Land-Applied Animal Waste and Water Quality

Linda Benedict, Wang, Jim Jian, Gaston, Lewis A.  |  11/23/2011 10:19:07 PM

Jim J. Wang and Lewis Gaston

The mantra of the environmental movement reads – Reuse, Reduce, Recycle. In this regard, the ancient farming practice of land application of animal waste is an environmental trifecta – excreta is reused as a fertilizer, thereby reducing the application of inorganic fertilizer while recycling organic matter back into the soil. However, the adage "all things in moderation" also applies. High rates of animal waste application to land or long-term application of high rates may lead to runoff or seepage of nutrients into streams, lakes and rivers and change the aquatic ecology for the worse.

Application of animal waste as fertilizer has been traditionally based on its nitrogen content, which is typically the most limiting nutrient for crops. However, animal waste contains many other nutrients, and they are not balanced with nitrogen in proportion to crop needs. In particular, the concentration of phosphorus is typically much higher than needed so that continual land application of animal waste tends to build up soil phosphorus. The result may be greater phosphorus loss (Figure 1), which could be a problem because phosphorus is typically the nutrient most limiting to algal and aquatic plant growth in freshwater. In addition, the loss of organic substances from animal waste could be also a concern because they generally enrich biological oxygen demand in the receiving water bodies.

Poultry is the largest animal agriculture enterprise in Louisiana and had a gross farm value of more than $800 million in 2010, most of which came from the 900 million pounds of broilers produced. A byproduct was about 200,000 tons of poultry litter. Dairy waste from confinement systems amounts to about 100,000 tons per year. To better ensure this amount of animal waste fertilizer does not become a nutrient source for water eutrophication, producers use a nutrient management plan. One aspect of it is the P-index – a simple assessment of the risk that some phosphorus in the fertilizer and already in the soil will be lost into nearby water bodies. The P-index used in Louisiana considers source factors such as amount of fertilizer applied, level of soil phosphorus, and loss factors such as soil permeability and slope. However, many factors affect nutrient conservation, and the ability to accurately predict the fate of fertilizer phosphorus and soil phosphorus is far from perfect. LSU AgCenter scientists are making efforts to improve it. In addition, LSU AgCenter scientists have developed a tool called the soil suspension turbidity test to account for particulate phosphorus in surface runoff based on simple turbidity measurement. This method works well especially for more eroded soils.

Furthermore, many different approaches for reducing potential phosphorus loss are being examined. These include:

  • Amending the animal waste or the soil with substances that decrease phosphorus solubility.
  •  Using hay harvest to phytoremediate high phosphorus soil.
  • Variable rate application depending on the spatially variable capacity of soil to retain added phosphorus. 
  • Alternative uses of animal waste fertilizer as in pine plantations and with potential bioenergy grasses.

One promising approach to reducing the loss of soluble nutrients and contaminants is to change the chemistry of animal waste and waste-amended soils by use of certain additives. This approach appears to have a broad spectrum of benefits, not only reducing the solubility and release of nutrients and organic materials in general but also specific organic substances such as veterinary antibiotics sometimes found in animal waste.

Both pure chemicals and some industry byproducts such as bauxite-refining residues, known as red and brown mud and containing iron and aluminum oxides, have been investigated. These amendments can strongly adsorb phosphorus as well as trace metals such as copper and zinc, immobilizing them into slowly released forms. Initial results showed that both red mud and brown mud decrease the leachability of nutrients and trace metals found in poultry litter and cattle manure. Brown mud showed strong reduction in water-soluble phosphorus, whereas red mud had strong reduction in water-soluble trace metals. In addition, bauxite residue reduced the survival rate of fecal coliform by 93 percent and 98 percent in poultry litter. See Figure 2.

Adding red mud to phosphorus-rich, manure-affected soils decreased phosphorus leachability by an average of 88 percent. Red mud also significantly decreased the leachability of trace metals and carbon and reduced five-day biological oxygen demand.

The effects of alum, montmorillonite and red mud amendments on reducing the runoff of tylosin, a commonly u veterinary antibiotic, from poultry litter were also investigated. Both alum and especially montmorillonite significantly reduced tylosin in runoff from small plots whereas red mud had no effect.

The results of this research demonstrate that different techniques and approaches can be used to manage animal wastes as well as animal waste-affected agricultural soils to minimize their potential impact on water quality.

Jim J. Wang, Professor, and Lewis Gaston, Associate Professor, School of Plant, Environmental & Soil Sciences, LSU AgCenter, Baton Rouge, La.

(This article was published in the fall 2011 issue of Louisiana Agriculture magazine.)

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