Nutrient Availability from Treated Manures

Chris Baxter, Carrie Laboski, John Peters, Larry Bundy
Extension Soil Scientists
UW-Platteville, UW-Madison

 Overview of Manure Treatment Systems

Composting

Composting can be simply defined as managed decomposition of organic materials. Benefits of composting include reduction of manure mass and volume, pathogen, weed seed and odor reduction, and concentration of nutrients. The amount of nutrients (particularly N) lost or retained during composting depends largely on the initial composition of the material, whether or not the compost pile is covered or left open to the elements, management of pile moisture, aeration, and temperature during the composting process. In general, a well-managed cattle manure compost can be expected to lose between 50-60% of the total N due primarily to volatilization of ammonia-N (NH₃-N), but the change in N concentration is usually much less because the decrease in mass (largely carbon loss) during composting and concentration of nutrients. Because the composting process is aerobic, composts can contain significant amounts of nitrate-N (NO₃-N). The organic matter in composts is more stable; thus, mineralization of organic N occurs more slowly. Composts generally do not immobilize soil N because there is little readily decomposable organic material. Concentrations of P and K in compost are usually higher compared to raw manures because these nutrients are not lost via volatilization; however some of these nutrients may be lost via leaching.

Liquid-Solid Separation

Liquid-solid separation systems vary widely in the size of solid particles removed and the efficiency of solids removal. Most of the N and P in liquid manure is associated with the smallest particles, and therefore separation systems that do not remove these particles can be expected to have little impact on the nutrient content of liquid manures. When used in combination with chemical treatment that include flocculants such as polyacrylamide (PAM) or precipitants such as aluminum or iron salts, separation efficiencies for both solids and nutrients can improve dramatically and result in much lower N and P contents in the liquid fraction.  It has not been clearly identified through research how separation systems impact the availability of N and P in either the solid or liquid fractions of separated manures, but some assumptions can be made regarding nutrient availability of the end-products. Because mechanical separation systems preferentially remove high-carbon particles, separated liquids typically have a lower C/N ratio compared to raw slurries, possibly indicating greater N availability in separated liquids. Conversely, the use of aluminum or iron salts to precipitate P may lower P availability through precipitation of P.

Anaerobic Digestion

Anaerobic digestion is the decomposition of manure in an oxygen-free environment. Many different types of anaerobic digesters exist and range in complexity from relatively simple covered lagoon digestion systems to temperature-controlled complete-mix systems. The anaerobic digestion process concentrates nutrients in the solids by converting the readily decomposable carbon into biogas. In addition, some of the organic-N in manure is converted to ammonium-N during anaerobic digestion, making it more readily available to plants compared to raw manures. In one study, anaerobic digestion of dairy manure resulted in either an increase or decrease in the water extractability of manure P and was dependent upon the innoculum to substrate ratio used during the digestion. A subsequent study found that digestion did not change the predominate inorganic P forms.

Nitrogen (N)

Nitrogen in any organic source can be categorized as inorganic or organic N. In typical manures and liquid treated manures, nearly all of the inorganic N is present as ammonium-N (NH₄-N) and is considered immediately available to plants. In treated solid manures (particularly composts), a considerable portion of the inorganic-N may be present as nitrate-N  (NO₃-N). Organic N occurs as various forms that must be converted into NH₄-N through mineralization before it is available to plants. The rate and total amount of organic N that is mineralized depends not only on soil and environmental factors (moisture, temperature, soil texture, organic matter content), but also on the forms and degradability of organic-N in the manure. Further, if there is an excess amount of degradable carbon in the manure, it may result in immobilization or “tie up” of available N. The addition of bedding materials that are high in C relative to N (i.e. straw, corn fodder, and sawdust) typically have a negative impact on the availability of N from manure due to slower mineralization and immobilization of soil inorganic-N. Any treatment system that alters the inorganic-N, organic-N, or carbon content of the manure is likely to affect N availability. Materials high in C relative to N tend to immobilize N, while those low in C relative to N tend to result in a net increase in available soil N. The use of carbon to nitrogen ratio (C:N) has been demonstrated to be effective in predicting whether the addition of plant residues will result in a net increase or decrease in available soil N. Plant residues with a C:N less than 25:1-30:1 typically result in a net increase in available soil N during the growing season, while those with a C:N greater than 50:1 have a greater potential to immobilize soil N. However, C:N has not been thoroughly tested as a method for predicting N availability from manures or treated manures.

