Manure Scoop: LSNT on Manured Fields, What Are We Really Measuring?

 

The Late Spring Nitrate Test (LSNT) has long been a useful in-season check on nitrogen availability, but manured fields have always sat a little differently in the interpretation guide. The original recommendation still says it plainly: soils receiving recent manure applications, or corn following alfalfa, don’t behave like “typical” soils and will get some slower nitrogen release. They tend to mineralize more plant-available nitrogen after sampling than a “typical” soil, meaning the standard LSNT critical values may not tell the full story. In practice, that means we’ve always known manured systems don’t fit neatly into a single threshold but we also want to try to use the LSNT as a tool to help us manage better.

The newer LSNT decision support work reinforces something many of us have seen in the field: even when we try to quantify nitrogen availability, there is still a lot of variability. That raises an important question, should we be thinking differently about manured fields altogether, rather than just adjusting the threshold or are the principles still the same? And the good news is, the principles are at least similar, but the bad news is we have to think a little bit deeper about the characteristics of the manure we are using, when we applied, and how the growing season has been in terms of mineralization potential thus far to improve interpretation.

One useful way to ground that question is to step back and look at soil nitrogen dynamics in-season. The FACTS soil mineralization tool is a helpful reference here, especially the “how are we comparing to average” thus far through the growing season comparison. If the season is tracking below average for mineralization, manure-derived organic nitrogen is likely also lagging behind expectations, meaning more of that nitrogen may show up later in the growing season and we won’t see as much as usual on our late spring nitrate test. If mineralization is trending above average, the opposite is true, we should expect manure nitrogen to be contributing more aggressively than a “typical year” assumption would suggest.

But not all manure behaves the same, and that is where interpretation gets more interesting. The ratio of ammonium-N to total-N is a simple but powerful indicator of how quickly a manure behaves like a fertilizer versus a slow-release organic source. I gave a summary of this in the Talkin’ Crap episode, Available or Not: The Nitrogen Guessing Game in Manure Planning, and you can see our summary of average Midwestern manure properties on this handout. In summary, high ammonium manures, such as liquid swine manure, tend to act much more like commercial nitrogen fertilizer at application, while more organic-heavy manures shift their contribution toward delayed mineralization.

That distinction shows up in the research. Woli et al. (2011), working with John Sawyer, evaluated liquid swine manure as a nitrogen source for corn and found LSNT critical values near 25 ppm, but also documented cases where yield response did not occur even at lower LSNT readings. One explanation is that manure history and mineralization dynamics were still supplying nitrogen beyond what the soil nitrate snapshot captured at sampling time. Is this a manure challenge? Maybe, but when you look at the latest data from the Iowa Nitrogen Initiative, we see fields that exhibit the same sort of response, that despite being below the critical threshold they don’t need as much N as an average field. Perhaps it is more common in manured fields because we are building soil health and quicker nitrogen cycling, but there are other ways to get there.

 

A similar pattern emerged in work by Ruiz Diaz, Sawyer, and Mallarino with poultry litter. They showed LSNT results were strongly tied to the ammonium fraction applied with the litter, while the organic nitrogen contribution was less immediately reflected in the test (probably because mineralization is relatively low until that May time period and starts to pick up rapidly around the time, we’d be taking the late spring nitrate sample). However, they also noted that this organic fraction does not disappear, it continues to mineralize and contribute nitrogen later in the season, which aligns with what we’ve seen in subsequent field work here in central Iowa.

Taken together, these results suggest that LSNT on manured fields is not just a question of “what is the number,” but “what nitrogen cycle are we sampling?” A manured field is often a moving target: part fertilizer response, part soil mineralization, and part delayed organic nitrogen release. That is exactly why variability in LSNT results is higher in these systems and why a single cutoff value will always feel a bit blunt, and a reason the newest tool focuses on the probability to a yield response.

So where does that leave us? LSNT is still valuable, but in manured systems it should be interpreted as one snapshot within a broader nitrogen story, not the full story itself. Understanding manure type (especially NH₄-N to total N ratio), current soil mineralization conditions, and field history becomes just as important as the number in how you tailor your nitrogen management decision. The LSNT tells us what nitrogen looks like today. Manure systems require us to also think about what nitrogen is still on its way.

What do Iowa Nitrogen Innovative on-farm trials say about optimal N rate and N per bushel?

 Every year, we ask the same question: “How much nitrogen do I really need to grow this corn crop?” And despite improving technology, we still often give the same, honest answer: “It depends.”

