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.

Key words:

• Phosphorus
• corn
• sudangrass
• sludge