Manure and Soybeans - How many soybean are using manure and what's the right approach?

 

The other day, I got the question, how much manure is soybeans using, and what type is it? Right when I got it, I asked – do you mean manure applied directly to soybean, or since this was a lifecycle/greenhouse gas question, how much mined and manufactured commercial fertilizer is being offset because of how we use manure in our crop rotations? That may seem like a slight distinction to some, but it me that is a big difference. Why? Because soybean is a legume and, as such, is capable of fixing much of its nitrogen needs, applying manure to it, especially nitrogen-rich manures, may not be the best use of our manure resources (though this depends, in some cases, applying low nitrogen manures that have high P and K may still make sense). However, using soybean in a rotation, with, for example, corn, can be a great way to better match crop nutrient removal to the amount of P and K applied.

Within this post, I'll try to answer a few points

1.      Benefits and challenges of applying manure to soybean.

2.      How banking P and K from manure application within a rotation supports fertility for soybeans.

3.      How many acres of soybean are receiving manure, and an Iowa-centric estimate of how manure supports soybeans within the state.

Manure Application to Soybean: Pros and Cons

Pros of Applying Manure to Soybeans:

 Nutrient Supply: Manure, especially from livestock like swine and cattle, provides essential nutrients such as phosphorus (P) and potassium (K), vital for soybean growth. While soybeans don't require as much nitrogen (N), the P and K in manure can boost soil fertility and improve crop yield.

Soil Health: Manure applications can improve soil organic matter, structure, and microbial activity. Manure improves soil health and water retention, benefiting the cropping system, including soybeans.

Cost-Effective Fertility: Manure is a cost-effective alternative to commercial fertilizers, especially for farms with ready access to livestock manure. Farmers can reduce reliance on purchased fertilizers by incorporating manure into their nutrient management plans.

Improved Soil Fertility Over Time: Manure can provide slow-release nutrients that benefit subsequent crops. When manure is applied to corn in a corn-soybean rotation, excess nutrients not utilized by the corn can become available to soybeans in the following season.

Cons of Applying Manure to Soybeans:

Nitrogen Misalignment: As a legume, soybeans can fix nitrogen through nodulation. Applying nitrogen-rich manure directly to soybeans can lead to inefficient nitrogen use, potentially increasing nitrogen losses through leaching or denitrification, as the soybeans won't need as much of it as they are fixing their own, or it means reducing fixation as a result of high nitrogen levels in soil when that nitrogen could have been used to replace nitrogen in other, non-leguminous production systems.

Soil Compaction: If manure is applied under wet conditions or if heavy machinery is used during application, soil compaction can occur, detrimental to root development and water infiltration in soybeans.

Manure Application in Corn-Soybean Rotations: "Banking" of Phosphorus and Potassium

Although applying manure directly to soybeans may only sometimes be the most efficient use of its nitrogen content, manure is often applied in corn-soybean rotations with the specific intent of providing phosphorus (P) and potassium (K) for both crops.

Manure Application Before Corn:

In a typical corn-soybean rotation, manure is commonly applied to corn because corn requires higher nitrogen. When manure is applied before corn, it supplies nitrogen and deposits significant amounts of phosphorus and potassium into the soil. Since corn may not utilize all of the P and K, these nutrients remain in the soil, available for the following soybean crop.

Phosphorus and potassium are less mobile in the soil compared to nitrogen. As a result, these nutrients persist and become accessible to soybeans in the year following manure application. This method effectively "banks" nutrients, supporting soybean growth without additional fertilizer applications.

By applying manure to corn with the understanding that residual P and K will benefit soybeans, farmers can maximize the use of manure nutrients across the two-year (or longer) crop rotation. This approach helps balance nutrient levels and avoid over-applicating phosphorus and potassium, which could lead to environmental concerns like eutrophication.

