Pre-Sample or Past Average? Choosing the Right Info to Set Your Manure Rate

 When it comes to setting your manure application rate, the golden rule hasn’t changed: test your manure. Every year. But when you’re planning your fall application in August and September, you might find yourself staring at two different numbers: a sample you just pulled from your storage before agitation, and the average of those carefully collected samples from the last few application seasons.

Which one should you trust more?

Let’s talk about what those numbers are really telling you.

The Pre-Sample: A Snapshot with Caveats

A pre-sample pulled late-summer or early fall, from a pit or lagoon that hasn’t been agitated yet, is tempting. It’s fresh, it’s this year, and it feels like it should be the most relevant. But unagitated storages stratify. Solids settle. Nutrients settle with them. That means what you sample near the surface in September might look different than what you actually apply on your field in October when the storages are agitated. How different the manure tests is a function of the manure we are working with, the nutrient we are most interested in, and the distribution of those nutrients between the solid and liquid fraction.

Nitrogen (N)

You might think nitrogen is easy to predict, but it's influenced by:

·         Time in storage (volatilization)

·         Diet shifts (especially protein levels)

·         Storage conditions (temperature, dilution from wash water or rain)

·         In most liquid manures ammonium nitrogen (a dissolved and water-soluble form) makes up about 50-75% of the total nitrogen, the other nitrogen (organic) is attached to solids. Sampling from liquid swine manure where it is 75% of the nitrogen is in the ammonium form, your pre-sample is probably reasonable (within 10% for deep pit storages from swine finishing operations). For dairy manure storages, you might notice a bigger difference of more like 30% in total nitrogen content when agitated.

Phosphorus (P₂O₅)

·         Phosphorus also has a dissolved and particulate bound form, however, in most manure storages greater percentages of phosphorus are particulate bound.

·         A sample drawn before full agitation may substantially underestimate P

·         Settling during pumping may cause uneven P distribution in fields

·         Agitated dairy manures will often test 50% higher than unagitated dairy manure for phosphorus content.

Potassium (K₂O)

·         Potassium is dissolved and mobile in slurry

·         Levels are still affected by dilution or bedding.

·         Generally, pre-samples are within 10% of samples from agitated manure storages

The Running Average: A Stable Forecast

In contrast, a running average of samples collected during past application events, when the manure was agitated and representative of what was applied, gives you a more stable estimate of what’s likely to come out of the tank this year, assuming nothing major has changed in your operation.

This kind of average smooths out year-to-year quirks and captures the manure you actually applied, not just what was floating on top in September. It reflects your real-world nutrient delivery, but doesn’t help you know for certain what was applied this year until after the application was done. If you have an out of barn manure storages, and rainfall amounts differ from year to year, you had a water leak in your barn this year, or made a management change to your manure handling on a change to the diet composition of your livestock the average may no longer be representative of what is in your storage this year.

Which Should You Use? Pre-sample or Running Average

We looked at data from six swine farms (finishing and gestation farrowing farms) with data series ranging from two to six years, with multiple samples (2 to 12) collected each year throughout land application. All sites used in-barn manure storages. Across all sites, phosphorus was the nutrient most likely to be misestimated using a single sample in any year, due to how tightly P is linked to manure solids. If solids settle or are not evenly agitated, a sample might not reflect the full picture.

In contrast, nitrogen and potassium, which are more often in dissolved forms, were similarly estimated whether using a single sample or a prior-year average, though no advantage was found to using a single sample from the current year as compared to running average nitrogen content to estimate the manures nitrogen content.

