PNW 508 R EVISED F EBRUARY 2015 Fertilizing with Biosolids A P ACIFIC N ORTHWEST E XTE N SIO N P UBLIC ATIO N O REGO N S TATE U N IVERSITY , W A SHI N GTO N S TATE U N IVERSITY , U N IVERSITY OF I DA HO Fertilizing with Biosolids D.M. Sullivan, C.G. Cogger, and A.I. Bary What are biosolids? Biosolids are a product of municipal wastewater treatment. Raw sewage solids must be processed to meet U.S. Environmental Protection Agency (USEPA) standards before they can be called biosolids. Biosolids contain organic matter and nutrients that are beneficial for soil, crop, and livestock productivity. This publication focuses University State Washington I. Bary, Andy on how biosolids can be used to supply nutrients for crop production. Table 1. Biosolids organic matter and macronutrients Biosolids as a nutrient source (dry weight basis).a Table 1 shows the typical macronutrient Usual range b content of biosolids produced by Pacific (%) Northwest wastewater treatment facilities. These Nutrient Low High nutrients include both rapidly and slowly available Organic matter 45 70 forms. Biosolids also provide plant-essential Nitrogen (N) 3 8 micronutrients, including copper (Cu), boron (B), Phosphorus (P)c 1.5 3.5 molybdenum (Mo), zinc (Zn), and iron (Fe). Sulfur (S) 0.6 1.3 Calcium (Ca) 1 4 Biosolids usually are applied at rates designed Magnesium (Mg) 0.4 0.8 to supply crops with adequate nitrogen (N). The Potassium (K)c 0.1 0.6 other nutrients they contain also reduce fertilizer aNutrient concentrations are expressed on a total elemental basis. requirements. Not all elemental content is in a plant-available form. bUsual range for freshly digested biosolids. Lagooned biosolids, For all nutrients except N, soil testing can composted biosolids, and alkaline-stabilized biosolids typically have be used to monitor the change in soil fertility lower nutrient concentrations. resulting from biosolids application. A long-term cPhosphorus and potassium are expressed on an elemental basis. Use the following conversion factors to convert to units used for monitoring plan to track soil nutrient levels over fertilizer marketing: To get P2O5 (phosphate), multiply P x 2.29. To get the years should use consistent sampling and K2O (potash), multiply K x 1.2. analysis methods. Extension publications listed in “For more information” (page 16) provide guidelines for designing a sampling and testing program. Dan M. Sullivan, Extension soil scientist, Oregon State University; Craig G. Cogger, Extension soil scientist, Washington See the sidebar “Fertilizer replacement value of State University; and Andy I. Bary, senior scientific assistant, biosolids” (page 3) for more information. Washington State University. 2 Biosolids nitrogen (N) produced by anaerobic digestion and dewatering (the most common product offered to farmers), Forms of N about 80 percent of total N is in the organic form, Biosolids contain organic and ammonium-N; with the remainder present as ammonium-N. nitrate-N is absent in most biosolids. Heat-dried biosolids contain more than 90 percent Ammonium-N is available to plants immediately organic N and a trace of ammonium-N. after application. Organic N provides slow-release Plant-available N N (Figure 1, page 4). Calculating biosolids application rates based on N The proportions of ammonium and organic A companion publication, PNW 511-E, N in biosolids are related to the stabilization Worksheet for Calculating Biosolids Application process (e.g., digestion, composting) used Rates in Agriculture, provides a step-by-step at the wastewater treatment facility. Liquid, process for calculating application rates that anaerobically digested biosolids often contain supply crops with adequate available N. The more ammonium-N than organic N. In biosolids worksheet estimates ammonium-N retained and Fertilizer replacement value of biosolids Table 2 estimates the fertilizer replacement value of biosolids. Soil quality benefits are difficult to express anaerobically digested biosolids N, P, K, and S for the in simple economic terms and are unique to every first year after application. This estimate is based on location (see page 10). typical biosolids analyses and estimates of nutrient To convert percentages given for P and K in Table 2 availability from university field trials. to units used in fertilizer marketing, use the following With the exception of N, the fertilizer replacement conversion factors: value for nutrients depends on existing soil test • To get P205, multiply P by 2.29. values and the cropping system. Typically, a biosolids • To get K O, multiply K by 1.2. application to meet crop N requirements eliminates the 2 need for annual P and S applications. For example, biosolids with 2.5 percent P and The fertilizer replacement values shown in Table 2 do 0.3 percent K contain 5.7 percent P2O5 (phosphate) and not include the potential benefits to soil quality from 0.36 percent K2O (potash). Table 2. Approximate first-year fertilizer replacement value of anaerobically digested biosolids. Biosolids nutrient content and value Available