Farmers can reduce or even eliminate the need for synthetic P and K fertilizers for soybeans when they take advantage of the nutrients from manure applied to corn. Doing so not only cuts production costs but also promotes more sustainable nutrient cycling within the cropping system and can lower the carbon footprint of soybean as both the energy associated with mining P and K fertilizers are eliminated as are application passes of these fertilizers.

While direct manure application to soybeans is not suggested due to the legume's nitrogen-fixing ability, it can still offer benefits in terms of phosphorus and potassium supply and soil health improvement. However, the common practice in corn-soybean rotations is to apply manure to the corn crop to use the excess P and K for the following soybean crop. This strategic approach ensures that nutrients are efficiently utilized over the rotation, benefiting both crops while minimizing the environmental impact and fertilizer costs.

Acres of Soybean Receiving Manure

The USDA ARMS survey estimates acres of different crops receiving manure. While not all done in the same year, it does provide a reasonable approximation of which crops are receiving manure. In 2020, they estimate that 2.3% of acres receive manure, or about 1.9 million acres of soybeans. It is ranked as the second most popular crop to receive manure, following corn, though corn receives about 80% of all manure produced, and soybeans receive only 10% of the manure. However, given the above conversation, it must be recognized that this is acres receiving manure in a given year and not accounting for carryover phosphorus and potassium from manure used to support soybean production in the following year.

 Table 1. USDA ARMS data on manure application rates by field crop across the US.

The second figure is also from a UDSA ARMS survey and estimates where the source of manure was for crop production. Of the manures applied to soybean, only about 7% was from swine, or about 130,000 acres across the US. At first, this may seem strange, but swine manure is typically the highest available nitrogen among manure sources and especially has much nutrient value (50%) related to nitrogen content. In contrast, other manures, cattle, and poultry generally only have 10-20% of their nutrient value tied to nitrogen with most value coming from phosphorus and potassium. Because of this, swine manure is probably the least likely to be applied to a legume like soybean. Beef and poultry manure also tend to be solid manures, which are more likely to be surface applied, especially in minimal tillage systems; timing the application after a corn crop can offer more residue and protection from runoff losses than applying after soybean (and before corn).

Figure 1. Manure source (by animal species) for the major crops receiving livestock manures.

However, that doesn't mean that swine manure isn't an essential source of nutrients for soybean production, just that it is being used as a multi-year fertilizer for soybeans in a corn-soybean rotation rather than the direct crop receiving manure. In Iowa, approximately 25% of all corn land receives manure, which is almost exclusively applied to fertilizer. However, even though continuous corn is more prevalent around livestock and swine farms than in other locations, most of the land (~80-85%) is in a corn-soybean rotation where manure is supplying P and K for the soybean crop). In Iowa, there are about 12.4 million corn acres, and assuming that 25% receive manure and 80% are in a corn-soybean rotation, there would be 2.5 million acres of soybean being fertilized by manure alone in Iowa. In Iowa, 80% of this would be from swine manure, so two million acres. The two million acres in a single state need to be reported on or captured in how USDA ARMS surveys farmers and collects manure application data. However, it represents a substantial amount of Iowa soybean acres (20%). While Iowa is a leader in acreage impacted in this way, similar patterns would be seen in Illinois, Minnesota, and Indiana, though with lower amounts of manure.

Soybean producers must proactively claim how this circular use of nutrients occurs across multiple years of a crop production system. It impacts the sustainability of soybean production in terms of both greenhouse gas scoring as related to reduced demand for mined phosphorus and potassium fertilizers, savings of energy use for a field pass specific to supply nutrients to soybean production, and a message of improved efficiency and reducing greenhouse gas footprints by more fully considering how manure is used.

Managing Manure Salts: Is it an issue, and when?

 When applying manure, there's more to consider than nutrients and hydrology. While they generally occupy most of my comments, salts in manure can significantly impact your soil and crops, particularly when combined with the wrong conditions. You must consider several key factors to ensure your fields stay productive and your soil remains healthy.