In work I’ve done at Nashua we routinely take pre-application samples from an unagitated manure storage and compared a single pre-sample to the manure results at the time of application. At this farm, we routinely test 20% higher for total nitrogen content at pre-sampling compared to what is obtained at the time of application. However, at this farm our pre-sample is generally collected before manure application occurs from the farm, while our as applied sample generally comes from manure applied after commercial manure applicators have been to the facility and emptied the pit. Steve Hoff suggested ammonia emissions during manure agitation were 4.5x higher during agitation than prior to agitation. While generally ammonia emissions are low from a deep pit barn (about 26 lb/day from a 1200 head barn); however, that means on the day of agitation this is 117 lb NH3 and it stays elevated for a short period (we’ll assume a week) after at about 40 lb N/day. These elevated emissions resulting from the agitation would result in about a 10% change, so not the 20% we saw, but similar in magnitude and a unique situation how we are operating at this farm.

Where does this leave us?

If you are trying to apply all your nitrogen with the manure, a pre-sample becomes a must. You need an estimate to set your rate. However, what I’ve started to do is just the pre-sample and compare to samples tanking during application and adjust accordingly. So, when my Nashua Iowa pre-sample comes back at 70 lb N/1000 gallons and I’ve historically, and consistently, seen 20% lower at the time of application, I adjust to 56 lb N/1000 gallons and roll with it.

It also means that if you're making decisions about application rates in advance of agitation, using your running average makes sense. It’s your best estimate of what you’re likely to apply, and it avoids the pitfall of making decisions based on unrepresentative samples, and as long you know the barns management is similar to previous years, it makes sense.

That doesn’t mean a pre-sample is useless, far from it. If you’ve changed diets, added water, or seen other operational changes, or are getting manure from a barn you for which you don’t know how this year’s management compared to previous, pulling a pre-sample can be a valuable early signal. Just use it as a flag to adjust expectations, not as the final say.

With that said, often times manure isn’t our only form of nitrogen. If this is the case and we are applying to be short on nitrogen, sampling during manure application and using those samples to know how much is applied is the best of both words. As long as we don’t exceed the amount of nitrogen, we want it informs us of how to adjust our commercial nitrogen application that will happen later.

How Many Samples Should You Collect?

While this question sounds vastly different than the one we asked earlier about collecting a manure sample, in many respects it is the same style of question. Again, it is about the value of information gained, in this case from every additional manure sample. The place to start is by understanding how variable samples are, but in this case, not from farm-to-farm but within a manure application event at a single farm.

Similar to what we saw earlier, the variation in manure samples is proportional to the average concentration of the manure, with higher sample concentrations having more variation. Generally, at the coefficient of variation I typically see for manures we additional samples to help hone into the correct amount of nitrogen supplied was worth around $3 an acre, however, even at this price a manure samples every 40-acres would pay for itself. To put this in perspective, this is approximately every 100,000 gallons of manure or three or four manure samples from a 1200-head barn. All this to say, we could be collecting more manure samples than we are in most cases to better understand variation while we are applying. Information is power, and this is a case were accessing that information will help us better understand variation or if there is a trend, like increasing nitrogen as we move to the bottom of the manure storage. Figure 1 and table 1 show the trends in variation of nutrient content while emptying a single storage and estimate the value additional samples offer.

Figure 1: Variation, as denoted by standard deviation, during a manure removal event as compared to the average nutrient concentration of the manure.

Table 1: Estimated value from additional manure samples that help you fine tune your manure application rate. While not as valuable these results still suggest that collecting additional samples provides enough benefit to get one average every 40 acres covered.

Tuning Up Your Manure Storage: Mid-Summer Maintenance That Pays Off

While summer is flying by, now is the time to tune up your storage, not just your equipment.

Waiting until September to check your pit, pond, or tank can leave you scrambling. A quick mid-summer storage review can help avoid headaches later and get you ahead of both environmental and logistic risks.

1. Review How Full You Are

Like most years, rainfall in Iowa has been variable throughout the state, but many of our livestock producing areas have seen substantially more rainfall than average, with some pushing 8-inches of rainfall above normal through this point in the year. While a few portions of Iowa had to deal with abnormally high rainfall additions to outdoor manure, the dry summer and fall helped alleviate some of the stress of full storages come application season. While it is too soon to know what the rest of this summer and fall will bring, reviewing how full your manure storage is and assessing to your storage needs to make it to manure application season this fall, it is critical to ensure storage success. Figure 1 provides a map of Iowa rainfall through July 18th as compared to normal, indicating that some locations are trending about 8.5 inches more than normal since January.

