Contribution of Inorganic Phosphorus Fractions to Plant Nutrition in Alkaline-Calcareous Soils

To evaluate the relationships between soil inorganic phosphorus P (Pi) fractions, the soil P test and plant parameters such as plant P uptake, dry matter yield, tissue P concentra-tion and relative yield, glasshouse experiments and chemical analyses were conducted on ۱۳ calcareous soils from six agricultural and seven adjacent bushland (virgin soil) sites. Four rates of P (۰, ۱۵, ۳۰, ۶۰ mg/kg soil) were applied as reagent-grade KH۲PO۴ to the soils in a randomised complete block design with three replications. Perennial ryegrass (Lolium perenne cv. Roper) was grown and forage was harvested five times over a period of ۲۱۰ days. Successive harvesting resulted in the depletion of plant available P as meas-ured by NaHCO۳-extractable P, which coincided with the decrease in the plant dry matter yield and P uptake. After five harvests, the order of reduction in Pi fractions induced by cropping without added P was Ca۱۰-P>Al-P>Ca۲-P>Ca۸-P>occluded-P>Fe-P for the vir-gin soils and Ca۲-P>Al-P>Ca۱۰-P>Ca۸-P>Fe-P>occluded-P for the agricultural soils. The order of abundance of Pi fractions for P treated-soils was non-occluded Al and Fe phos-phate (Al-P+Fe-P)>secondary Ca-bound P (Ca۲-P+Ca۸-P)>acid-extractable P (Ca۱۰-P)>occluded-P for both virgin and agricultural soils. Although a marked proportion of added P was transformed into less soluble Al and Fe phosphates, successive harvesting had depleted considerable percentages of these fractions. Highly significant (p<۰.۰۰۱) re-lationships were found for P uptake vs. Olsen-P, P uptake vs. Pi fractions (Ca۲-P, Ca۱۰-P, Al-P, Ca۸-P, Fe-P) and Olsen-P vs. Pi fractions. NaHCO۳-extractable P seems to be ade-quate for evaluating plant available P in calcareous soils. However, the closer relationship for the Fe-P fraction vs plant P uptake than for Olsen-P versus plant P uptake indicates that NaHCO۳ may not provide the best estimate of plant available P for calcareous soils. Using stepwise regression analysis, it was found that the Ca۲-P fraction was most predic-tive of P uptake (۶۰%), total dry matter (۶۸%), relative yield (۷۴%) and Olsen-P (۶۹%), followed by the Fe-P fraction.

کلیدواژه ها:

Calcareous soils ، Availability ، Inorganic P fractions ، NaHCO۳-extractable P ، Plant P uptake