 

How Salts Affect Crop Growth

Crops need a balanced environment to thrive. When the salt concentration in the soil becomes too high, it can interfere with the plant's ability to absorb water. Water moves from low salt concentration to areas of high salt concentration—a process called diffusion. Plants use this process to their advantage, maintaining higher ion concentrations in the plant than the surrounding area and wanting to draw water in – a process called osmosis. When the soil around the roots is salty, plants struggle to take up water, even if there's plenty of moisture in the soil (saline). Such conditions can lead to water stress, reduced growth, and lower yields.

 

Crop Tolerance to Salts

Different crops have varying levels of tolerance to salts. For example, corn has a moderate tolerance, with a threshold of about 1,700 microS/cm in the soil water. Once the salt concentration exceeds this level, you'll likely see a drop in yield. Other crops, like soybeans, are even less tolerant (1,000-1,500 microS/cm) and can be affected at lower salt levels. Understanding the salt tolerance of the growing crops can help you make better manure and water management decisions.

 

Impact on Soil Structure

High salt levels can also damage soil structure. Sodium, a common component of salts, can cause soil particles to disperse, leading to poor water infiltration and drainage (sodic). Such soil dispersion can create a hard crust, making it harder for crops to establish roots and access nutrients. Over time, this can reduce soil fertility and make it more challenging to manage your fields.

 

Understanding Salt Leaching in Your Fields

In Iowa, we average about 26 to 38 inches of rain annually. Around 4-8 inches typically drain away through tile or natural soil drainage. This drainage helps remove salts from the soil as they leach from the root zone and, for the most part, prevents them from building up to harmful levels. However, adding irrigation water higher in salt or applying manure with high salt content can change this. In particular, the leaching fraction is often used to help control salt levels within a field as the soluble salts will move with the water.

 

To calculate the Leaching Fraction, Use the formula

Leaching Fraction = (Drainage Water) / (Total Water Applied)

 

Let's start to think about these balances throughout Iowa. In that case, we'll average around 4 inches of drainage to 26 inches of water in Northwest Iowa and 8 inches of drainage to 38 inches in southeast Iowa. The leaching fractions are 15-21%

Suppose you rely only on rainfall; about 15-21% of your water leaches and removes salts. When you add irrigation, this fraction can change. However, even with more water applied, your soil might not drain any better, especially if it's naturally slow-draining or you are using more water to meet the transpiration demand of your crop, i.e., the water is evaporated or transpired. Even though you added water, the leached volume didn't change.

 

Balancing Salt Application with Crop Tolerance

When applying manure or irrigation water, limiting the salt levels to what your crops can tolerate is essential. For example, corn has a salt tolerance level of around 1,700 microS/cm. Soybeans are less salt tolerant at around 1,000 – 1,500 microS/cm. To understand what salt levels you'd expect, you need to know the salt added to the soil through irrigation or manure and estimate what leaching removes.

Think of it as a balance: the amount of salt you add should equal or lower than the amount you could expect to leach out. If you're leaching 4 inches of water with a salt level of 1,000 microS/cm, you can safely apply an equivalent amount of salt. But if you add more, especially during dry years, you could set yourself up for trouble. It's often a good idea to be conservative with your salt applications—cutting them in half gives you some breathing room in case of a dry spell.

 

Timing is Everything

Timing your applications is crucial if you're trying to manage salty wastewater. You'll want to avoid applying salty water during critical growth stages, like when your corn is tasseling or forming ears. Instead, aim for the spring or fall, when rainfall is more likely to help dilute and leach those salts. There's also a difference between using water for irrigation and disposing of salty water. When irrigating, the goal is to supplement the crop's water needs, which means you're adding water when the soil is dry. But with salty wastewater, it's better to apply when there's more water in the soil to help with dilution.

 

How Much Salt is in Manure?