To help put in perspective, what this amount of water of rain means, if you had a 150-foot diameter circular manure storage (the same as the ISU dairy) and received 8.5 inches of direct rainfall more than normal it would add 93,600 gallons more water than normal to the manure. This is approximately equivalent to the manure produced over the year by 13 dairy cows.

Figure 1. Comparison of January 1 to July 18th precipitation as compared to normal, indicating that some areas of Iowa have seen much higher rainfall than average through this period of the year.

2. Project Your Fall Application Window

If you are in a situation where your manure storage volume might be tight, start communicating with your custom applicator or cooperating landowners now. If you’re looking at needing an early application, it’s also time to review nitrogen stabilization strategies or soil nitrate retention tools. Cover crops are especially effective at capturing early applied nitrogen.

3. Inspect Safety Features & Clean Up Around the Storage

Manure often isn’t the first thing on our mind as we are busy with spring field work and then again with fall harvest, but with summer hopefully it gives some time to think about maintenance of the manure storage. You see it every day, but have you really looked deeply at it to see how it is working?

• Check fences for wear and areas that need repair.
•  Look over or add signs around access points to ensure safety around open storages or pits.
•  If you have a push ramp, make sure it is in good repair and will stop you from rolling into the pit as you are pushing in manure.
• Check over agitation and pump-out ports so they secured and in good repair so when it comes time to move them in the fall you can quickly get them out of the way.
•  Clear weeds and brush to improve visibility and reduce pest risk. Check for signs of erosion, cracking, or damage around embankments or pit walls.
• Evaluate roads and paths to the manure storage to ensure equipment can access critical areas and movement of mud to roadways will be minimized
• Review your emergency response plan; make sure items are up to date and you are prepared for pumping season with critical contact numbers.

Figure 2. Clearing away brush and debris and keeping the area around the manure tank mowed allows easier assessment of storage conditions and risks.

Bottom Line:

Just like a planter check in February saves stress in April, a manure storage tune-up now pays off in smoother, safer application this fall. You’ll avoid overflow risks, reduce emergency pumping costs, and give yourself time to plan smarter.

Why Manure Sampling Is Still Your Most Valuable Tool

I spend a lot of time talking about managing manure as a resource, and while manure sampling has become very popular in Iowa, with more than 90% of farmers reporting they take at least one manure sample a year. But here’s the hard truth: no other management change gives you more immediate, practical value than a good manure sample. It’s not about checking a box. It’s about unlocking real dollars in nutrient value and reducing risk.

The Value of Information

There’s a concept economists use called the value of information. In simple terms, it means that better decisions are made when you have better data. If you’re applying manure without sampling, you’re basing application rates on assumptions, and those assumptions often cost you.

Let’s say your manure is actually 40 pounds of nitrogen per 1,000 gallons, but you guessed 32. If you’re applying 5,000 gallons per acre, that’s a 40-pound-per-acre miss. In this case, it’s 40 lb N/acre more than you wanted, and that means you wasted about $19 worth of N fertilizer value. In a paper we wrote a few years back, we walked through how just one manure sample per year, interpreted correctly, could return $8–28 per acre if applying at a nitrogen limited rate and $1-20 per acre when applying at a phosphorus limited rate, depending on nutrient prices and application rates. Now scale that across a 1,000-acre operation. Sampling starts to look like a no-brainer.

The Manure Database: Variability Is Real

You might think your barn is like your neighbor’s, or that one pit looks like another. But ManureDB, a Manure Composition Database says otherwise. Lately it feels like I’ve been looking at the database a lot to determine what an “average” or “typical” manure looks like and to determine if we can determine differences in nutrient content between different types of manure. However, here we want to do something different, we are more interested in how variable the manure samples are within a single system. This is because manure types that are consistent might be able to be estimated with a book value with reasonable accuracy, while samples that are more variable need to be sampled more frequently to ensure a representative sample.

