Liquid swine manure is one of the most nitrogen-rich manures used in Midwestern cropping systems, but not all nitrogen in manure is equally available to plants. Understanding how nitrogen behaves after application is critical to making the most of this resource — and to minimizing its environmental footprint. In this article, we’ll dig into why liquid swine manure tends to be highly plant-available, what makes Iowa’s and Minnesota’s availability recommendations different, and how those differences matter (or don’t) depending on your crop rotation and nutrient strategy.
Forms of Nitrogen in Liquid Swine Manure
Most of the nitrogen in liquid swine manure exists in two forms:
- Ammonium (NH₄⁺): This is readily plant-available and behaves similarly to commercial fertilizer nitrogen.
- Organic Nitrogen: Found in proteins, cells, and other organic residues — this N must mineralize to become plant-available.
One of the key indicators of nitrogen availability is the NH₄⁺: Total N ratio. In liquid swine manure, this ratio often ranges from 70–85% for typical manure storages, though exceptions can happen. High NH₄⁺:TN ratios signal a greater portion of nitrogen is immediately available for plant uptake, making it more predictable and efficient as a fertilizer source. Using information from ManureDB, Iowa liquid swine manures average 75% ammoniacal nitrogen while Minnesota averages 74%, so basically no difference.
The Role of C:N Ratio in Predicting Organic N Behavior
When it comes to the organic portion of nitrogen, carbon-to-nitrogen (C:N) ratio is critical. Organic nitrogen in manure with a low C:N ratio (<15:1) tends to mineralize, releasing nitrogen over time. High C:N ratios (>30:1), in contrast, promote immobilization, where soil microbes tie up nitrogen to digest the carbon.
Liquid swine manure tends to have low C:N ratios in its organic fraction — meaning it contributes more nitrogen than it "costs" microbes to break it down. This is another reason why liquid swine manure is considered a high-availability material.
Iowa vs. Minnesota: Availability Recommendations Differ in Meaning
A key point of confusion arises when comparing Iowa and Minnesota nitrogen availability estimates:
- Iowa calculates first-year availability of manure N as a function of mineralization only and has a second factor for volatilization losses. In other words, Iowa breaks these two processes apart, accounting for losses (like ammonia volatilization) separately from how much of the total nitrogen becomes plant-available. In so doing, the range that Iowa provides isn’t dependent on the application method selected.
- Minnesota, on the other hand, includes volatilization loss within its availability values. That means a Minnesota “availability” percentage is often lower, but also that it is a function of the application method used.
Current Iowa guidelines in PMR 1003, Using Manure Nutrients for Crop Production, suggest swine manures in Iowa are between 90-100% first year available. The second-year availability would then be the remaining fraction that wasn’t claimed in year 1, i.e., 0-10% of the N applied.
In Minnesota they have a publication “Manure Management in Minnesota” that is used for estimating manure nitrogen availability, and I’ve often heard it quoted that it says that Year 1 swine manure is 80% available. I wanted to look closer at their table and clear this up, they effectively have a few different things going on. They show a different availability with different application methods, such as surface broadcast with no incorporation, surface broadcast with incorporation in less than 12 hours, and injection (which they have two categories, one for sweep and one for knife). These categories were selected because they are the closes parallels between Iowa’s direct injection, immediate incorporation, and no incorporation application methods discussed in PMR 1003.
The first thing to notice, is that to make the availability of Minnesota’s publication similar to that of Iowa’s 90 to 100%, we should be summing what they say is available in year 1 and their losses, to get a number comparable to what we suggest, that is 85% as compared to 90-100%. To me, when comparing this, the bigger difference is how different we are on estimating volatilization losses (which their footnote also suggests includes dentification, presumably denitrification above what would have occurred with commercial fertilizers).
Table 1. Suggested year 1, year 2, and gaseous nitrogen losses (volatilization and denitrification) from liquid swine manure in Minnesota.
Table 2. Estimated year 1, year 2, and gaseous nitrogen losses (volatilization) from liquid swine manure in Iowa.
Bottom line: The 90-100% availability in Iowa is accounting differently than the 80% often cited for Minnesota, though they seem to apply 85% first year availability.
When Does This Matters
In a continuous corn rotations using manure every year, this difference makes almost no impact on what you’d apply, because year-to-year mineralization overlaps and the resulting N carryover.
Let’s take a look at this to see it illustrated. Say I have a swine manure that tests 60 lb N/1000 gallons. I’m going to inject and have minimal volatilization losses (let’s say 2%) and I want to put on 190 lb N per acre. I’m going to compare what rate I’d use if I was assuming 100% available based on Iowa guidance or 85% available based on Minnesota guidance.
Table 3. Estimated impact of using Iowa and Minnesota availability recommendations on manure application rates in continuous corn rotations receiving manure annually (estimates are based on the average median field and yield response curve for Story County, Iowa).
So, in this case, because of how the math works out on the losses, Minnesota’s availability would suggest a slight (10 gallon per acre) lower rate than the Iowa assumption of 100% available.
