably by biochar application. Biochar application significantly increased other soil properties including total porosity, saturated hydraulic conductivity (Ksat), soluble and exchangeable potassium (K⁺), K⁺/Na⁺ ratio, available P, and total C. Compost application also improved BD, total porosity, and available P, but not exchangeable Na⁺ and ESP. Total aboveground biomass of rice showed a trend of FRR + BC > FRR + Comp > FRR > RRR. Relatively higher K⁺ uptake and lower Na⁺ uptake in rice straw in FRR + BC resulted in a significant two times higher K⁺/Na⁺ ratio over other treatments. Our results highlight that biochar amendment is a beneficial option for reducing ESP and providing available K⁺ and P under salinity-affected P-deficient conditions, hence improving straw biomass.