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.

Comparing N-FACT to the Yield Goal Method: A Look at Nitrogen Recommendations

A few years ago, I took a deep dive into two common approaches for determining nitrogen application rates: the Yield Goal method and the Maximum Return to Nitrogen (MRTN) approach, illustrating which was higher in each county in Iowa and how similar or different they were. That comparison highlighted the strengths and limitations of each method, offering insights into how they influence manure management planning and nitrogen use efficiency. With Iowa State University’s recent release of the Nitrogen Fertilization Application Consultation Tool (N-FACT), it seems like the perfect time to revisit this discussion, comparing N-FACT to the Yield Goal method.

 

N-FACT represents a step forward in nitrogen management, leveraging updated on-farm data and modeling techniques to refine nitrogen recommendations. But how does it compare to the long-standing Yield Goal approach in the Iowa Manure Management plan form? My goal here isn’t to summarize either method but to compare what the two methods suggest. In so doing, it gives some insight into who may be selecting each technique and why that may be.

In this post, I’ll walk through how N-FACT differs from the Yield Goal approach, present some figures to illustrate their recommendations across different scenarios and share key takeaways on their practical applications.

 

Manure planners are probably extremely familiar with the Yield Goal method. Still, the county average corn and soybean yield were briefly estimated using the last five years of county yield data (2019-2023). The amount of continuous corn and corn following soybean in each county was calculated from the area of corn planted and the area of soybean grown in each county. Corn following soybeans are set equal to the amount of soybeans in each county (but not allowed to exceed the acreage of corn for that county); continuous corn acres were calculated from the excess corn acres not yet accounted for in each county. Corn yields in continuous corn and corn following soybean were estimated by assuming a 10% yield drag in continuous corn as compared to corn following soybean and requiring the county-weighted average corn yield to equal the reported county average corn yield from the survey of agriculture. Yield goal methodology was then applied, setting the yield goal for corn equal to the estimated yield for that rotation plus ten percent, multiplying by the area-weighted nitrogen use factor for corn (typically 1.2 lb. N/bu for most Iowa counties, but adjusting as necessary for areas that are 1.1 lb. N/bu and 0.9 lb. N/bu, see figure 1). For corn-soybean rotations, a “rotation” credit of 1 lb N/acre per bushel of soybean yields up to a maximum of 50 lb. N/acre was used.

Figure 1. Nitrogen use zones for corn in Iowa for use in the Yield Goal method in Iowa Manure management Planning. Zone 1 has a nitrogen use factor of 0.9 lb. N/bu, Zone 2 has a nitrogen use factor of 1.1 lb. N/bu, and Zone 3 has a nitrogen use factor of 1.2 lb. N/bu.

 

N-FACT works differently, following an approach similar to Maximum Return to Nitrogen (MRTN) but based on modeling efforts that simulate the yield response of corn to nitrogen in each county under various weather conditions, planting options, and management scenarios. For this tool, you select a county, the rotation, weather conditions for spring and summer, the amount of residual nitrate in the soil, a planting date, and a nitrogen and corn price. Inputs used in these simulations were done in all counties for corn following corn and corn following soybean, but then consistent weather patterns (average spring moisture, average summer moisture, a residual soil nitrate of less than 20 lb. N/acre, a planting date before April 30^th, a nitrogen price of $0.45 per pound N, and a corn price of $4.64 per bushel. N-FACT provides a range of outputs from the 25^th percentile economic optimum nitrogen rate, the 75^th percentile economic nitrogen application rate, and the average economic optimum nitrogen application rate. A modeled yield is provided for the 25^th and 75^th percentiles and the average economic optimum nitrogen application rates. The N-FACT tool uses a concept from MRTN: the response from applied N determines the economic optimal rate, not the yield itself. As such, even if actual yields differ from those suggested by N-FACT and the underlying modeling behind it, the response curve generated is still the best estimate of the N rate required. All that to say, when the N-FACT reports the 25^th percentile economic optimum N-rate, it suggests that 25% of the modeled site-weather years in the county would require a lower nitrogen rate to be economically optimal. Similarly, when it reports the 75^th percentile N rate, it suggests that 75% of site-weather years within the modeled scenario would have a lower economically optimum N rate.

