A field study was conducted on a Decatur silty clay loam (clayey, kaolinitic, thermic rhodic paledult) soil field plots which received Huntsville and Chicago sludges at 20 Mg ha#x2010;1for five consecutive years and also on field plots which received an single application of Chicago and Huntsville sludges at 100 Mg ha#x2010;1over a ten year period. Indicator crops corn (Zea mays, L.) and sudangrass (Sorghum sudanenses, L.) were grown on these plots for one season to assess the availability to crops of residual P from sludge#x2010;treated soils. The results of this study indicated that P accumulation in corn tissue was greater at the early clipping stage (39 days after planting). However, P accumulation was greatest at the milk to dough stage (79 days after planting). The extractable soil P in top 0#x2013;15 cm depth by using various extracting procedures generally followed the order of: Bray P#x2010;1 Mehlich 3 Mehlich 1 Morgan (before planting), and Bray P#x2010;1 Mehlich 1 Mehlich 3 Morgan (after harvesting). For all extractants, both Huntsville and Chicago sludges when applied at 20 Mg ha#x2010;1yr#x2010;1split treatments resulted in significantly higher extractable P than the single 100 Mg ha#x2010;1treatment. However, the Huntsville sludge treatments generally resulted in higher P levels than the Chicago sludge treatments at each corresponding application rate. Regression analysis between tissue concentrations and extractable soil P concentrations indicated that the highest correlation (r=0.62**) for corn occurred with the Morgan extractant at the early clipping stage. However, for sudangrass the highest correlations were found with the Mehlich 1 (r=0.77**) and Bray P#x2010;1 (r=0.74**) extractants, both also at the early clipping stage. This suggests that these extractants could be useful in predicting the P uptake by crops, especially when done during a specific stage of growth.
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