Phosphate and micronutrient behaviour under flooded and aerated soil conditions
Phosphorus and micronutrient (Fe, Mn, Cu & Zn) behaviour was investigated under flooded and aerated soil conditions, typical of paddy rice and wheat rotations in Pakistan. Pyrophosphate was used in small proportions with orthophosphate to investigate its effects on P and micronutrient, particular Zn availability, during both laboratory incubations and glass house experiments. Hydrolysis of pyrophosphate, its effects on P sorption by lime and different soils with or without added "organic acids" were also studied. Flooded conditions during both laboratory incubations and glass house trials greatly increased the extractability of soil P in comparison to controls and orthophosphate treated samples. Moist aerated conditions enhanced the sorption of applied P, with further sorption being observed in alternating flooded and moist aerated conditions. Increases in the extractability of soil P were related to iron oxidases/hydroxides. Soil Fe, Mn and Cu increased with flooding whereas soil Zn decreased. Soil P availability increased with the addition of lime, the 10% level being more effective than 5%, either due to increases in pH and/or due to precipitation of iron, the active site for P sorption. Pyrophosphate with or without added Zn decreased p sorption by lime and the soils studied, with the greatest effects being observed in the presence of lime. Added pyrophosphate, in the presence of lime, also maintained lower pH values in the solution compared with orthophosphate treated samples. Among the soils, pyrophosphate proved to be more effective when added to a calcareous Pakistani soil rather than the local soils (brown earth + brown podzolic). Added "organic acids", like extracts of soil and rice straw, decreased P sorption whereas the addition of (0.1M) citric acid increased it. Added Zn decreased P sorption by agricultural lime and the soils studied. In the presence of added pyrophosphate, sorption of Zn by agricultural lime decreased whereas sorption increased in soils.