The range of salt in manure is highly variable, but a typical range from deep pit swine manure may be 5,000 to 15,000 microS/cm, while dairy manure may range from 2,000 to 10,000 microS/cm. Regarding application rates, deep pit swine manure rates are often 3,000-6,000 gallons per acre (0.11 to 0.22 inches), while dairy manure is often 10,000 to 15,000 gallons per acre (0.37 to 0.55 inches). At first glance, this can look concerning as these concentrations are much larger than what we said plans could handle, but as they get mixed in with rainwater and flushed with leaching, so where do we end up?

For the examples, we will assume dairy manure at 10,000 microS/cm at 0.55 inches as it has the highest salt loading. I'll also work with the Northwest Iowa data and take 4 inches of drainage to 26 inches of water. We'll work with a 5,000 microS/cm limit to be safe.

Drainage: 4 inches (5,000 microS/cm) = 20,000 (microS/cm)(inches)

Added: 0.55 inches (10,000 microS/cm) = 5,500 (microS/cm)(inches)

Essentially, we are checking that potential salt removal would be higher than salt additions, and if that is the case, we are in good shape, at least over the long term.

If you are willing to look over shorter periods, things can change. For example, from 2021-2023, parts of Iowa were about 15 inches below average on moisture, or about 5 inches a year. As a result, drainage water fell drastically, in the worst places averaging about 1 inch of drainage. If you take that change, it would have only leached 5,000 (microS/cm)(inches) per year, and that means we'd have added to our salt pool and maybe had an issue in some parts of fields.

All this illustrates that if you live in an area where less drainage occurs or have poorly draining fields, the issues of salts from added manures can become an important consideration in your management plan.

 

A Few Practical Considerations for Farmers

Monitor Your Salt Levels: Keep an eye on the salt content of your manure, especially during drier years.

Calculate Your Leaching Fraction: Use the formula Leaching Fraction = (Drainage Water) / (Total Water Applied) to estimate how much water—and salts—are being leached out. It is more coincident that it may be an issue if dry years start stacking together.

Consider Soil Type: Remember that poorly drained soils may not leach salts as effectively, increasing the risk of salt buildup. You may need to adjust your application rates or frequency in these cases.

While for the most part, salt buildup in most Iowa soils won't be a concern with manures, as you think about other practices like irrigation, poorly drained soils, or working with different industrial by-products that may have

 

By understanding the balance between salt application and leaching, you can manage manure salts more effectively, ensuring healthy crops and long-term soil productivity, as this is a case where an ounce of prevention is worth a pound of cure.

The Power of Manure Timing: Enhancing Nitrogen Use Efficiency

 

Corn's response to manure application can be a challenge to predict. Manure nutrient uptake is highly variable and impacted by both the year's crop growth conditions and the manure characteristics. Nitrogen (N) losses and corn N needs to be supplied by fertilizers vary significantly by year. N losses are maximized during warm, wet springs, as nitrate-N readily moves through the soil with water. Additionally, these conditions promote denitrification losses to the atmosphere if nitrogen is present in the nitrate form. Various tools exist to assist with N application rate decisions, such as the Maximum Return to Nitrogen (MRTN) Calculator. The MRTN Calculator is available in many Midwestern states but does not account for the season of N application and provides a nitrogen recommendation based on well-timed spring and side-dress applications.

Generally, it is best to apply manure as close to the growing season as possible to limit nitrogen loss opportunities before crop uptake. A literature review and meta-analysis were conducted using various studies investigating different N rates during different application seasons to evaluate the impact of nitrogen application timing on crop response to nitrogen. For a study to be considered and used in this analysis, the study needed to use at least three nitrogen rates and have performed nitrogen applications in at least two of the three application seasons. Yield results were normalized to the percent of the maximum yield obtained within a study year to facilitate analysis across studies and time. The results of this review are shown in Figure 1.

The high variability in yield illustrates the difficulty in accurately predicting N needs in a given year. Fall applications, on average, achieved the lowest yield at a particular N application rate. Sidedress applications, on average, had the highest yield at a nitrogen application rate. This appears to be related to the risk of nitrogen loss between the time of application and crop uptake. However, this wasn't the case in every study or every year, as within at least one of the included studies, spring manure application had the lowest yields.