This same concept can be continued to determine how often you should be manure sampling or how many samples you should collect during manure application. If samples are consistent, fewer samples are necessary, if nutrient concentrations vary substantially from the start of the manure application event to the end, either with a changing trend with time or with high variation, more samples are necessary. While this second question is useful, we will focus on the first, how valuable is sampling in general in this article, and then on the second question of variability of nutrient content during manure application a little later when we explore when to use a running average of samples or when samples from the current year are preferred.

How Variable is Manure Between Farms

Manure variability can be expressed in multiple ways, but the two most prominent are either the standard deviation or the coefficient of deviation. The coefficient of variation is the standard deviation divided by the average, while the standard deviation is a measure of the dispersion of the data relative to its mean. While both these concepts are useful, there is another concept called mean deviation that quantifies the average distance between each data point and the mean. For normal distributions, the mean deviation is about 80% of the standard deviation.  Figure 1 shows the relationship between the average nitrogen concentration of a manure type and the standard deviation of the nitrogen concentration in the manure type; effectively as average concentrations get bigger, so does the variation.

Figure 1. Relationship between the average N content and the standard deviation of that manure types nitrogen concentration.

For our purposes we will use the mean deviation (both positive and negative), which is 80% of the standard deviation, and the mean value averaged together to estimate the potential value of a manure sample. While this makes the math much quicker and easier, it isn’t a perfect answer as the value of the sample is a non-linear function due to the yield response being non-linear, but it gives a quick approximation. 

Table 1 provides an estimate of the value of knowing the true nutrient content of the manure, i.e., from sampling adequately to estimate the true mean of the manure being applied by different animal manure storage types, storage types, along with the coefficient of variation and the value of a manure test in the continuous corn and corn-soybean rotation. Manures that have higher coefficients of variation (essentially the standard deviation divided by the average) get more benefit from sampling because our guess about what would be in the manure if we didn't have a sample, is worse.

Table 1. Estimated value of knowing the nutrient content of the manure for different rotations (CC – Corn following corn, CS- Corn following soybean) and livestock/manure storage combinations.

Make It Count

When you know your manure’s nutrient content:

• You match rates to crop needs
• You reduce commercial fertilizer inputs
• You track nutrient balances
• You have proof of value for the manure user

Doing these is the difference between managing manure as a waste and managing it as a fertilizer. Now, more than ever, it is critical that those of us using manure that we document the value of manure and prove we are using it to the best of our ability.

Bottom Line:

The cost of guessing is higher than the cost of a test. A manure sample might be your most valuable tool this year, and every year. It pays for itself, protects your crops, and helps you manage manure smarter. The time and effort of getting a manure sample you can trust and using those results pays for itself in the first 2 to 5 acres.

Understanding Nitrogen Availability from Liquid Swine Manure: Why Ratios, Timing, and State Guidance Matter

 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.

Solids Separation in Dairy Manure: Digging Deeper into the Tools for Phosphorus Control

 Is solid separation needed to help you manage phosphorus accumulation in soils in your field? Dairy manure tends to have more phosphorus than most cropping systems need when we apply nitrogen-based rates. Over time, this leads to nutrient loading, P Index concerns, and tougher decisions about where to haul your manure.

My last article discussed strategies for tackling this imbalance, like targeting low-P fields or adopting practices that hold soil in place. But what if your fields are already saturated, your neighbors aren't lining up to take solids, and you're still trying to manage more manure than you have places to put it?

Let's dig into solids separation—not as a silver bullet, but as a tool that gives you more control over where your nutrients go. We'll focus on the nuts and bolts of how these systems work, what kind of phosphorus they capture, and what the real-world tradeoffs look like.

