Evaluation of iron oxide impregnated filter paper method as an index of phosphorus availability in paddy soils of Tanzania

Phosphorus is a major yield limiting nutrient in rice production and yet most soil test methods for predicting P availability to plants have a number of shortcomings especially under flooded conditions. The objective of this study was to evaluate the iron oxide impregnated filter paper (Pi) method for assessing changes in P availability in soils subsequent to flooding and to determine the suitability of the method in assessing P status in paddy soils. The results indicated that available P increases considerably (between 34% and 256%) subsequent to flooding and this affected responsiveness of the soils to P application. Two versions of the Pi method namely; the Pi method used under flooded conditions (Pi-P_f) and Pi strips embedded directly in flooded soils (Pi-P_fe) were effective in assessing P availability in paddy soils. The two Pi methods were better correlated with rice dry matter yield than the traditional soil tests and are hence recommended for use in soil testing under flooded conditions. The tentative critical levels of P for the two procedures under pot conditions were 22 mg/kg for Pi-P_f and 15 mg/strip for Pi-P_ef.