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.