From Agronomists of the
Potash & Phosphate Institute
655 Engineering Drive, Suite 110
Norcross, Georgia 30092-2837
Phone (770) 447-0335

Winter 1999, No. 1


Biosolids, fly ash, paper mill sludge…many waste materials are available today. They are produced in amounts too large to continue to landfill forever. For this reason, they are increasingly being applied to cropland, just as farmers have been doing with animal manures for centuries. These materials contain nutrients that can substitute for fertilizers. Yet, their nutrient supply is not balanced. Some materials, in fact, may decrease rather than increase the availability of certain nutrients.

    About 100 million tons of coal combustion by-products such as fly ash are produced each year across North America. They contain many elements, just like the coal they were derived from. Fly ash materials have corrected plant nutritional deficiencies of boron, magnesium, molybdenum, sulfur and zinc. Some can correct soil pH. Sometimes they supply too much boron, at levels toxic to plants. Recent research at the University of Georgia found that fly ash additions to soil aggravated deficiencies of phosphorus and magnesium in plants, and did not supply adequate potassium.

    Coal combustion by-products applied to soils can diminish the soluble forms of phosphorus more than other fractions. In recent Pennsylvania research, flue gas desulfurization by-products or fluidized bed combustion fly ash reduced water-extractable phosphorus by 48 to 71 percent, while reducing Mehlich-III soil test phosphorus by 8 to 13 percent. These materials can potentially keep the more soluble forms of phosphorus out of runoff water, but they could also affect its availability to plants.

    Biosolids can improve degraded soil by increasing organic matter. The potential to enhance soil productivity with materials like paper mill sludge is substantial, but building soil organic matter requires nitrogen and affects the availability of many micronutrients. Sewage sludge biosolids are often low in potassium. Depending on how the sludge was separated from the effluent, the phosphorus may or may not be available to plants. Research in Ontario found the extractability of phosphorus in several sewage sludges ranged from 1 to 6 percent, compared to 30 to 70 percent in composted livestock manure.

    Waste materials vary not only in nutrient content, but also in nutrient availability. A complete analysis can help predict what will be needed to bring plant-available nutrients into balance, but it is not simple. Large amounts of calcium, for example, can precipitate phosphorus and imbalance potassium. Adding materials at high rates can change the interpretation of the soil test.

    Best management of waste materials on cropland requires:

Many waste materials contain valuable nutrients for crops. Managed well, and balanced with fertilizer, they can help keep soils productive. Be sure to monitor carefully what they do for your crop.


For more information, contact Dr. Tom Bruulsema, Eastern Canada and Northeast U.S. Director, PPI, 18 Maplewood Drive, Guelph, Ontario N1G 1L8, Canada. Phone: (519) 821-5519; E-mail:
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