Nutrient Fertilizer replacement value Total nutrient nutrient value of biosolids nutrient Nutrient Expressed as (% dry wt) (% of total nutrient)a ($/lb)b ($/dry ton) Nitrogen N 5.0 35 0.57 19.95 Phosphorus P 2.5 40 1.09 21.80 Potassium K 0.3 100 0.69 4.14 Sulfur S 1.0 35 0.38 2.66 Total — — — — 48.55 aEstimated plant-available nutrient released in the first year after biosolids application, based on Pacific Northwest field research byashington W State University and Oregon State University. bBased on 5-year average national inorganic fertilizer prices (2009–2013) compiled by USDA Economics Research Service: urea ($521/ton), diammonium phosphate ($643/ton), potassium chloride ($641/ton), and ammonium sulfate ($421/ton). The actual cost of a pound of nutrient from inorganic fertilizer varies, depending on nutrient form and analysis, transportation charges, market conditions, and the quantity purchased. Cost of fertilizer application is not included. 3 Figure 1. Nitrogen cycling: Forms and pathways following biosolids application. Biosolids contain both organic-N and ammonium-N. If biosolids are not immediately incorporated by tillage, some of the applied ammonium-N is lost as ammonia to the atmosphere. In the soil, ammonium-N is rapidly converted to nitrate-N by soil microbial activity (indicated by dashed arrows). Both ammonium-N and nitrate-N can be used by crops. Nitrate-N can be lost to the atmosphere through denitrification or to groundwater via leaching. Nitrogen loss can be minimized by matching the biosolids application rate to crop N need. Organic N must be converted to ammonium-N by microbial activity before plants can use it. Figure by Dan Sullivan. organic N mineralized from different types of the biosolids ammonium-N is lost as ammonia biosolids. It uses the following general equation to gas. Ammonia loss is very rapid during the first forecast plant-available N release from biosolids: hours after application. a (ammonium-N in biosolids) + b (organic N in biosolids) Tillage or sprinkler irrigation with clean water + c (organic N from previous biosolids applications) immediately following biosolids application where: increases ammonium-N retention in soil. a = fraction of ammonium-N retained after application However, tillage or immediate irrigation is not b = fraction of organic N mineralized during the first feasible or desirable in many cropping situations. growing season When biosolids are not incorporated, c = fraction of organic N mineralized from previous ammonium-N retention is greater for liquid biosolids applications on the same field biosolids than for dewatered cake biosolids. Ammonium N retained after application With liquid biosolids, some of the ammonium-N When biosolids are surface applied (not tilled infiltrates below the soil surface, thereby reducing or injected into soil at application), a portion of ammonia loss. 4 Organic N mineralized during mineralization occurs within the first 3 to 6 weeks the first growing season after application following biosolids application. Following field application, organic N in Organic N mineralized biosolids is converted to plant-available forms from previous biosolids applications (ammonium and nitrate) by soil microorganisms The organic N not mineralized during the first through a process known as mineralization. The year after application is mineralized slowly in biosolids treatment process affects the organic succeeding years. Thus, when biosolids are applied N mineralization rate after land application. repeatedly to the same site, the annual application Freshly digested biosolids usually contain more rate needed to meet crop needs is reduced mineralizable N than do biosolids produced with (Figure 2). more intensive stabilization processes (composting or long-term lagoon storage). Field trials have demonstrated that crop N fertilizer requirements are reduced for several The organic N mineralization rate also is years following biosolids application. For example: affected by soil temperature and moisture conditions. Mineralization is most rapid when soil • In dryland cropping systems in the is moist and warm (above 60°F). When biosolids 10- to 14-inch precipitation zone in central are applied to dry soil and not incorporated by Washington, the amount of N fertilizer tillage, N mineralization is delayed until moisture needed to attain maximum yield was is present. reduced from approximately 60 lb N/a to 20 lb N/a for the second crop following Mineralization proceeds most rapidly an agronomic biosolids application immediately after biosolids application, (3 dry ton/a). provided soil temperature and moisture are favorable. Usually, more than half of first-year N • In western Washington, N fertilizer needs for a cool-season grass forage (tall fescue) were reduced by about 60 lb N/a during the second year following a one-time application of biosolids at 4 dry ton/a. The worksheet used for estimating agronomic application rates of biosolids (PNW 511-E) explains how to estimate credits for N mineralized from biosolids applied in previous years.