Manure Minute: Why you need a manure management plan
Planning ahead will help in handling the complexities of manure.
August 9, 2020 by Chryseis Modderman
When applying manure, the main goals are to apply at an accurate rate and to avoid nutrient pollution. But this isn’t always easy, as manure, in general, is complex. There are five main factors that make manure complicated – often even more so than commercial fertilizer. Following a manure management plan will help combat these challenges.
Overall nutrient content is low
Total nutrient content of manure is low – rarely above 10 percent – whereas commercial fertilizers have a much higher nutrient concentration by weight. The low nutrient content of manure can be a problem, because you need a larger volume of manure than commercial fertilizer to achieve the same nutrient application rates. This increases time and transportation cost, making it more economical to apply to the field nearest the barn. Over time, repeated overapplication to the same field can lead to nutrient buildup and subsequent pollution. It is quite common to see fields nearest a livestock operation with very high soil test phosphorus levels.
Nutrient ratio is fixed
Unlike commercial fertilizers that can be mixed and adjusted to reach desired nutrient balance, manure nutrients are fixed. Let’s do some quick math to illustrate this. Let’s say you have turkey manure with 30 pounds (lbs.) of plant-available nitrogen (N) and 40 lbs. of plant-available phosphorus (P) per ton, and your agronomist says to apply 180 lbs. N per acre for your corn crop. You’d need to apply manure at six tons per acre (180 / 30 = six).
However, this application rate poses a risk for nutrient pollution. At six tons/acre, you will apply 240 lbs. P/acre (40*6=240). Corn only uses 0.29 lbs. P per yield unit. So, even a really high corn yield of 250 bushels per acre would only require 72.5 lbs. P/acre, and that’s including what is already in the soil. Adding 240 lbs. of P is way too much! Over-application of phosphorus can lead to phosphorus buildup, which can lead to pollution.
Nutrient availability is difficult to estimate
Nutrient availability, especially the availability of nitrogen, can be challenging to accurately estimate. Manure supplies two forms of nitrogen: inorganic and organic. Inorganic nitrogen is immediately available to the plant, while organic nitrogen is not. Organic nitrogen can become inorganic nitrogen over time through a process called mineralization.
The challenge here is estimating how much organic nitrogen will become inorganic nitrogen and how quickly. This can be tricky because mineralization is a microbial process, meaning that how fast or slow it processes organic nitrogen depends heavily on the environment – and we know how fickle the environment can be!
Nutrient content is not uniform
Unlike commercial fertilizers, which are fairly uniform in nutrient content, manure uniformity varies spatially and over time. This can make accurate rate calculations tricky. To meet this challenge, it is very important to take a good representative manure sample for testing. But even then, slight over- or under-application can occur.
Nutrient timing may not be ideal
In a perfect world, manure would only be applied when the nutrients are necessary and when it poses the least risk to the environment. Unfortunately, we don’t live in a perfect world. Often, manure application timing is driven by storage limitations and wet weather, harvest or planting, rather than when it is best for the crop and environment. Nutrient loss from manure is higher when it is applied in late winter, around the time of snowmelt.
How to meet these challenges
While we may never be 100 percent perfect with manure management, there are ways to make these challenges as small as possible. The most significant one is to have a manure management plan which encompasses many best management practices, such as accurate rate calculations, soil and manure sampling, setbacks and buffers, spreader calibration, and more.