 

An essential consideration for manure planning is how different locations will be impacted if a switch from the yield goal method to the N-FACTs recommendation is evaluated. For both rotations, we grouped counties into three groups: counties where the yield goal nitrogen estimate was less than the 25^th percentile economic N rate from N-FACTS (N-FACTS suggests considering higher nitrogen rates). In these counties, the yield goal nitrogen estimate is between the 25^th and 75^th percentile economic optimum N (N-FACTS and the yield goal method are similar), and locations where the yield goal estimate exceeds the 75^th percentile economic optimum N rate. These counties were colored blue (Yield goal lower than the 25^th percentile N-rate from N-FACTS), white (Yield goal is between the 25^th and 75^th percentile N-rates from N-FACTS), and orange (Yield goal is higher than the 75the percentile N-rates from N-FACTS), respectively.

 

(a)	(b)
Figure 2. Comparison of the Yield Goal Method (based on county average yields) and N-FACTS nitrogen suggestions for (a) corn following soybean and (b) continuous corn. Counties colored blue have a yield goal N rate below the 25th percentile economic optimum N suggestion from N facts, counties left white have a yield goal N rate between the 25th and the 75th percentile economic optimum N rate suggestion from N facts, and counties in orange have a yield goal N rate above the 75th percentile economic optimum N rate suggestion from N facts.

 

The maps in Figure 2 suggest that counties on the Des Moines Lobe, the Iowa Erosion Surface, and much of the Northwest Iowa Plains farms should continue to fill out their manure management plans using the yield goal method, but at the time of manure application should be considering opportunities to lower their actual nitrogen application rates (through both manure and commercial manure fertilizer) to be more in line with the guidance suggestions coming from N-FACTS. In the remaining regions of Iowa, primarily the Alluvial Plains, the Loess Hills, the Southern Iowa Drift Plain, and the Paleozoic Plateau, we should carefully consider our nitrogen application rates and obtained corn yields to evaluate improvement opportunities. Specifically, N-FACT suggests that higher nitrogen application rates than those indicated by the Yield Goal method of the Iowa Manure Management Plan form may be required to reach optimum economic thresholds. However, the N-FACTS tool suggests that these locations can achieve higher yields. As such, choosing to apply higher nitrogen rates should be accompanied by evaluating corn yields and practices that may help increase corn yields.

While the monolithic evaluation provided in Figure 2 helps answer the question of writing a manure plan with a yield goal compared to writing a manure plan with N-FACT, it doesn’t address the magnitude of the change. Overall, the results are similar; the counties in red on the Des Moines Lobe and the Iowa Erosion Surface are locations where the yield goal is higher than the 75^th percentile N suggestion from the yield goal. In many of the remaining counties, the green color indicates that the yield goal would need to increase by this much to reach the 25^th percentile N suggestion. While the graphs are similar, these figures illustrate the magnitude of change that could cause.

(a)	(b)
Figure 3. Comparison of the Yield Goal Method (based on county average yields) and N-FACT nitrogen suggestions for (a) corn following soybean and (b) continuous corn. The scales demonstrate the magnitude of difference between the yield goal method and the N-FACT suggestion. Counties in red show N-FACT is lower than the yield goal suggestion, and counties in green show N-FACT above the yield goal suggestion.

 

One important point to clarify is writing your manure plan based on a method and selecting a nitrogen application rate. Writing a manure plan to allow flexibility in decision-making is often helpful. Choosing methods that would enable higher nitrogen application rates and include more acres than the minimum required to write an acceptable plan provides the needed flexibility. Such an approach puts us in a position to make the best agronomic decisions within a growing season to respond to unknowing weather patterns; this means not treating the manure plan rate as a maximum rate but choosing a target rate below this to make the best use of manure. It also plans to increase the number of acres our manure touches to maximize its value.

Writing plans for higher nitrogen application allows flexibility without having to write amendments to the plan should weather conditions in any specific year put us in a situation where more nitrogen is required. An example of this might be if a wet spring occurs and additional nitrogen is needed to address this concern. One example of how to do this is that plans choosing to use N-FACT as their methodology could be written for average moisture conditions, but should wet spring conditions occur, the farm would be allowed to modify to wet weather conditions without filing an amendment.

One important question that remains is how a change from yield goal to N-FACT recommendations would impact overall nitrogen recommendations in the state. In the map, you can see areas where more nitrogen is required and areas where less nitrogen would be suggested, but it is hard to say what this means for state nitrogen use. Weighting the state by the number of corn acres in each county suggests that if all acres were applied based on a yield goal methodology and switched to the N-FACT suggestion, the average nitrogen application rate would decrease by about ½ lb. N/acre. As I follow up, it will be important to compare what this means for some of the manure-rich areas in the state and the implications for manure management planning.

