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DIVISION S-4—SOIL FERTILITY & PLANT NUTRITION

Inorganic and Organic Phosphorus Fertilizer Effects on the Phosphorus Fractionation in Wetland Rice Soils

^a Soil Science Division, Bangladesh Rice Research Institute, Gazipur 1701
^b Natural Resource Group, CIMMYT, Bangladesh

^* Corresponding author (c.meisner{at}cgiar.org)

Long-term effects of rice (Oryza sativa L.) cultivation with varying nutrient management on soil P fraction are important to understand from soil nutritional and environmental point of view. Soil P fractionation gives an idea about the soil P supplying capacity to plants. The present experiment was conducted to evaluate the effect of different nutrient management in wetland rice on the changes of soil P fraction at different depths. Soil samples from five depths (0–5, 5–10, 10–15, 15–30, and 30–50 cm) were collected from a long-term experimental field classified as a Chhiata clay loam, hyperthermic Vertic Endoaquept. The field received six treatments for 10 yr: absolute control with no fertilizer applied (T₁), one-third of recommended fertilizer doses (T₂), two-thirds of recommended fertilizer doses (T₃), full doses of recommended fertilizers (T₄), T₂ + 5 Mg cow dung (CD) and 2.5 Mg ash ha^–1 (T₅), and T₃ + 5 Mg CD and 2.5 Mg ash ha^–1 (T₆). The apparent balance of P compared with the initial P status after 10 yr varied from –115 kg ha^–1 under T₁ to 348 kg ha^–1 under T₆. The P fractionation study was conducted over the treatments and soil depth. Treatment and depth had no significant effect on solution P. Larger concentrations of NaHCO₃ soluble P, NaOH extracted inorganic P (P_i), and acid P were observed under treatments with organic fertilizers (T₅ and T₆) than with other treatments at 0- to 5-, 5- to 10-, and 10- to 15-cm depths. The concentrations of NaHCO₃–P, NaOH-P_i and acid P fractions were lowest under T₁ and T₂ treatments. At 15 to 30 cm or lower soil depths, none of the P fractions were affected by treatments. The change in NaOH organic P (P_o) and residual P (extracted with HNO₃ + HClO₄) with soil depth was not significant, and the differences in these P fractions under the tested P treatments were not large. The depletion of NaHCO₃–P and NaOH-P_i at the 0- to 15-cm depth under control and T₂ suggests that the rice plant depends upon these fractions of P. The P depletion profile in wetland rice appears to be confined within the first 15-cm depth. The mean P uptake by rice showed a polynomial relationship with NaHCO₃–P and NaOH-P_i (average of 0–15 cm) and it was linearly correlated with acid P (0–15 cm).

Abbreviations: CD, cow dung • HYV, high yielding varieties • P_i, inorganic phosphorus • P_o, organic phosphorus • T₁, absolute control with no fertilizer applied • T₂, one-third of recommended fertilizer doses • T₃, two-thirds of recommended fertilizer doses • T₄, full doses of recommended fertilizers • T₅, T₂ + 5 Mg cow dung and 2.5 Mg ash ha^–1 • T₆, T₃ + 5 Mg CD and 2.5 Mg ash ha^–1