Predicting the available N in treated manure requires an accurate analysis of all forms of N present and knowledge of the potential for immobilization to occur. At a minimum, liquid treated manures should be analyzed for NH₄-N and total-N, and solid treated manures should be analyzed for NH₄-N, NO₃-N, total-N and C:N. In liquid treated manures, it can be safely assumed that inorganic N will be primarily in the NH₄-N form. However, in solid forms such as compost and separated manure or digester solids both NO₃-N and NH₄-N may exist. Predicting the N availability from the organic-N is difficult without sufficient data to derive specific availability factors for treated manures. Previous research has indicated that between 25-50 % of organic N from manures and biosolids are available during the first year, so a conservative estimate of 25 % organic N availability should be adequate in these situations. If the treated manure has a relatively high C:N (> 50:1) then a more conservative estimate of 10 % may be appropriate because of slower organic N mineralization in these materials. When applying materials with C:N ratios greater than 50:1, it is also a good idea to check soil N with a pre-sidedress nitrate test because of the potential for N immobilization. If the treated manures are surface applied, decrease the availability of NH₄-N by 50% to account for ammonia volatilization losses. Equations for estimating N availability are provided in Table 1.

Table 1. Guidelines for estimating first-year N availability of treated manures.¹

¹ These calculations are based on the best available data for determining N availability of treated manures. Modifications may be necessary pending the outcome of future research. The estimate of available N should be multiplied by the manure application rate to determine the amount of nutrients applied to a field in lb N/a.

²  Incorporation must be within three days (72 hours) of application.

³ When using solid treated manures with a C:N greater than 50:1, check soil nitrate levels with a pre-sidedress nitrate test.

Phosphorus (P)

Phosphorus in manures is present in both inorganic and organic forms. For most animal species, the inorganic P forms are dominant. The form of P that plants use is inorganic P.

Availability of manure P can be defined in two ways: 1) agronomically as the ability to supply P to a crop; and 2) environmentally as the ability to increase soil test P levels. Currently manure P is considered to be 60 % available in the first year after application. This means that 60 % of the total P in manure is available to a crop. For comparison, fertilizer is considered 100 % available. Ongoing research is suggesting that assuming manure P is 60 % available may not be appropriate for all manure sources on all soil types with regard to crop growth and changing soil test P levels.

At one time it was generally assumed that the amount of P available from manure was equal to the amount of inorganic P in the manure. Current research is finding that P availability is more complex and can be dependent upon both inorganic and organic P along with the type of carbon compounds (short chain organic acids vs. large humic or fulvic acids) in the manure. For example, organic P in swine slurry has been shown to preferentially bind to the soil leaving more inorganic P in solution and potentially increasing availability to a plant. This explains why some studies have found 100 % availability of swine slurry P with regard to changes in soil test level upon application to soil.

As stated previously, anaerobically digested manures may contain the same forms of inorganic P as raw manure, but the water extractability of the digested manure P may be greater or less than raw manure.  Carbon reduction in digested manures would likely change the interaction between inorganic P, organic P, and he remaining carbon containing compounds to alter how P binds with soil and subsequent P availability. For liquid-solid separation systems, P in the solid fraction may be less available if treated with iron or alum because the P will precipitate with these materials and settle out. When applied to soil, these precipitates may be unreactive or slowly dissolve to available forms. As stated previously composting can increase the P concentration of the manure. Phosphorus availability may be altered by composting because there is a change in the carbon containing compounds in the manure and P may cycle between organic and inorganic forms.

Because research in P availability of manures is in its infancy compared to N availability and because detailed chemical composition of treated manures is not known, it is difficult to say with any certainty exactly what P availability of treated manures might be. Thus, assuming treated manure P availability is 60 % would be prudent until more information is known.

Potassium (K)

Potassium in manures is in the inorganic form, largely contained in the liquid fraction, and is readily available to plants. For all manures first-year K availability is considered to be 80 % of total manure K. It is reasonable to assume that treated manures would have a similar K availability as raw manures as the treatment processes likely don’t influence K very much. In outdoor, uncoverd composting operations K can be lost by leaching from the compost, but these losses should be reflected in an analysis of the compost before application. 

For more information on manure nutrient credits, see Chapter 9 – Nutrient Credits in UWEX publication A2809 Nutrient application guidelines for field, vegetable, and fruit crops in Wisconsin. It can be purchased or downloaded from the web at: http://learningstore.uwex.edu/Nutrient-Application-Guidelines-for-Field-Vegetable-and-Fruit-Crops-in-Wisconsin-P185C43.aspx