Effect of Poultry Litter to Water Ratios on Extractable Phosphorus Content and its Relation to Runoff Phosphorus Concentrations
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Source factors with regard to phosphorus (P) loss in runoff waters are often soil test P and P content of fertilisers applied. More recently, water extractable P (WEP) fractions in fertilisers and animal manure have been the focus of many field and plot-scale studies. This study evaluated changes in the WEP content of six poultry litters by varying the extraction ratio from 1:10 (20 g fresh poultry litter to 200 ml water) to 1:200 (1 g fresh poultry litter to 200 ml water), and also the relation between WEP content determined at these different ratios and P concentrations in runoff water collected during rainfall simulations. Six different poultry litters were surface-applied at equivalent total P (TP) application rates (67 kg [TP] ha-1) to 152 by 610 m grass plots on a 5% slope which received artificial rainfall at 5 cm h-1 until 30 min of continuous runoff was observed. Four plots were used per treatment including four control plots (no poultry litter application). Water-extractable P content of the various poultry litters increased with a decrease in the amount of poultry litter used in the extraction, i.e. from an extraction ratio of 1:10 to 1:200. However, these results also suggest that the 1:10 extraction ratio may extract different fractions of WEP compared to the 1:200 extraction ratio, and that some variability in dry weight extraction ratios existed when using fresh poultry litter. Water-extractable P application rates were positively correlated to P concentrations in runoff waters from the small plots. This study reaffirmed the importance of WEP content of poultry litter when determining the potential for P loss in surface runoff but demonstrated differences in the relation between runoff soluble reactive P (SRP) concentrations and WEP application rates related to the runoff to rainfall ratio.
