Assessment of Phosphorus Requirements of Wheat on Different Textured Alluvial Soils through Freundlich Type Equations

Once soil solution phosphorus (P) level optimum for plant growth is identified, P adsorption isotherms or their equations can further be used to estimate fertilizer P rates required to adjust this desired soil solution P level to obtain maximum yield. Surface soil samples were collected from a farmer's field area and research area. An adsorption study was conducted on Ustic Endoaquerts (S1 soil), Typic Calciargids (S2 soil), and Typic Torripsamments (S3 soil) to develop the Freundlich-type equations. Phosphorus adsorption data were obtained by equilibrating 10-g soil samples in 100 mL of 0.01 M calcium chloride (CaCl2) containing various amounts of monopotassium phosphate (KH2PO4). Values of 1/n (slope) ranged from 0.4827 to 0.6452 L kg-1. Based on 1/n values, it was inferred that each of the two S1 and S3 soils was homogeneous and S2 was not. The KF (mg P kg-1) values of S1, S2, and S3 soils were 92.45, 55.81, and 23.38, respectively. The highest amount of P (92.45 mg kg-1) was adsorbed at unit EPC in S1 soil, whereas the lowest amount (23.38 mg P kg-1) was adsorbed in S3 soil. Thereafter, 11 P fertilizer doses were calculated by these Freundlich-type equations to adjust different estimated soil solution P levels that were designated as treatments (0.05 to 0.90 mg L-1). Then field experiments on wheat (cv. Inqalab-91) were conducted according to a randomized complete block design (RCBD) on these soils to determine internal (plant tissue), external (soil solution), and fertilizer P requirements. Maximum wheat gain yield (Mg ha-1) was 6.82 with T5 (0.25 mg P L-1) on S1 soil, 5.96 with T5 (0.25 mg P L-1) on S2 soil, and 4.97 with T7 (0.40 mg P L-1) on S3 soil that was obtained by application of 196 kg P2O5 ha-1 on S1 soil, 142 kg P2O5 ha-1 on S2 soil, and 78 kg P2O5 ha-1 on S3 soil. Internal P concentration (%) associated with 95% of maximum wheat yield at booting stage was 0.32 in S1, 0.21 in S2, and 0.33 in S3 soil. In straw, it was 0.123% in S1, 0.080% in S2, and 0.108% in S3 soil. The internal P requirement of wheat grain was 0.39% in S1, 0.40% in S2, and 0.37% in S3 soil. External soil solution P requirement (ESPR) for 95% of maximum yield of wheat was 0.45 mg L-1 in S1 soil, 0.34 mg L-1 in S2 soil, and 0.44 mg L-1 in S3 soil. Quantity of P2O5 corresponding to ESPR values were 217 kg ha-1 on S1, 123 kg ha-1 on S2, and 60 kg ha-1 on S3 soil. Putting ESPR values in the respective Freundlich-type equation, P fertilizer rates (kg P2O5 ha-1) were estimated that were 282 on S1, 167 on S2, and 83 on S3 soil; Practically, 262, 156, and 78 kg P2O5 ha-1 was applied in the field to adjust soil solution P level (mg L-1) at 0.40 (T7), 0.30 (T6), and 0.40 (T7) in S1, S2, and S3 soil, respectively, that are somewhat less than determined ESPR values. Phosphorous doses applied to achieve a desired EPAS value or estimated from graphs against predicted ESPR values, or calculated from corresponding Freundlich-type equations using desired ESPR values are in close proximity to one another. Therefore, any of the techniques can be used interchangeably to estimate the P fertilizer requirement for optimum wheat yield.