Submitted to: National Cotton Council Beltwide Cotton Conference
Publication Type: Proceedings
Publication Acceptance Date: 1/6/2006
Publication Date: 6/9/2006
Citation: Funk, P.A., Armijo, C.B., Lindemann, W.C., Lewis, B.E., Flynn, R.P. 2006. Fertility and toxicity of potting soils prepared for ginning and dairy wastes anaerobic digestate. National Cotton Council Beltwide Cotton Conference. In: Proceedings of the Beltwide Cotton Conferences, January 3-6, 2006, San Antonio, Texas. 2006 CDROM. p. 402-409. Interpretive Summary: Distributed energy production from a renewable source is a good thing. Obtaining revenue from a waste treatment process is a good thing. The only possible drawback to the ginning and dairy waste conversion technology that is presently being developed is disposal of the solid residue. This study looked at the soil amendment potential of anaerobic digestate from a two-phase system by mixing the residue with sand and using it as a potting soil to grow lettuce. The amendment was compared to a conventional compost made from the same raw materials (dairy manure and cotton gin trash) by counting emerged seedlings (seed germination) and weighing the crop after six weeks (yield). The anaerobic digestate used in this experiment had water percolating through it each day, washing out nutrients that might remain in a commercial scale system where the water would be recirculated. Consequently, there were very low levels of nutrients and yield in the anaerobic digestate, compared to high yields and nutrient levels in the conventional compost. But the anaerobic digestate resulted in better seed germination, so it is not toxic to plants. And it is a good source of carbon. Anaerobic digestate from this process would be useful as a soil amendment where adding nutrients is undesirable, such as where existing nutrient levels meet expected crop uptake requirements and applying additional amounts may result in surface or ground water contamination.
Technical Abstract: Methane gas resulting from combining cotton gin trash and dairy manure in a two phase anaerobic digester is easily marketed. Digestate solids are not. This study was conducted to determine anaerobic digestate toxicity and its potential as a soil amendment. The same mixture of dairy manure and cotton gin waste was digested anaerobically in solid phase piles leached daily, and composted conventionally. Lettuce was planted in pots filled with different proportions of anaerobic digestate or conventional compost mixed with washed sand or native soil. Emergence was used as an indication of germination. Dry leaf mass was used to measure yield. Emergence was greater in anaerobic digestate mixtures, indicating that it is not toxic. Potting soil mixtures made with various amounts of conventional compost had one to two orders of magnitude greater nitrate (NO3-N) and produced one to two orders of magnitude greater yields. Anaerobic digestate may be needed where carbon is preferred over nutrients.