Availability to crops of residual p from a sludge‐treated soil

Abstract

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‐1 for five consecutive years and also on field plots which received an single application of Chicago and Huntsville sludges at 100 Mg ha‐1 over 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‐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 0–15 cm depth by using various extracting procedures generally followed the order of: Bray P‐1> Mehlich 3> Mehlich 1 > Morgan (before planting), and Bray P‐1 > Mehlich 1 > Mehlich 3 > Morgan (after harvesting). For all extractants, both Huntsville and Chicago sludges when applied at 20 Mg ha‐1 yr ‐1 split treatments resulted in significantly higher extractable P than the single 100 Mg ha‐1 treatment. 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‐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.