Another aspect of N-FACT is the on-farm nitrogen rate trials. The on-farm trials provide data on the economic optimum nitrogen rate, yield at the economic optimum nitrogen rate, and the nitrogen use efficiency at the optimum nitrogen rate for each trial conducted. Thus far, data from two trial years, 2023 and 2024, have been available. The nitrogen use efficiency at the economic optimum nitrogen rate provides another point of comparison for the yield goal method. In 2024, the on-farm trials for corn following corn averaged an N use efficiency of 1.00 ± 0.19 lb. N/bu (ave.  ± s.d.). One interpretation of these results is that 84% of fields (assuming the fields evaluated represent Iowa fields) are below the 1.2 lb. N/bu.

As we look at the on-farm trials for corn following soybean, the obtained N use efficiency was 0.92 ± 0.19 lb. N/bu (ave.  ± s.d.). Again, this can be interpreted as 84% of fields (assuming the fields evaluated represent Iowa fields) are below 1.12 lb. N/bu, that on average corn following soybean crops would be allowed in Iowa Yield Goal based plans. This value is based on the 231 bu/acre corn yield (average of corn following soybean plots) and a 50 lb. N/acre rotation effect that allowable N rate is 1.10 N/bu.

Data is also available for 2023, but the previous crop isn’t listed. The measured N use per bushel of corn at optimum N rate was about 90% of the 2024 value, and the standard deviation was 0.14 lb. N/bu. As 2023 was drier in much of the state, these results are consistent with what we’d expect. Higher rainfall led to some nitrogen losses in 2024 and required higher nitrogen inputs per bushel. Moreover, the drier conditions led to more consistent results as the soil variation was slightly reduced. Assuming the 90% reduction is consistent for continuous corn and corn following soybean, then 0.92 ± 0.14 lb. N/bu was required. In continuous corn, 97.5% of all fields would be below the 1.2 lb. N/bu is used for much of the state for yield goals. For corn following soybean, the N needed was 0.85 ± 0.14 lb. N/bu. Given the yields in 2023, the YG method would have allowed 0.98 lb. N/bu. In 2023, 84% of fields in corn following soybean would be below the N level allowed by the yield goal method.

A few things come to mind as I look at the totality of the results. The yield goal method is essentially performing how it should be. It limits N application based on agronomically allowable amounts without constraining yield limits by limiting nitrogen application. I.e., those constraints were initially set to be a guide that didn’t limit N to the point yields would be unduly constrained, but it wasn’t meant to be a nitrogen recommendation system. It sets a maximum limit based on the best available science at the time, not an application rate suggestion. We should be thinking about facilitating better manure management and moving our rates to not the maximum allowable but more towards the best recommendation or best available guidance methodology. In some cases, this could mean increasing nitrogen application rates, but often, these must be accompanied by practices that are increasing corn yields.

Spring Manure Application: Setting Up for Success

As winter fades and the ground begins to thaw, spring manure application moves to the top of the priority list. But applying manure effectively in the spring requires more than just an open weather window—it demands planning to maximize nutrient availability, avoid compaction, and minimize environmental losses.

Evaluating Manure Storage and Application Timing

For many farmers, the first concern with manure is manure storage capacity. Earthen manures storages can be reaching near-full levels by the end of winter, creating urgency for spring application. While deep pit storages, which are often designed to hold close to a years’ worth of manure, won’t be full, it is important to evaluate how full the storage is and if there is sufficient space available to make it to fall or if you should be considering getting some manure used this spring. When soils are frozen there is greater risk of nutrient loss from both in both runoff and from volatilization. Even when the surface appears thawed, subsurface frost can prevent infiltration, leading to manure running off with early spring rains.

Soil Conditions: The Foundation of Smart Application

Beyond soil temperature, moisture content is another key consideration. Applying manure to saturated soils increases the risk of runoff and compaction. Wet soils soak manure in more slowly, and getting the manure nutrients in contact with the soil is critical for holding nutrients. Moisture conditions also play an important role in soil compaction. Compacted soils can reduce corn yields by up to 10% due to poor root development and restricted water movement in some growing conditions.

You can find more information on how soil moisture impacts field activities from Iowa State University Extension and Outreach and I talk more about how  moisture impacts manure movement in soil during injection in a classic The Manure Scoop post.