Why Separate?

The basic idea is this: when we separate manure, we split the nutrients into two different forms:

·         Solids, which are enriched in (especially the organic and particulate-bound forms)

·         Liquid, which contains most of the ammonium-N

This gives us flexibility. Suddenly, you're not stuck applying the full nutrient load to every acre. You can:

·         Haul solids to further fields without hauling water

·         Retain nitrogen-rich liquids for near-barn applications

Let's look at the tools we've got—and what they do regarding phosphorus removal and system compatibility.

Tools of the Trade: Separation Systems and Their P-Capturing Power

Slope Screen or Static Screen

The slope or static screen might be your entry point if you're looking for the simplest mechanical manure separator. These systems rely entirely on gravity—no moving parts, no motors, and minimal maintenance.

How It Works

Manure is pumped or flows across an inclined, slotted screen, usually mounted at a 15–30° angle. As the liquid manure flows over the screen, coarse solids catch on the surface and slide down while the liquid passes through into a collection tank or lagoon.

This method works best with thick, undiluted manure and a relatively high amount of large fiber.

Screw Press

The screw press is one of the most common and accessible solids separation systems on dairy farms today. It strikes a balance between simplicity, cost, and performance—making it a solid choice for farms looking to improve manure handling and pull out some phosphorus without diving into full-scale treatment systems.

How It Works

At its core, the screw press is a mechanical filter. Manure is pushed into a cylindrical housing where a rotating auger presses the material against a screen (often wedge wire or perforated steel). As pressure builds, the liquid is squeezed out through the screen, and the remaining solids are discharged at the end of the auger as a damp, fibrous cake.

The design is straightforward, with relatively low energy and maintenance requirements. No chemicals, no high-speed moving parts, and no need for complex control systems.

Centrifuge

On the other end of the solids separation spectrum, you've got the centrifuge—a high-speed, high-cost solution aimed at serious nutrient recovery. Centrifuges are more common in commercial or tightly regulated livestock operations, but they're becoming increasingly relevant where phosphorus management is mission-critical.

How It Works

A centrifuge spins manure at high speeds—often thousands of revolutions per minute—causing particles to separate by density. Heavier solids (and the phosphorus they carry) are flung to the outer wall and collected, while the lighter liquid stays near the center and exits through a different outlet.

Expected Performance

Separation performance was surveyed by Hjorth et al. (2009). The summarized performance is shown in Table 1. While all separators had the potential to alter the N:P ratio and create a solid fraction that could move phosphorus further from the barn, centrifugation had a much higher potential.

Table 1. Separator performance and variation in performance for solids, total nitrogen, and total phosphorus separation into the solid fraction.

	Separation Percent (% in solid fraction) ave. (s.d.)
	Dry Matter	Total N	Total P
Sloped Screens	47 (28)	34 (28)	30 (22)
Screw Press	34 (15)	15 (12)	13 (7)
Centrifuge	63 (15)	36 (14)	69 (17)

 

Table 2. Estimated characteristics of the separated solids and the remaining liquid after separation. Centrifugation has the potential to substantially alter the N-to-P ratio of both the liquid and solid fractions.

	Solids Characteristics (lb/ton)				Liquid Characteristics (lb/1000 gallons)
	Solids (%)	Total N	P2O5	Available N:P	Solids (%)	Total N	P2O5	Available N:P
Sloped Screens	14	8	3	1.3	4	16	6	1.9
Screw Press	23	8	3	1.3	4	18	7	1.9
Centrifuge	33	16	11	0.6	2	14	2	4

 

Thinking Ahead: What's Separation For?

Solid separation doesn't eliminate your phosphorus problem—it reshapes it. It works best when it:

·         It buys you the flexibility to put nutrients where they're needed

·         Enables cost-effective solids hauling to further fields or sell to other farmers.

Final Thoughts

Solid separation isn't magic, and it won't make phosphorus disappear. What it can do is give you options. It lets you make smarter nutrient management decisions, especially when your fields are overloaded, or your P Index is getting tight.