That’s why the Iowa Nitrogen Initiative (INI) is running hundreds of on-farm trials. By testing in real fields under real conditions, we can get closer to answering the “how much N” question in a way that fits Iowa corn growers. I dug into nearly 500 of those trials from 2023 and 2024 to see what they tell us about yields at the economic optimum N rate (EONR), how much N it takes to get there, and how efficient that N is when measured on a per-bushel basis.

I’m going to make this article a little different, I’m going to highlight what I found first, and if you want to see the statistics behind it and the data processing, I did to get there read to the end.

Why Does This Matter for Iowa Farmers?

·         Rotation helps because it boosts yield, and higher yields mean better N efficiency. On average, when corn followed soybeans, yields were about 19 bu/ac higher (239 vs. 220 bu/ac) than when corn followed corn.

·         Higher yield potential means more total N, but less N per bushel. When I looked at the economic optimum N rate across trials, a pattern popped out: for every additional bushel of yield, it took about 0.45 lb more N per acre.

·         Regional and year effects are real. Your neighbor’s optimum N rate might not match yours, because soils and weather shape the response.

This is why INI trials matter: they help us put real numbers to what we’ve all seen in the field. It’s not a perfect crystal ball (the models only explained about 25% of the variation), but it’s a step closer to giving farmers confidence in their N plans, and showing that chasing higher yield potential, when realistic, can also improve nitrogen efficiency, and more importantly, that there is work to do to understand what is driving the other 75% of the variation in need.

Alright, that’s the 30,000-foot view. But if you’re like me, you want to peek under the hood and see how we got those numbers. Here’s how the data was pulled together and what the statistics say.

Data & Scope

I analyzed 493 on-farm nitrogen (N) response trials from the Iowa Nitrogen Initiative (downloaded from N-FACT). Trials with alfalfa as the previous crop (n=1) was changed to an unknown crop, to alleviate the non-replication struggle I was having, and one 2024 Western Region outlier (99 bu/ac yield at 226 lb N/ac; 2.8 lb N/bu) was excluded. Four factors were available to explain variation: region, year, previous crop, and yield at the economic optimum N rate (EONR).

That leaves 499 trials left in the dataset that we can use to try to understand the yield, optimum N rate, and the N-use factor. In the currently existing dataset, there are four variables that we can use to understand these factors; they are MLR, year, the previous crop, and the yield of corn at optimum N rate.

Yield at Optimum N Rate:

We can write a statistical model of:

Using this statistical model, we can describe 22% of the variation in the corn yield at Optimum N. The Region, region x year interaction, and previous crop were all significant (p = 0.0001, 0.0005, & p < 0.0001) respectively.

The previous crop indicated that corn following soybean averaged 239 ± 4 bu/acre, while corn following corn averaged 220 ± 5 bu/acre. That 19 bu/ac bump isn’t just nice, it’s the main reason corn-after-soybean looks more efficient on a per-bushel basis. It’s yield driving efficiency.

The year x region interaction was driven by a large yield increase in 2024 relative to 2023 in the “Illinois and Iowa Deep Loess and Drift” (27 bu/acre) and “Central Iowa and Minnesota Till Prairies” (14 bu/ac) while other landform regions didn’t change significantly with time, though the “Northern Mississippi Valley Loess Hills” declined by 26 bu/acre in ’24 relative to ’23, the low sample size kept it from being statistically significant.

Figure 1. Average corn yield by region and year. 2023 yields not sharing the same lower-case letter are statistically different α = 0.05, 2024 yields not sharing the same upper-case letter are statistically different, and yields within the same landform region not sharing the same number are statistically different.

Optimum N rates:

This analysis describes 26% of the variation in optimum N rate. The previous crop term was not significant so it was removed from the analysis. The yield at economic optimum and the region x yea interaction were all significant or near significant (p < 0.0001, p = 0.396) respectively.

In this analysis, the economic N rate was a function of the expected yield at economic N. For every bushel increase in yield at optimum N, the estimated increase in the economic optimum N rate was 0.45 ± 0.06 lb N/acre. In this analysis there was no difference between the economic optimum N rate for corn following corn and corn following soybean, though trial did indicate that corn was planted approximately 12 days earlier on average following soybean than if it followed corn. That means if you’re pushing 250-bushel yields, don’t be surprised when the EONR is a good 20–25 lb higher than a 200-bushel field, yield does matter.

Again, their was significant interaction between year x region was primarily driven by the large increase in Economic Optimal N Rate in the “Eastern Iowa and Minnesota Till Prairies” (27 lb N/acre) and no other regions seeing a statistically significant difference, though the “Iowa and Minnesota Loess Hills, Iowa and Missouri Deep Loess Hills” and the “Iowa and Missouri Heavy Till Plan” both saw large optimum nitrogen need in ’24 relative to ’23 (16 & 32 lb N/acre respectively).

