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
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