·         Screens and screw presses are relatively affordable and easy to operate—but don't expect miracles.

·         Centrifuges can remove a lot more phosphorus, but they come with a big price tag and more management.

·         None of them solve the problem unless you also solve the logistics of where those solids go.

For most dairy farms, separation makes the most sense when it's part of a bigger strategy:

·         Matching nutrients to fields that can use them

·         Using agronomic practices to reduce runoff and improve soil uptake

·         Thinking ahead about storage, labor, and timing

And if you're not ready to invest in new hardware? You can still manage phosphorus smartly by focusing on P Index risk reduction. Cover crops, ensuring erosion control, using thoughtful application timing, and squeezing every bushel of uptake you can out of your crop system.

Solid separation isn't a silver bullet. But it can be a good wrench in the toolbox—especially when you know what job you're trying to get done.

Revisiting Nutrient Separation: What's Changed in Manure Management and Where We're Going Next

 In 2016, I wrote a blog post titled Can Nutrient Separation Reduce Manure Application Costs? At the time, I explored how separating and concentrating nutrients in manure might help farms reduce land application costs — especially when facing long hauling distances or limited land access nearby. The central idea was simple: nutrient-dense manure is cheaper to haul per pound of nutrient than diluted manure. If we could concentrate those nutrients into a smaller volume, we could make manure a more cost-effective fertilizer.

Nearly a decade later, I've been getting more questions than ever about manure treatment and nutrient recovery. That old article still holds up, at least in principle. But times have changed — and so have prices, technologies, regulations, and perhaps most importantly, our understanding of the manure system itself.

Because of that, it feels like it's time to revisit this question: When — and where — might nutrient separation make sense on Iowa livestock farms today?

Why This Topic Is Worth Revisiting

When we think about manure, we often compare it to commercial fertilizer — a source of nitrogen (N), phosphorus (P), and potassium (K) with added soil health benefits. But here's the hard truth: we don't capture the full agronomic or economic value of manure nutrients as well as we do from synthetic fertilizers. And that gap is worth revisiting.

Let's start with the nutrient profile of livestock manure. In cattle and poultry systems, manure tends to have a low nitrogen-to-phosphorus ratio compared to what most crops need. Corn, for example, typically requires about 6 to 7 pounds of nitrogen for every pound of phosphorus. But finishing swine manure often has a ratio closer to 2:1 or 3:1. That mismatch creates a problem: if you apply manure to meet the crop's nitrogen needs, you'll overapply phosphorus — sometimes dramatically. Over time, this builds up soil P levels beyond agronomic need, increasing the risk of runoff losses and environmental scrutiny. However, if you run a corn-soybean rotation, this can even out with a crop demand closer to the 3:1 swine manure offers.

Conversely, if you apply manure to meet phosphorus recommendations, you'll underapply nitrogen — and probably have to supplement with commercial fertilizer. In either case, you're leaving value on the table: excess phosphorus and potassium beyond crop need doesn't generate crop yield but still costs you time, fuel, and application effort to haul and apply while having to make a supplemental nitrogen fertilizer passes (admittedly this is of minor consequence).

Now, let's consider nitrogen. On paper, manure contains a lot of it — but in practice, the effective nitrogen supply from manure is more variable and often less predictable than synthetic fertilizers. That's partly due to application timing. Commercial fertilizers go on when the crop needs them, or in the case of anhydrous ammonia, they are often self-inhibitory to nitrification to some levels. Manure, on the other hand, frequently goes on when storage is full, fields are accessible, or when equipment and labor are available. In the Midwest, this usually means fall, months ahead of crop uptake. This disconnect increases the potential for nitrogen loss via leaching, denitrification, or volatilization.

