Forms of Phosphorus in relation to Soil Maturity along a Toposequence under Hot, Dry, Subhumid Agro-ecological Subregion of West Bengal

Forms of soil phosphorus (P) are characterized in relation to soil properties in five pedons belonging to Inceptisols and Alfisols, occurring along a toposequence in Hot Dry Subhumid Agroecological Subregion (AESR 12.3) of West Bengal, falling under Chotanagpur plateau region. The soils are shallow (<0.5 m) to very deep (>1.5 m), sandy clay loam to clayey in texture, moderately acidic to neutral in reaction (pH_aq. 5.3–6.6), low to high in organic carbon content, low in NaHCO₃ extractable P (2.1–3.0 mg kg⁻¹ in the surface horizon), low to medium in NH₄OAc–extractable K (32.0–43.0 mg kg⁻¹ in the surface horizon) and fairly high in Fe₂O₃ content (15.0–18.8%). Based upon overall profile weighted mean (PWM) values maximum amount of P is in the form of reductant soluble P (RS-P) [109.9 mg kg⁻¹] while minimum amount is in the form of saloid P (S-P) [1.9 mg kg⁻¹] contributing 31.2% and 0.5% to the total P content, respectively. Sequential extraction of inorganic soil P fractions reveal that the mean relative abundance follow the order: reductant soluble phosphate (RS-P) > iron phosphate (Fe-P) > occluded-P = calcium phosphate (Ca-P) > aluminium phosphate (Al-P) > saloid phosphate (S-P). The overall relative abundance of the different inorganic P fractions in terms of their average ratios are: S-P (0.1): Al-P (0.8): Fe-P (2.2): Ca-P (1.0): RS-P (6.0): occluded-P (1.0). The PWM - total P content suggest that the soils are fairly rich in P reserve ranging from 279.3 to 533.2 mg kg⁻¹ with an overall mean of 352.3 mg kg⁻¹. Significant positive correlation was observed between S-P with OC (r = 0.504**) and total P (r = 0.653***). Close relationship between total P and S-P (r = 0.653***), Fe-P (r = 0.520**) suggested the existence of equilibrium between the former and the latter. The study reconfirms the poor P availability status of the soils under Chotanagpur plateau region of West Bengal. Soil maturity based upon PWM of total P is in the sequence: P5 (most matured) > P4 > P3 > P2 > P1 (least matured) which did not corroborate the maturity sequence inferred from soil taxonomy viz., P4 (most matured) > P1, P2, P3, P5 (least matured). Maturity sequence derived from weathering index (WI) based upon inorganic soil P fractions viz., P4 (most matured) > P3 > P2 > P5 > P1 (least matured) is in good agreement with that obtained from soil classification indicating a close relationship between the former and the later. The study emphasized the suitability of inorganic soil P based weathering index (WI) for adjudging soil maturity in terms of pedogenic development of the soils.