But in manure-to-soybean systems or when rotating manure use so that it isn’t applied every year, the results can be quite different. We’ll rework the problem above, but in this case say the target N demand is 146 lb N/acre (if you are curious about where the N demands I’m using come from, they are the Story County median rates for corn following corn and corn following soy). In this case, the N carry over to a corn crop, occurring in the 3rd year after manure application in both cases is about 20% higher than when using assuming 100% availability.
Table 4. Estimated impact of using Iowa and Minnesota availability recommendations on manure application rates in corn soybean rotations receiving manure (estimates are based on the average median field and yield response curve for Story County, Iowa).
This isn’t to say one number is right and one is wrong, but rather to illustrate when the assumption matters and the magnitude of impact it can have. In case you are wondering, the N-FACT suggested impact of only getting 85% availability when you assumed 100% is about 22 pounds of nitrogen, or 3.5 bu/acre on average (the year makes a difference). If you went the other way, and assumed 85% available and got 100% it would be worth that would have added 26 lb N/acre and resulted in about 1 more bushel of corn per acre.
A second version of this is how much environmental impact would this have. While that is a tough and nuanced question, I’m going to simplify and say the field when fertilized at 146 lb N/acre yields 200 bu per acre, so when we planned on 100% availability and only got 85% availability, we went from a “nitrogen use efficiency” of 0.73 lb N/bu to 0.63 lb N/bu. On the other hand, when we assumed 85% availability and got 100%, we went from 0.73 lb N/bu to 0.86 lb N/bu. While these calculations don’t tell us everything, we need to know to estimate losses, they give an estimate about the slack or potential for loss.
Timing and Temperature Impacts
Availability isn’t just about the manure it’s about what happens after application. Nitrogen mineralization depends on:
- C:N ratio (low = more mineralization)
- Soil moisture
- Microbial community activity
- Temperature (higher = faster microbial activity)
I wanted to comment only briefly on manure characteristics and spend most of our time on temperature effects, but the manure database suggests on average 75% of swine manure nitrogen is in the ammonium form for both Iowa and Minnesota farms. That is to get to the 85% available Minnesota is assuming 40% of the organic nitrogen is mineralizing during the first growing season, while Iowa is suggesting 60-100% of the organic nitrogen will mineralize during the 1st growing season.
With that said, let’s focus on a different characteristic temperature. Because microbial activity doubles for every 18°F increase in temperature, timing matters. A fall-applied manure on October 1 in southern Iowa will experience more microbial activity compared to a November 15 application in northern Iowa. As we move further north, to Minnesota, this difference grows even greater.
We’ve developed a microbial growing degree day (μGDD) scale to help compare how much microbial activity, and thus potential manure organic N conversion, might occur depending on when you apply. This scale accounts for how microbial activity speeds up with warmer temperatures (doubling every 18°F) but shuts off below freezing. It doesn’t tell us what percent of the N becomes available, but gives us a method to estimate mineralization potential in different locations and different application timings and compare them to each other.
Where:
t is a day count with t=1 as the of manure application
Tt is the 4-inch soil temperature that day in degrees F
Tref is a reference temperature to benchmark activity against and for our purposes here set to 32ºF
It should be noted
that the absolute value of µGDD has no meaning and this term is only use as a measure of relative microbial activity compared to another µGDD.
Thankfully, Iowa has invested in a soil temperature monitoring network (4-inch depth) where you can get soil temperature data. Currently there are 26 stations in the network. We assumed a November 15th manure application and evaluate Microbial Degree Days though June 15th. From the northern part of the state, to the southeastern edge of Iowa, there was a 20% difference in microbial degree days (Figure 1). While I didn’t continue this analysis into Minnesota (there temperature network appears to be at a 6-inch depth) given the spatial change over distance, it appears that there would be about 10% less microbial degree days in southern Minnesota than northern Iowa. So, if you were wondering about why there is an assumed difference in nitrogen mineralization, this would be a big part of it.
I did want to look at this one more way, how would our microbial growth degree days vary as a function of the date the manure is applied. Not surprisingly, manure that is applied earlier is exposed to more microbial growth degree days (Figure 2). But what I wanted to point out is, if you applying in fall vs spring there was about a 40% difference in microbial activity the manure was exposed to, which is bigger than the spatial differences in Iowa at a single application date and that most of the estimates behind swine manure nitrogen availability are based on what is available assuming a spring manure application (because we want to know about availably and not losses of N related to application timing) .
Figure 1. Relative microbial activity as estimated based on microbial growth degree days from Northern Iowa to Southern Iowa for manure applied on November 15th until June 15th.
Figure 2. Relative microbial activity as estimated based on cumulative microbial growth degree days from the manure application date through June 15th.
Conclusion
Liquid swine manure is an excellent fertilizer, offering high N availability, especially as it has a high NH₄⁺:TN ratio and a low organic C:N ratio. Understanding how Iowa and Minnesota calculate nitrogen availability can help farmers and advisors use those numbers appropriately. Additionally, understanding regional differences in microbial activity (as proxied through 4-inch depth soil temperatures and microbial activity growing degree days) provides insight into why availability estimates may be lower in Minnesota than in Iowa.
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