An important point to consider is how this impacts your farm, your bottom line, and how your crop will yield. For example, 200 lb N/acre applied in the fall yields 89% of the maximum on average, but switching to a side-dress application results in 98% of the max yield. This 9% increase correlates to an 18-bushel improvement if the field's yield potential is 200 bushels per acre. While application timing significantly impacts yields in the "average" year, varying weather conditions can make this timing effect more or less significant in different growing conditions and different years. Farmers need to weigh the pros and cons of each scenario to determine which application strategy best suits their fields and the probable growing conditions for the upcoming cropping year.

Figure 1. Scaled nitrogen response curves where application timing is coded by color and each study used is coded with a different symbol. Yield response was more significant at lower nitrogen application rates for nitrogen fertilizer applications closer to the corn growing season, but with higher nitrogen application rates similar yields could be obtained with different application timing.

Farmers benefit from longer, typically drier, application windows in the fall. Soil conditions are generally favorable, providing an ideal environment for application. As long as soil temperatures have cooled to 50°F and are trending colder, microbial activity and mineralization to nitrate are limited. Additionally, for organic matter-rich manures (often solid manures), a fall application allows some time for the mineralization of organic N, allowing it to break down into forms usable by plants.

However, fall applications come with their challenges. Snowmelt and other early spring weather introduce the risk of N leaching or denitrification, leading to a greater risk of potential nitrogen losses. As seen in Figure 1, fall applications, on average, tend to need higher nitrogen application rates to obtain similar yields. Moreover, fall application has the highest uncertainty about the exact amount of N that will be available to the crop, complicating the decision-making process for farmers.

Spring applications offer their own set of advantages. Less time between when the nitrogen is applied and when crops uptake the nitrogen means fewer opportunities for N losses. By avoiding the nitrogen being exposed in the field during early spring rains, leaching losses are minimized. Improved N retention can lead to higher crop yields, and farmers can better predict weather conditions for the season ahead, providing a higher certainty level of potential nutrient need than fall.

Spring manure applications come with their own set of difficulties. Wet spring conditions can hinder fieldwork schedules, leading to delays or missed opportunities. Planting demands may leave little time for additional field operations, creating conflicts in timing. Compaction is another risk associated with spring applications. Heavy agricultural machinery on moist soils can reduce soil porosity and negatively impact future crop growth. While most of the N available in swine manures is available in the first year, organic N takes time to mineralize, meaning only ammoniacal N is immediately available to plants in spring application scenarios, potentially delaying nutrient uptake.

In-season sidedress applications have become another attractive option for manure application, avoiding some of the challenges associated with fall and spring timings. Farmers have a clearer picture of seasonal conditions by summer, facilitating adjustment to planned nitrogen application rates to meet the expected demands of a particular growing season better. Precision fertilizing allows tailored N application to meet crop needs, while immediate crop uptake minimizes N loss opportunities.

However, side-dress applications come with their challenges. Depending on the crop growth stage, specialized equipment may be required to avoid crop damage during application. Dragline and tanker applicators may be used up to the V4 stage in corn but should be avoided beyond V4 to prevent crop damage as the corn matures unless specific equipment is used to avoid running over corn. Specialized high-clearance irrigation equipment may be used for late-season nutrient application. Without pre-plant N, fertilizer response may be reduced, affecting overall crop health and yield. Additionally, seasonal conditions can pose application challenges and compaction risks, limiting effectiveness in wet summers.

Every application scenario comes with its benefits as well as its risks. As fertilizer prices continue to change, weather conditions become more unpredictable, and environmental stewardship becomes a higher priority, it is crucial to recognize the pros and cons of each nutrient management option. One strategy to mitigate some risk involved with fertilizer applications could be multiple split applications, e.g., providing a partial N rate in the fall and the rest in the spring. Predicting corn's response to manure application can be a significant challenge. While the optimal manure application strategy may change from farm to farm and year to year, it is necessary to weigh the pros and cons of each strategy to determine the best fit for your operation.

