Phosphorus and zinc availability in selected calcareous soils from the Khost Province of Afghanistan.

摘要

Phosphorus (P) and zinc (Zn) deficiencies are common in calcareous soils. Calcareous soils make up some of the most productive agricultural lands in the Mandozai district of the Khost province in southeastern Afghanistan and little is known about the phosphorus and zinc fertility status of these soils. Our objectives were to determine the soil fertility status of 6 soils collected from the Mandozai district and also to conduct P and Zn incubation studies on these soils to determine the fixation and availability of added P and Zn. The experiment design was a randomized complete block with 3 replications of each treatment. Soils used in this study had pH > 8.0, free CaCO3 contents of 6-10% and texture ranged from loam to clay loam. Seven rates of P (0, 5.6, 10.9, 16.4, 21.8, 32.8, 43.7 mg kg-1) or Zn (0, 1, 2, 3, 4, 5, 6 mg kg-1) were added as either monocalcium phosphate [Ca (H2PO4)·2H2O] or zinc sulfate (ZnSO4·7H2O). Soils were incubated at approximately 0.03 MPa soil tension for 15, 30, 45, 60, 75, 90, 105, and 120 days at 25 ° C and the Mehlich 3 soil test was used to determine available P and Zn. Mehlich 3 extractable P and Zn did not change consistently with time so data were averaged over all 8 sampling periods to determine the effects of P and Zn rate on Mehlich 3 P and Zn soil test levels. Soils segregated into two groups of three soils each that responded similarly in their response to P applications. Approximately 45 percent of the applied P remained available in one group of soils, while in the other group of soils, approximately 55 percent of the applied P remained available. The availability of added Zn was similar in four of the six soils. In these soils nearly 80% of the added Zn remained available. In another soil there was no detectable fixation, and in one other soil nearly all of the added Zn was fixed except at high application rates. None of the soil characteristics measured completely explained the observed differences in P or Zn availability following application of these important plant nutrients.