Manure

Animal manure is an excellent source of nutrients and organic matter. Many of the nutrients in fresh livestock manure, especially nitrogen, are readily available.  Nutrient content varies by animal species, their diets and the form of their manure. There are times when readily available nitrogen is needed, but many people prefer to compost manure before field application (see Compost section below). Manure application rates are now regulated in many New England States (see Nutrient Management Regulations).

Nitrogen in manures and other waste products: The N content of manures is highly variable. Differences are due to the species of animal, the animal's age and diet, the moisture content of the manure, handling and storage, and the amount of bedding in the manure. The N fertilizer equivalent of a manure varies not only with the total N content of the manure, but also with the timing and method of manure application. Manure samples can be analyzed by the Universities of Maine and Vermont Laboratories. The values in Table 7 are based on analyses of Vermont manures as well as published data from other states. If specific manure analysis data is not available, growers should estimate N credits using these or other book values. The time elapsed between spreading and incorporation of manure is also important. About half of the N in dairy manure and three quarters of the N in poultry manure is in the form of ammonium (NH₄), which easily turns to ammonia gas (NH₃) and is volatilized (lost to the air). The longer that manure is left on the soil surface, and not incorporated, the greater NH₃ volatilization losses become (Table 7a). Broadcast application of slurry manure without incorporation should always be avoided because this method increases air contact and allows time for all ammonia to be lost. Research has shown that in reduced or no-till fields where manure must be surface applied without incorporation, ammonia can be best conserved if applied during cold temperatures, low wind speeds and especially to a growing cover. A growing cover also reduces manure run-off and leaching losses.  NOTE: Manure often contains human pathogens. Serious illness has occurred from eating produce where fresh manure was applied without an adequate waiting period (see Produce Safety).

Previous manure applications: Up to 50% of the total N in cow manure is available to crops in the year of application. Between 5% and 10% of the total N applied is released the year after the manure is added. Smaller amounts are furnished in subsequent years. The quantity of N released the year after a single application of 20 tons per acre of cow manure is small (about 15 lb N per acre). However, in cases where manure has been applied at high rates (30-40 tons per acre) for several years, the N furnished from previous manure increases substantially. The buildup of a soil's capacity to supply N resulting from previous applications of manure has important consequences for efficient N management, including: 1) The amount of fertilizer N needed for the crop decreases annually; and 2) If all the crop's N needs are being supplied by manure, the amount of manure needed decreases yearly.

With poultry manure (as compared with manure from cattle) a higher percentage of the total N in the manure is converted to plant-available forms in the year of application. Consequently, there is relatively less carry-over of N to crops in succeeding years. This does not mean, however, that there is never any carry-over of N from poultry manure applications. If excessive rates of poultry manure (or commercial N fertilizers) are used, high levels of residual inorganic N, including nitrate (NO₃), may accumulate in soil. High levels of soil nitrate in the fall, winter and spring have the potential to pollute groundwater and coastal seawater.

Table 7: Nitrogen Credits from Manure Applied Before Planting

Adapted from Nutrient Recommendations for Field Crops in Vermont (2018). Dairy manure values are from Vermont samples analyzed by University of Maine, 2012-2016, others are adapted from University of Nebraska-Lincoln NebGuide G 1335 and Penn State Agronomy Guide (2016). Values do not include bedded pack. Manures vary greatly, so obtaining a manure analysis is always best practice. n/a = data not available.

Table 7a: Availability of ammonium nitrogen from spring or summer applied manure (% fertilizer N equivalent)