Do Iowa Counties Have Sufficient Crop Land to Meet Manure Management Plans Requirements Without Exporting Manure?

Iowa is known for its significant agricultural production, including crops like corn and soybeans and livestock such as hogs, cattle, and eggs. However, given the size of the Iowa livestock industry, a common question becomes whether there is too much manure. In large part, manure from these livestock operations is often used as a nutrient source for crops, so people are asking two questions at its core. 1. Is there enough crop ground to utilize produced manure and 2. Are farmers taking credit for this manure and reducing fertilizer purchases?

Whether Iowa counties have sufficient cropland to utilize all the nutrients from manure without exporting it depends on various factors, including:

Livestock Density: Areas with high concentrations of livestock will generate more manure and, therefore, have more manure nutrients.

Cropland Availability: The amount of available cropland, its proximity to livestock operations, and the productivity and nutrient need determine the feasibility of using all manure locally.

Manure Management Practices: Effective manure management practices can maintain more nutrients in the manure, but they require more land to use the manure nutrients. Manure practices a farmer picks could be influenced by the amount of manure they need. With that said, we want to pick practices that conserve manure nutrients because they increase the circularity of agricultural systems.

Within this work, we will look at these concerns in several ways. The first two are at a state level. This work is built off Andersen and Pepple's (2017) A County-Level Assessment of Manure Nutrient Availability Relative to Crop Nutrient Capacity in Iowa: Spatial and Temporal Trends. Within that work, I defined algorithms for using Census of Agriculture data to estimate livestock populations and from this both manure nutrient excretion and available manure nutrients for land application, with the former being an estimate of what is excreted by the livestock, and the latter being an estimate manure nutrient recovered for land application and corrected per Iowa State suggestions for nutrient availability and application losses. I also estimated a state level of nutrient needs using the Census of Agriculture production statistics and the USDA Crop Nutrient Removal Database for evaluating the nutrient content of the harvested material.

A summary of nitrogen and phosphorus comparisons between manure (excreted and available for crop use) and crop capacity is provided in Figures 1 and 2. Crop capacity focuses on corn, corn silage, soybean (for phosphorus only), hay and haylage (phosphorus only), and small grain. It represents the amount of nutrients estimated to be harvested and removed, not the amount to support the crop. As such, it is a low estimate of nutrient requirement. Pastureland nutrient needs were not considered, though, for animals estimated to be on pasture (beef cows), only a fraction of the manure was estimated to be recovered, with the remaining being on pasture at approximately nutrient need. Manure production system, nutrient excretion, and availability were estimated based on production practices standard from 2000-2025. As a result, manure estimates earlier in history (predating approximately 1990) may not be as representative as animals may have spent more time on pasture (especially dairy cows), or other production styles (open lot pigs) may have been more prevalent. 

Figure 1.  Comparison of crop nitrogen need and both manure nitrogen excretion and manure nitrogen retained and available to support crop production.

Figure 2.  Comparison of crop phosphorus need and both manure phosphorus excretion and manure phosphorus estimated to be retained and available to support crop production.

Results indicate nitrogen excretion with livestock manures has returned to levels last seen in 1970. Today, a more significant amount of this nitrogen is estimated to be retained and available to help support crop production. In at least part, this represents a shift from cattle systems (which, given the open lot nature, generally had higher ammonia volatilization losses than current swine systems and lower nitrogen availability due to differences in ration). Manure phosphorus levels have also returned to levels seen in the 1950s-1970s. Available phosphorus mirrors excretion due to limited means of nutrient loss during storage.

Over the same period, crop nutrient needs have significantly increased due to significant crop yields per acre increase.

