Estimating crop nutrient availability of manure
All manure should be applied at the rate and time that assures maximum utilization of the nutrients by the crop. Manure testing will provide the quantity of manure nutrients, but during storage and after land application, there are many factors that affect how much will become available to the current crop.
Not all of the nutrients in manure are directly plant available. Organic forms must be mineralized into inorganic, or “plant-available” forms—such as nitrate. The rate of this mineralization changes depending on soil type, moisture, temperature, manure composition and other factors.
Researchers continue to improve and refine predictions, but it is still necessary to use assumptions and estimates when defining mineralization rates. Organic forms of nitrogen will continue to mineralize and become available to crops in years 2, 3, and even 4 after the initial application. Most availability tables will provide calculations to use when calculating nutrient availability in subsequent years.
There are some inherent losses of nutrients to the environment during and after manure application. The three macronutrients, nitrogen (N), phosphorus (P), and potassium (K), each have unique characteristics with respect to expected losses. These losses vary by manure application method (subsurface injection versus surface application).
Nitrogen is the most dynamic nutrient when it comes to potential loss. Some of the N in manure is in ammonia form or is readily converted to this form after application.
Ammonia is volatile—it quickly changes to gas and disperses to the atmosphere—and losses of ammonia from land-applied manure can be significant. Many of the state availability tables (mentioned below) have coefficients describing ammonia and other nutrient losses based on animal species, manure handling system, and manure application method. Some states will vary the N-availability tables for ammonia based on the time, usually hours or days, after manure application if the manure has been left on the ground surface.
How manure is managed on the farm will affect the nutrient availability once it is land applied. The more highly treated or processed the manure is, the more the nutrients are bound up chemically or organically and the less plant available they become. Some treatment and storage processes enhance organic nutrient development, whereas others decrease it.
Composting manure is a treatment that results in a high degree of manure processing with a high resulting level of organic nutrients. Thus, the nutrients (especially nitrogen) are usually much less plant available at the time of application and must be mineralized over the course of several years to become plant available. Some forms of solids removal, including chemical flocculation or precipitation, also result in manure nutrients being physically or chemically tied up and thus more slowly plant available.
Manure application is generally scheduled to accommodate the crop planting and harvesting schedule. The size of manure storage may also factor in to the timing of manure application. Occasionally, lack of storage can require that manure be applied at times that are less than ideal for maximum crop nutrient uptake.
Organic nutrients applied in the spring will have a higher mineralization rate than the same nutrients applied during the fall. A nutrient management planner should apply the appropriate mineralization rate coefficients for these application periods. Fall applications could result in nutrient losses if they are simply preplant applications for the following spring. The nitrate form of nitrogen is very mobile in the soil and subject to leaching losses. Depending on the climate, significant conversion of ammonia to nitrate may occur, with subsequent leaching of nitrate.
As mentioned, N is the most dynamic of the three macronutrients. Availability factors for P and K are fairly consistent between states because these nutrients have minimal loss potential with volatilization or leaching as compared to N, and they remain relatively stable in the soil. Most regions of the U.S. show that 80-100 percent of the P and K in manure is considered plant available.
There are many factors that ultimately affect the amount of N that will be plant available. Manure management and application method, as stated above, are two of the key factors. Other factors are mineralization and volatilization factors. These are determined by the environmental conditions at the time of manure application (air temperature, humidity, wind, soil temperature, soil C:N ratios, etc.).
Because N dynamics have many variables, several factors can be applied to the manure N analysis to assess its availability for crop use. Some states assign a flat 50 percent ammonia loss factor for manure that is surface applied without incorporation. Some states will segregate these factors based on time of year. Others will apply an increasing scale of volatilization (hence a decreasing scale of availability) depending on how long manure sits on the soil surface before incorporation.
Typically, volatilization losses are assumed to cease after seven-10 days. Depending on the state, manure application method, manure type (animal species), and handling system, nitrogen availability can range from 10-95 percent. Producers who wish to minimize N losses and thereby maximize the manure value will use application methods such as injection or immediate incorporation.
Most states or regions have developed tables for estimates of mineralization rates based on the common types of manure handling methods, typical manure application windows, and typical environmental conditions that affect mineralization. These tables can vary significantly from state to state.
You find the resources based on research in your state in a number of ways: •Contact your local or state extension service; •Do a web search for “manure nutrient availability” plus your state’s name and select extension publications (.edu addresses) from the list; •State-specific nutrient management resources. — Karl Shaffer, North Carolina State University Extension