Animal manure is a great source of nutrients for crops, but it is also complicated. Luckily, one of the complications is something that can be controlled: manure application method. As a reminder, nitrogen (N) occurs in two forms in manure: 1) Organic N is not available to plants. 2) Inorganic N, on the other hand, can be broken down into ammonium and nitrate (both are available to plants). This will be important as you consider the various types of application methods and how they impact nutrient losses.
Application methods for solid and liquid manures
The following are two application methods for both solid and liquid manures:
- Broadcasting manure onto the surface of a field is the oldest method of spreading. It is easy, cheap, and can be done during almost any season. But, there are also some significant disadvantages. From a nutrient standpoint, a substantial amount of nitrogen can be lost within a few days of application. This happens when ammonium (inorganic, plant available N) converts to ammonia gas, a process called volatilization. The organic N is not lost, however, and some of it may become available later in the growing season. Other concerns are odors and the possibility of nutrient or pathogen runoff in large rain or snowmelt events.
- Broadcasting with incorporation means mixing, or incorporating, the manure into the soil immediately or within a few days after broadcasting. This method greatly reduces ammonia gas losses, especially if the manure is incorporated quickly. Because the manure is thoroughly mixed into the soil, it also promotes conversion of organic N into inorganic N, a process called mineralization. That means you can expect more N to be available for plant growth than when manure is left on the soil surface. One drawback is that this requires tillage, which disturbs the soil surface and may not fit in all agricultural systems. Double disk application is considered a form of broadcast application since it applied manure on the soil surface, but it is immediately incorporated as the disks throw soil on top of the manure to cover it up.
Injection methods for liquid manure
The following methods are types of manure injection used for liquid manure. Injection is when the manure is placed below the surface of the soil. It was developed to reduce odors and issues with ammonia gas losses. It is more expensive because it takes longer and requires more tractor horsepower and fuel.
- Knife injection is when vertical shanks, similar to knives, pull through the soil. This creates a vertical slot approximately six to eight inches deep for the manure to go. Using this method reduces ammonia volatilization loss, but concentrates the manure and nutrients into small strips in the soil, which may not be optimal for plant growth.
- Sweep injection systems were developed to reduce the concentrated zones of manure beneath the soil surface. Instead of creating a vertical band of manure where a knife shank ran through the soil, this method creates a broad, horizontal band. Sweep injection is more effective at reducing ammonia volatilization losses and does a better job of mixing manure and soil. It does require more tractor horsepower than other types of injection.
- Disk or Coulter injection systems use a rolling disk or a wavy disk called a coulter to open a vertical slot in the soil for the manure to go. Some people use closing disks to close the slot and reduce ammonia volatilization losses. This method requires less horsepower than knives or sweep injection, but manure applied at high rates may still end up on the soil surface if it overflows.
Although there are several things that affect N availability from manure, like the weather and soil characteristics, the method of manure application is one of the most influential factors that producers can control. See Table 1 for examples of how manure application method affects N availability.
Table 1: Manure nitrogen availability as affected by method of application and animal species.
|Year||Broadcast + incorporate
|Broadcast + incorporate
|Broadcast + incorporate
(including double disks)
|Injection with sweeps||Injection with knife or coulter|
|N available/ year||N available/ year||N available/ year||N available/ year||N available/ year|
Reviewed in 2021