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Nutrient Management

All manure should be applied at the rate and time that assures maximum utilization of the nutrients by the crop. Unfortunately, there are factors that reduce our precision when trying to make these calculations.

Organic Forms of Nutrients

Not all of the nutrients in manure are directly plant available. Organic forms of nutrients must be mineralized into inorganic, or “plant-available” forms--such as nitrate. The rate of this mineralization is variable and depends on soil type, moisture, temperature, manure composition, and other factors. It is easy to see that assumptions and estimates have to be made when attempting to define mineralization rates.

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 between different regions of the country. More discussion on this issue is found below in the specific sections on N, P, and K.

Environmental Losses of Nutrients

There are some inherent losses of nutrients to the environment through manure application. The three macronutrients, N, P, and K, each have unique characteristics with respect to expected losses as a function of manure application method. Also, each manure application method produces variations in nutrient losses.

Nitrogen is the most dynamic nutrient when it comes to potential loss. The reason is that some of the N in manure either is in the ammonia form or can readily be converted to this form after application. Ammonia is volatile and losses of ammonia from land-applied manure can be significant. Many states have tables with coefficients describing nutrient losses as a function of animal type, manure handling system, and manure application method. Some states will vary the N-availability tables for ammonia based on the time (days) after manure application that the manure has been left on the ground surface.

Manure Collection, Storage, and Treatment

How manure is managed on the farm will affect the nutrient availability once 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 of manure is a treatment mechanism that can result in a high degree of manure processing with a high resultant organic nutrient component, depending on the duration and factors involved in the composting. 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 composting with old hay.

More information is available through the LPE Learning Center topics:

• Manure Handling and Storage
• Manure Treatment Technologies

Timing of Manure Applications

Manure application is generally scheduled to accommodate the crop planting and harvesting schedule. Additionally, the volume 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. This is because most confined animal operations have some form of manure storage structure with limited capacity.

Ideally, the amount of storage that is available is reasonably matched to the cropping system so that manure can be applied at appropriate times. One cannot make presumptions as to exact manure application timing when writing a nutrient management plan, and thus standard factors must be assigned. A system manager can make adjustments to manure application rates based on specific conditions surrounding the timing of manure application (environmental factors) and relevant loss factors associated with a current manure lab analysis and proposed application method.

Another issue related to timing and nutrient availability is seasonal applications. Organic nutrients applied in the spring will have a higher mineralization rate than the same nutrients applied during the fall. The planner must apply the appropriate mineralization rate coefficients for these application periods. Fall applications could result in nutrient losses if they are simply pre-plant 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.

Availability of Nitrogen (N), Phosphorus (P), and Potassium (K)

These are the three nutrients most commonly tracked in manures as they are the ones that are needed by crops in the largest amounts. Hence they are the ones typically purchased as a fertilizer. As stated above, N is the nutrient that is the most dynamic of the three. Standard coefficients 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 US show that 80-100% 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% 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 7-10 days. Depending on the state, manure application method, manure type (animal species), and handling system, nitrogen availability can range from 10% to 95%. Producers that wish to minimize N losses (maximize the manure value) will use application methods such as injection or immediate incorporation.

Author: Karl Shaffer, North Carolina State University