Research and farmer experience show that we often apply more manure nitrogen than needed to "hedge our bets" against such losses. For example, ISU guidance on nitrogen use efficiency suggests that fall-applied manure might only deliver 70–80% of its total nitrogen to the crop. That means you need to apply more to get the same result. This sometimes means applying 30 to 50 pounds more nitrogen per acre than you would with well-timed synthetic fertilizer. That's more hauling, more labor, and more nutrient losses — and ultimately, more cost per unit of nutrient used. You can dispute this – managing your manure well and nutrient use efficiency can be very similar to synthetic fertilizers. Still, while most ISU research on synthetic nitrogen fertilizers shows partial factor production (lb N/bu) decreasing (a good thing), those using manure plans following the yield goal method in corn-soybean rotations have been applying more N.

This points to a core challenge: while manure provides valuable nutrients, we don't always use them efficiently or economically – sometimes due to our choices and sometimes due to the practical realities of the Midwestern agricultural system and the integration between crop and livestock production. That's where the promise of nutrient separation and manure treatment technologies comes in.

If we could separate solids and nutrients, concentrate nitrogen into a more stable and transportable form, or even create dischargeable water, we could reshape how, when, and where we apply nutrients originating from manure.

Imagine if a treatment system allowed you to reduce the volume of material you needed to haul by 50–75% — either by removing water or concentrating nutrients. That could mean:

·         Fewer tankers on the road, reducing fuel and labor needs;

·         More timely applications on fields that are farther away but still agronomically valuable;

·         Greater flexibility to store treated manure or separate nutrients until conditions are right;

·         The possibility of irrigation-like systems for applying treated liquid fractions in-season when crops can use nitrogen.

In systems where treated effluent meets water quality standards, some farms are beginning to explore discharge or reuse options, which could further reduce the storage and hauling burden. While my reading of the Iowa code suggests this wouldn't be allowed, the future potential is there if we ensure the water is clean enough. A high bar but a technically feasible one.

A Tool, Not a Silver Bullet

Let's be clear: nutrient separation or manure treatment won't make sense on every farm. The technology is still evolving, the capital investment is significant, and the return depends heavily on your hauling distance, soil nutrient levels, land access, and long-term nutrient management strategy. The result may be similar to what I concluded the last time, a significant engineering challenge that we could do, but something that doesn't make economic sense for many Iowa farms due to the strong integration with crop production.

But as livestock farms grow, land availability tightens, fertilizer prices remain volatile, and water quality pressures increase, the economics and logistics of manure use will keep changing. The value of a pound of phosphorus or a gallon of water you don't have to haul may look very different in the future than in 2016 (or we might understand how well we are currently using manure nutrients relative to fertilizer nutrients better).

A New Look with Updated Costs, Technology, and Context

I still believe the best manure use happens when livestock and cropping systems are integrated — something Iowa does well. But the pressures on and great questions push us to move forward and relook at things we thought we knew. I'm dusting off my old spreadsheet, giving it an upgrade, and asking: What do today's hauling, application, and treatment costs tell us about the potential for nutrient separation on Iowa farms? Over the next eight months, I'll be:

·         Updating the original tool with current cost data and more flexible assumptions about transport logistics and application timing.

·         Exploring new technologies for nutrient separation and nutrient recovery — including which are proven, which are promising, and which are mostly hype.

·         Sharing new case studies that show how and where separation systems might pencil out — especially in phosphorus-saturated regions or when land application windows are tight.

·         Publishing a decision-support tool to help you evaluate if and when separation systems might reduce costs or create agronomic value on your farm.

What's Next

This project isn't about selling anyone on a piece of equipment. It's about helping farmers, advisors, and policymakers think more critically about how the science of manure management connects to economics — and how we might adjust our approach when costs, constraints, and conservation goals shift.

If you're a farmer, manure manager, consultant, or researcher thinking about these same questions — or better yet, trying these technologies in the field — I'd love to hear from you. The more perspectives we bring, the better this new tool will be.

Until then, stay tuned. The math may be similar, but the conversation around nutrient separation is more important than ever.