Manure Minute: Why you need a manure management plan
May 19, 2023 by Chryseid Modderman, University of Minnesota Extension
When applying manure, the main goals are to apply at an accurate rate and to avoid nutrient pollution. But this isn’t always easy because manure, in general, is complicated. There are five main factors that make manure complicated; often, more complicated than commercial fertilizer. Following a manure management plan will help combat these challenges. Read on for the five challenging factors.
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 is a potential problem because you need a lot more 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 over-application to the same field can lead to nutrient build up 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. It is what it is. Let’s do some quick math to illustrate this. Let’s say you have turkey manure with 30 pounds of plant-available nitrogen and 40 pounds of plant-available phosphorus per ton, and your agronomist says to apply 180 pounds of nitrogen per acre for your corn crop. You’d need to apply manure at six tons per acre (180 / 30 = 6).
Does this application rate pose a risk for nutrient pollution? Yes. At 6 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 yield of 250 bu/ac corn 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 nitrogen. The inorganic nitrogen is immediately available to the plant; while the organic nitrogen is not. Organic nitrogen can become inorganic nitrogen over time through a process called mineralization. The challenge is estimating how much organic nitrogen will become inorganic nitrogen, and how fast. 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 that are fairly uniform throughout, 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, it is likely that 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 working around wet weather, harvest, or planting rather than when it is best for the crop and environment. Nutrient loss from manure is higher when application occurs 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 minimize these challenges. The most significant is to have a manure management plan which encompasses best management practices such as accurate rate calculations, sampling, setbacks and buffers, spreader calibration and more! •