Effect of poultry litter to water ratios on extractable phosphorus content and its relation to runoff phosphorus concentrations

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 (approximately 67 kg TP ha(-1)) to 1.52 by 6.10 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.