Figure 2. Average economic optimum N rate by region and year. 2023 yields not sharing the same lower-case letter are statistically different α = 0.05, 2024 yields not sharing the same upper-case letter are statistically different, and yields within the same landform region not sharing the same number are statistically different.

N Use Factor:

Using this statistical model, we can describe 24% of the variation in the Nitrogen Use Factor at Optimum N. In this model most terms were significant or near significant, including year (p = 0.1044), previous crop (p = 0.0599), year x region (p = 0.0394), and yield at economic optimum nitrogen rate (p < 0.0001).

One of the key findings of this analysis is that the N use factor decreases with increasing yield, by about 0.0021 ± 0.0002 lb N/bu. While this may not sound like much, it suggests a decrease of about 0.1 lb N/bu in moving from 175 bu/acre corn to 225 bu/acre corn. In other words, the higher your yield potential, the less N each bushel needs to get made. That’s the story behind the push for better genetics, drainage, and management; it’s not just more yield, it’s better efficiency.

We also saw there was a significant difference in lb N/bu in corn following corn (0.97±0.03 lb N/bu) as compared to corn following soybean (0.93 ±0.02 lb N/bu). Earlier we reported that there was a 19 bu/acre yield advantage that corn following soybean had relative to corn following corn, which corresponds to this difference in nitrogen use efficiency.

Figure 3. Average nitrogen per bushel by region and year. 2023 yields not sharing the same lower-case letter are statistically different α = 0.05, 2024 yields not sharing the same upper-case letter are statistically different, and yields within the same landform region not sharing the same number are statistically different.

Conclusions

The Iowa Nitrogen Initiative is helping us move beyond rules of thumb and into real-world, Iowa-based numbers. We’re learning that yield is the biggest driver; more yield means more N total, but less N per bushel. Rotation helps mainly by boosting yield. And soil and weather patterns mean your field’s story may not match your neighbor’s.

The other lesson? We can only explain about a quarter of the variation in optimum N. That means three-quarters is still a mystery; soil differences, hybrid genetics, weather patterns, timing, and maybe things we don’t even have on our radar yet. That’s why more trials, more farmers, and more data are the only way we’ll keep sharpening this picture.

Weather Variability and Storage Capacity: Are You Designed for the Year You Actually Get?

 

We don’t farm averages. We farm variability. Livestock produce manure every day, mostly predictably. Weather adds water whenever it wants. Storage has to handle both. If we want fall application into cooling soils to remain an agronomic decision, not a reaction to storage pressure, we need to run the math now, not in September.

In our Spring Storage Planning article, we talked about the concept. Here, I want to punch some numbers and run the math.

Start with manure production.

How many head do you have? How many gallons per head per day are you generating? How many days until November 1?

Swine Finishing

For finishing swine in a deep pit, this is relatively predictable. Multiply head count by gallons per head per day and by days to your desired fall window, then divide by the surface area of your pit. Finishing pigs in a wean-finish barn, relatively tight in terms of water use, are often around 1 gallon per pig per day (your mileage may vary — I see barns from 0.85 up to 1.4 gallons per head per day). As I write this on February 23, there are 251 days until November 1. That’s roughly 300,000 gallons of manure.  doesn’t feel like much until you convert it to inches of pit depth. Have a 50x 190 barn holding 1200 head, that’s 51 inches of manure. Do you have that in your barn?

Liquid Beef

For liquid beef deep pit systems, it’s the same kind of math, just change the manure production number. Deep pit beef barn, put your manure production at around 6.5 gallons per head per day. That’s about 7.3 feet of storage space needed. But now we introduce uncertainty.

Are roof downspouts tied into the system? Over the next 251 days we average about 30 inches of rain. If about 80% of this turns into runoff and you are catching water in the pit from half your downspouts, that’s another foot of rainwater added. Do you have 8 feet of usable space so you can make it to your fall application window?

Liquid Dairy

For dairy, you add layers. Manure volume. Parlor wash water. Loafing lot runoff. Silage bunker runoff and leachate. What is the shape of the manure storage?

Let’s start around 22 gallons per head per day for manure and generated wash water. Looking at the ISU dairy — no loafing area outside, so I’m in luck. Silage bunker runoff is directed through vegetative filter strips. Yes, I chose this farm to make it easier.

With around 400 cows, I need roughly 18 feet of slurry storage space in our tank to get to November, plus whatever rainfall accumulates on the surface, maybe close to zero in a dry year, maybe several inches or more in a wet one.

But what if we were also handling runoff from the silage bunker area? That’s around 40,000 square feet. If roughly 80% of rainfall becomes runoff, I’d need close to another 5 feet of storage space in our manure storage.

