| Abstract: | If soil solution phosphorus (P) optimum levels for plant growth (external P) are known, P adsorption isotherms or their equations could further be used to assess how much fertilizer P may be needed for optimum plants yield (QFPN) by adjusting this known external solution P requirement in the soil (ESPR). 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 two-surface Langmuir-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). Thereafter, 11 P fertilizer rates were calculated by two-surface Langmuir-type equations to adjust different estimated soil solution P levels (EPAS) that were designated as treatments (0.05 to 0.90 mg L?1). Then field experiments on lentil (cv. Niab Masoor 2002) 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 lentil seed yield (Mg ha?1) was 0.87 with T4 (0.17 mg P L?1) in S1 soil, 1.8 with T3 (0.20 mg P L?1) in S2 soil, and 0.73 with T7 (0.28 mg P L?1) in S3 soil, obtained by applying 170 kg P2O5 ha?1 in S1 soil, 110 kg P2O5 ha?1 in S2 soil, and 78 kg P2O5 ha?1 in S3 soil. Internal P concentrations (%) of the whole plant associated with 95% of maximum lentil seed yield at flowering stage were 0.245, 0.210, and 0.315 in S1, S2, and S3 soils, respectively. Internal P requirements of lentil seed were 0.290 in S1, 0.245% in S2, and 0.380% in S3 soil. The ESPRs for 95% of maximum yield of lentil were 0.16 mg L?1, in S1 soil, 0.23 mg L?1 in S2 soil, and 0.27 mg L?1 in S3 soil. The QFPN estimated from graphs corresponding to these ESPR values were 160 kg P2O5 ha?1 in S1 soil, 125 kg P2O5 ha?1 in S2 soil, and 74 kg P2O5 ha?1 in S3 soil. The QFPNs estimated from corresponding two-surface Langmuir-type equation by using respective ESPR values were 164, 127, and 75 kg P2O5 ha?1 in S1, S2, and S3 soil, respectively. Field-applied P2O5 amounts to adjust soil solution P levels (mg L?1) at 0.166 (T4), 0.229 (T4), and 0.281 (T7) were 170, 126, and 78 kg ha?1 in S1, S2, and S3 soil, respectively. Based on the results of these studies, we propose that QFPNs estimated by graphs against identified ESPR values or calculated by the use of corresponding two-surface Langmuir-type equations are in close proximity to the field-applied P to adjust desired EPAS value. Therefore, either of the two techniques may be used to estimate QFPN for optimum lentil yield. Close |
