Water Treatment Sludge Influence on the Growth of Sorghum-Sudangrass

Disposal of water treatment sludges (freshwater coagulant sludges), which are primarily amorphous Fe and Al compounds, poses a management problem for most municipalities in Colorado and other states. Previous studies indicate that water treatment sludges have a high capacity to fix P and that plant P deficiencies develop when plants are grown in sludge-soil mixtures. No attempt has been made to quantify the ability of water treatment sludges to fix P to determine specific application rate recommendations. This research was initiated to determine acceptable application rates of three types of water treatment sludges added to the Colby (Ustic Torriorthent; pH = 7.5) and Red Feather (Lithic Cryoboralf; pH = 5.5) soils. A greenhouse study was conducted utilizing mixtures of an alum, iron, or organic polymer sludge combined with each of the two soils at rates of 0, 5, 10, 15, 20, and 25 g kg⁻¹. Three cuttings of sorghum-sudangrass [Sorghum bicolor L. Moench ‘NB280S’-S. sudanense (Piper) Stapf] were taken. The Langmuir adsorption isotherm approach gave an accurate prediction of the relative fixation capacity of different sludges and consequently the effect of different sludge rates on P uptake by plants. Total plant yield and plant uptake data indicated that positive plant growth responses to sludge above those in the control (0 g kg⁻¹) were the result of improved Fe availability in the Colby soil, and possibly increased soil pH in the Red Feather soil. Yields declined at higher sludge application rates (15 – 25 g kg⁻¹) as compared to low application rates (5 – 10 g kg⁻¹) probably due to increased P fixation by sludge. Plant concentrations of Al, Cu, Mn, Ni, and Pb were not high enough to decrease plant growth; however, Cd concentrations associated with 20 and 25 g alum sludge kg⁻¹ Red Feather soil were greater than 2 mg kg⁻¹. This is the upper limit reported for normal concentrations in plants and it may pose a threat to livestock that consume this forage. Contribution from the Dep. of Agronomy and Colorado Agric. Exp. Stn. Project 15-2924.