APPLYING BIOSOLIDS: ISSUES FOR VIRGINIA AGRICULTURE

The value of biosolids and the potential for environmental damage from surface runoff and subsurface
leaching varies by the mineralization rate for the site and the characteristics of the biosolids. Mineralization
rates, or the rate at which nitrogen (N) tied-up in organic matter becomes available to plants in ammonium
(NH₄) and nitrate forms (NO₃), varies with the source as well as site-specific characteristics such as
temperature, rainfall, elevation, slope, aspect, latitude, and ground cover. Mineralization rates are generally
greater in the coastal plains than in the Piedmont or mountains and valleys because of the warmer, wetter
climate. Lime stabilized and aerobically digested biosolids have mineralization rates that are faster than
anaerobically digested biosolids (Table 2). Other nutrients, including phosphorous, potassium, and
micronutrients, are also available in subsequent years to benefit later crops. In drought years, these
residual nutrients are harder to estimate. But their benefit in the soil after a drought year is pronounced
simply due to lowered crop uptake.

Table 2. Plant Available N after Biosolids Application

Source: Gregory Evanylo, Land Application of Biosolids for Agricultural Purposes in Virginia,
VCE Pub. 452-300, 1999, p.18

BIOSOLIDS

The nutrient content and percent solids varies by type of biosolids (Table 3). To estimate the pounds of
nutrients per ton of dry biosolids, the nutrient content percent is multiplied by 2,000 pounds. For example,
lime-stabilized biosolids average around 3 percent total N, including both organic and inorganic forms;
therefore, one ton will contain approximately 60 pounds total N (0.03 * 2,000 pounds = 60 pounds total
nitrogen). Five to seven dry tons of dry biosolids are typically applied per acre of crop or pasture land in
Virginia; therefore, approximately 360 pounds total N per acre are commonly applied through biosolids
(0.03 * 2,000 pounds * 6 tons = 360 pounds total N). However, total N and plant available nitrogen
(PAN) from biosolids are not the same. Most of the N contained in biosolids is organic, which is a slow
release form. Thus, the PAN is a fraction of the total N contained in the material. The amount of PAN
is a function of the NH₄ and NO₃ that are already available to the plant and the mineralization process.
For lime-stabilized biosolids, the PAN during the first year after application is about 30 percent. In the
360 pounds total N applied example, approximately 108 pounds of organic N (360 pounds * 0.3 = 108
pounds PAN) would be mineralized into inorganic forms and be available for plant uptake during the first
year after application.

Source: D. Steven McMahon, Technical Services Administrator, Wheelabrator Water Technologies Inc.,
Bio Gro Division, personal communications, February 19, 1996.

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