If your projected level in September or October approaches your limit, the fall window is already compromised. Planning now can save some headaches latter.

Is Spring a Strategic Drawdown?

If the math is tight, what is the plan? Is our best approach to move some manure this spring to protect our fall window? If you do get full before November, what will you do? Do you have acres available? Are you confident harvest will start early? Will cover crops be established in time to receive early fall manure and protect water quality? This is not an argument that everyone should switch to spring application. It is an argument that some operations may need to use spring strategically to protect fall.

Fall application can work very well, if we actually reach cooling soil temperatures. That only happens when storage capacity gives us the ability to wait. Run the numbers. Then decide intentionally.

Why Booster Pump Placement Matters Near Streams

Umbilical manure application has become a go-to option for moving large volumes of manure efficiently while keeping heavy tanks out of the field. But with long hose runs, connections, and high pressures, failures can and do happen. One of the more concerning scenarios is a hose rupture or leak near a stream or drainageway.

Where we place the booster pump relative to that stream can make a big difference in how much manure is released if a failure occurs, and whether that release becomes an environmental incident or not. Hopefully you’ve heard the best management practice is to place the booster pump across the stream so the pressure is lower in the hose as it crosses the stream, but why?

The Scenario

Imagine an umbilical system crossing a small stream. Somewhere near the stream crossing, the hose develops a hole. This could be from abrasion, a weak spot, damage during setup, or even normal wear. The key question is:

·         Is that section of hose under high pressure or low pressure when the failure occurs?

That depends entirely on where the booster pump is located.

Two Booster Pump Options

Option 1: Booster pump on the near side of the stream

·         Pump → high-pressure hose → stream crossing → toolbar

Option 2: Booster pump on the far side of the stream

·         Pump → high-pressure hose → toolbar

·         Stream crossing is on the suction / low-pressure side of the booster

From a manure movement perspective, both setups can work. From a spill risk perspective, they are very different in how quickly we are going to leak manure.

Figure 1. Demonstration of how to fix a leak in an umbilical hose at the Manure Expo.

 

Why Pressure Matters When a Hose Leaks

The flow rate through a hole is directly related to pressure. For a small hole or tear, the leak rate is approximately:

Where:

QL is the leak flow rate

Cd is the discharge coefficient (typically 0.6 to 0.7 for sharp-edged holes, which we will assume (thought it does mean there is enough pressure to keep the hose rigid, which may not actually be the case

AL is the area of the leak (the cross sectional area of the hole)

Delta p is the pressure difference inside the pipe versus outside, which I’m being lazy and assuming doesn’t chance from the pre-leak conditions compared to when the leak starts (it does change and the pressure will go done, but how much depends on the relative resistances and gets a bit more math heavy then we need).

Rho is the density of the fluid

That square-root relationship is important. If pressure increases by a factor of four, the leak rate roughly doubles, and that is the basis of the recommendation. By controlling the location of the booster pump, we are controlling the pressure should a leak occur in the worst possible location, the hose in the stream.

A Simple Pressure Comparison

Let’s put some reasonable numbers to this.

Typical umbilical operating pressure downstream of a booster pump: 120 psi

Pressure upstream (suction side) of a booster pump: 30 psi

Let’s assume a 1” diameter hole, that manure has the same density of water, and we’ll set C_d = 0.65.

Option 1

Option 2

 

All that to say, by quadrupling the pressure (120 psi vs 30 psi) we doubled the rate of leakage (212 gpm vs 106 gpm).

 A low-pressure leak may be slow, noticeable, and more easily stopped before reaching water

Why the Stream Crossing Should Be Low Pressure

Placing the booster pump across the stream ensures that the hose segment near the waterway is operating at the lowest pressure in the system. That does three important things:

·         Reduces leak flow rate if damage occurs

·         Buys response time to shut down the system

·         Limits environmental consequences if manure reaches the stream

This is a classic example of risk-based design: assuming failure can happen and designing the system so that when it does, the consequences are minimized.

Positioning booster pumps so that environmentally sensitive areas, streams, ditches, intakes, and tile outlets, are exposed to as low-pressure hose as practically possible.

Practical Takeaways for Applicators

·         Always identify stream crossings during hose layout

·         Place booster pumps after the stream crossing, not before to minimize risk should a break occur.

 One last thought, is there a tank equivalent to this? Yes. If loading near the barn or in the field, look for the location that offers the least risk should a tank overflow occur. In particular this means looking for surface inlets around the loading area and trying to move as far from them as possible.

We can’t prevent every spill, but we can manage to minimize their occurrence and any negative environmental impact they may have.