AVAILABILITY TO CROPS OF RESIDUAL-P FROM A SLUDGE-TREATED SOIL

A field study was conducted on a Decatur silty clay loam (clayey, kaol initic, thermic rhodic paledult) soil field plots which received Hunts ville and Chicago sludges at 20 Mg ha(-1) for five consecutive years a nd also on field plots which received an single application of Chicago and Huntsville sludges at 100 Mg ha(-1) over a ten year period. Indic ator crops corn (Zea mays L.) and sudangrass (Sorghum sudanenses L.) w ere grown on these plots for one season to assess the availability to crops of residual P from sludge-treated soils. The results of this stu dy indicated that P accumulation in corn tissue was greater at the ear ly clipping stage (39 days after planting). However, P accumulation wa s greatest at the milk to dough stage (79 days after planting). The ex tractable soil P in top 0-15 cm depth by using various extracting proc edures generally followed the order of: Bray P-1> Mehlich 3> Mehlich 1 > Morgan (before planting), and Bray P-1 > Mehlich 1 > Mehlich 3 > Mo rgan (after harvesting). For all extractants, both Huntsville and Chic ago sludges when applied at 20 Mg ha(-1) yr(-1) split treatments resul ted in significantly higher extractable P than the single 100 Mg ha(-1 ) treatment. However, the Huntsville sludge treatments generally resul ted in higher P levels than the Chicago sludge treatments at each corr esponding application rate. Regression analysis between tissue concent rations and extractable soil P concentrations indicated that the highe st correlation (r=0.62*) for corn occurred with the Morgan extractant at the early clipping stage. However, for sudangrass the highest corr elations were found with the Mehlich 1 (r=0.77*) and Bray P-l (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 c rops, especially when done during a specific stage of growth.