Manure Nutrient Content: Test Before You Apply

One of the biggest mistakes in manure application is assuming nutrient values without testing. The nutrient content of manure varies significantly based on animal diet, storage method, and agitation practices. The University of Minnesota, recently released a tool, ManureDB, showing results from many manure samples. One thing we can say is manure concentrations are highly variable from farm to farm. It is critical to know what is in the manure you are applying by getting it samples and tested.

·         Swine manure ranged from 30–65  lbs of nitrogen per 1,000 gallons, 10–30 lb P2O5, and 15–35 lb K2O

·         Dairy manure averaged 10–20 lbs of nitrogen per 1,000 gallons, 5–10 P2O5, and 15–25 lb K2O

·         Beef manure averaged 10–20  lbs of nitrogen per ton, 5–15  lb P2O5 per ton, and 10–20 lb K2O per ton.

·         Poultry litter contained 40–65  lbs of nitrogen per ton, 30–50  lb P2O5 per ton, and 26–45 lb K2O per ton.

Without testing, there’s a risk of either over-applying (wasting nutrients and increasing leaching risk) or under-applying (leading to nitrogen deficiencies in crops). Taking multiple samples and averaging, or compositing before analysis, helps reduce variability in sample results and ensure a more representative measurement of manure nutrient content.

Calibrating Equipment for Even Application

Uneven manure application can result in poor crop performance and increased environmental losses. Before heading to the field, check equipment to ensure:

·         Flow rates and pressure settings are appropriate for the manure consistency.

·         Injectors are distributing manure evenly across the application width.

·         GPS or guidance systems are calibrated to prevent overlaps or skips.

Minimizing Nutrient Losses and Protecting Water Quality

Spring manure application comes with an increased risk rainfall and wet soil conditions. To minimize loss:

·         Setbacks: Follow setback distances from streams, water sources, and designated areas.

·         Incorporation: Injecting or lightly incorporating manure reduces ammonia volatilization by 30–60% compared to surface application.

·         Cover Crops: Research shows cover crops can reduce nitrate leaching by up to 40%. We are currently evaluating cover crop control of nutrient loss with swine manure application data and will have more data available this fall on how it performed.

Regulatory Considerations and Recordkeeping

Keeping records of application dates, rates, field location, and application method is required for those with a manure plan. It is also a best practice to record field and weather conditions at application that can assist with compliance and nutrient management planning.

Final Thoughts

Spring manure application is about more than just emptying storage—it’s an opportunity to optimize nutrient use, improve soil health, and support crop productivity. By waiting for the right soil conditions, testing manure, calibrating equipment, and using best management practices to reduce losses, farmers can turn manure into a valuable asset rather than an environmental liability. With a little planning and attention to detail, manure can provide the nutrients crops need while protecting water quality and maintaining soil health for the long run.

How much manure is applied by commercial manure applicators in Iowa?

 The manure industry in Iowa is expansive, involving over 550 businesses that manage both liquid and solid manure applications across the state. In the spring of 2024, we surveyed 562 commercial manure application business. We received responses from 90 commercial manure applicators (16% response rate), offering a detailed look at the industry's practices, scale, and significance. These findings are valuable for farmers, businesses, and stakeholders engaged in manure management and decision making around manure utilization and fertilization. Over the next few months, we’ll be sharing information we learned from these surveys.

Manure Application Businesses by Manure Handling Type in Iowa

The survey revealed that the majority of manure applicators in Iowa deal with liquid manure. Of the 90 businesses that responded:

• 58 businesses handle liquid manure exclusively.
• 11 businesses handle liquid and solid manure
• 21 businesses handle solid manure exclusively

This implies for Iowa’s manure industry that 64% of businesses focus on liquid manure, 23% focus on solid manure, and 12% apply both liquid and solid manure. Taking this a step further, assuming this is a representative sample of the overall industry, this implies that of the 562 commercial manure businesses in Iowa 362 business apply liquid manure, 131 apply solid manure, and 69 apply both liquid and solid manure.

Market Share of Liquid Manure Application

Iowa generates approximately 13 billion gallons of liquid manure each year excluding rainfall. Given 2023 was a dry year, this estimate should be an accurate representation of manure volume available in the year the survey was conducted. According to the survey data, the responding companies apply about 21% of the state’s total liquid manure. With an estimated 562 commercial manure businesses in Iowa, and assuming 77% apply liquid manure, approximately 431 businesses are involved in liquid manure application. Extrapolating from the survey, commercial manure applicators handle roughly 8 billion gallons of the state’s total liquid manure output—about 62%.

If you are interested in more information contained within the survey, take a look at our video presentation where data is summarized.