published in


July 2011

Soil amendments to reduce pH of bahiagrass pastures that received excessive biosolids application

Maria L. Silveira
University of Florida/IFAS

photo of Dr. Maria Silveira

For questions or comments regarding this publication contact: Maria L. Silveira

Proper application of biosolids on agricultural land can provide essential nutrients and increase crop yields. Biosolids can be used as a source of plant nutrients (especially N and P), and organic matter for pastures. Lime-stabilized biosolids are used by some livestock producers as a cost effective alternative to purchasing lime and fertilizer separately. During the biosolids treatment process, lime can be added to control pathogens, insect vectors, and odor. The pH of lime stabilized biosolids can range from 7 to 11. Nitrogen concentrations are usually between 3% and 5% (dry matter basis), and P concentration between 2% and 4%. Lime-stabilized biosolids can correct low soil pH. However, when improperly managed, repeated application of these materials can increase soil pH to levels that cause negative effects on forage production. Several bahiagrass pastures in Florida have reportedly received excessive amounts of lime-stabilized biosolids, resulting in pH values greater than 7. At this pH, bahiagrass roots cannot function properly to absorb sufficient iron, manganese, and other micronutrients. As a result, substantial portions of grass sod are lost to weeds in a manner similar to the symptoms of bahiagrass decline due to soil acidity. 

Various soil amendments have been suggested to reduce soil pH; however no information is available about their effectiveness or reliability on soils treated with lime-stabilized biosolids. A pilot study was conducted at the Range Cattle Research and Education Center to investigate the potential of various soil amendments to reduce soil pH of bahiagrass pastures with long-term histories of lime-stabilized biosolids application. Soil samples (0-6 inches) with pH of 8 were collected from four sites in central-south Florida: Hendry, Pasco, and Polk counties. Treatments included:

  • Control (no amendment)
  • Aluminum sulfate
  • Ammonium sulfate
  • Elemental sulfur (applied as tiger 900CR®)

Soil amendments were applied at a rate of 500 lb sulfur per acre. Samples were incubated in the laboratory for approximately 22 wks. Results showed that the acidifying potential of each amendment varied across locations. This is probably due to differences in site history (i.e. biosolids application loads). On average, ammonium sulfate reduced soil pH by 1.2 units (range varied from 0.4 to 1.7 pH units), while aluminum sulfate and elemental sulfur reduced soil pH by 0.5 and 0.3 units, respectively. Results showed that the effects of elemental S were transient, which suggests that repeated applications are likely required to maintain lower pH. Ammonium sulfate appeared to be an effective amendment for reducing pH of biosolids-impacted soils. Further investigations at field-scale are necessary to determine the impacts of amendments on forage production, validate application rates and recommendations, and assess economic practicality. We thank the county livestock agents and producers for providing the soil samples for the study.