Overall, while Iowa has a significant amount of cropland and livestock, the balance between manure production and cropland capacity varies by region and depends on the specific practices employed by farmers and regulators to manage nutrient cycling effectively. To understand these results and contextualize them, I look at two other variables: the percent of N or P that is excreted, recovered, and available to be used as a crop fertilizer. In general, this estimate has been trending up and I currently estimate it at around 60% of N and 80% of P. These estimates are slightly low, as they aren’t crediting N and P deposited on pasture in grazing systems where the nutrients could be used. The other factor I look at is what percent of nutrient need is supplied by livestock manures. In 2022, this is about 38% of the N and 30% of the P. Again, this represents the crop removal rate, not the nutrients required to support crop nutrient production.

I estimate this is sufficient nitrogen to supply between 4.1 and 4.8 million acres of corn production (if all the manure was applied to the ground for corn production). In 2022, Iowa had about 12.9 million acres planted to corn, so manure should account for 32-38% of all nitrogen fertilizer use in Iowa. In the fall of 2021, the National Agricultural Statistics Service collected nitrogen fertilizer use for corn in the Agricultural Resource Management Survey. They reported 87% of Iowa corn acres received fertilizer (presumably, the other 13% were manure only). Manure should be about 1/3 of the nitrogen fertilizer use; 13% sounds too low. But many acres would get some of their fertility from manure and be supplemented with commercial fertilizer, so that data doesn’t tell the whole story. The survey estimated the total commercial N fertilizer applied to corn at 834,650 tons of N. I estimated 393,000 tons of N from manure. Based on these figures, manure was at 32% of the nitrogen fertilizer supplied by the state. For phosphorus, the ARM survey reported that 49% of corn acres received phosphorus fertilizer (presumably, the other 51% were either manure only or had high-testing soils that didn’t need additional P applied). In this case, the estimate is that there were 201,500 tons of P from fertilizer. I estimated 89,000 tons of P from manure, making manure about 30% of the P applied in the state, in agreement with the phosphorus budget proposed earlier.

I also like to look at this data on a county level. Again, I’ll be using my estimate of crop nutrient removal and comparing that against the amount of manure I estimate to be produced, retained, and available for crop production within that county. While the work assumes no manure is moved from one county to another and gives a low level of crop nutrient need, but still serves as a helpful indicator of nutrient budgets. In general, we see a continuation of the trends we’ve been seeing; some counties are getting more manure-rich, and others continue to get a smaller fraction of their nitrogen and phosphorus needs from manures. Again, this figure shouldn’t indicate whether we have sufficient land for manure but more an indicator of potential areas where giving a closer look makes sense. In particular, you could question plenty of assumptions – the percent of manure I’m collecting on cow-calf farms and the type of storage. While probably reasonable for the state, I think these assumptions might have outsized effects in this area. Specifically, the southern region of Iowa may be more likely to choose lagoon manure storage because of both location and associated lagoon performance (a warmer area of the state), and many of the more extensive swine facilities in this area may be related to gestation-farrowing operations or nursery farms.

Figure 3. County level comparisons of crop nitrogen removal with harvest (excluding legumes and hay) as compared to amount of manure estimated to be available to support crop production in the county. Example, the dark green counties indicate that less than 10% of the nitrogen removed in the harvested fraction of crops could be supplied by livestock manures.

Figure 4. County level comparisons of crop phosphorus removal with harvest as compared to amount of manure estimated to be available to support crop production in the county. Example, the dark green counties indicate that less than 10% of the phosphorus removed in the harvested fraction of crops could be supplied by livestock manures.

I also wanted to look at this another way: if we use my estimates of manure production and available for land application, would it be possible for all the manure within a county to be used within that county in compliance with manure management plans (and for ease I’m going to assume all manure would require manure management plans). To do this, I obtained corn, corn silage, soybean, and alfalfa acres from the USDA Census of Ag at the county level. I got hay production numbers from the Census of Ag and divided them by hay acres to get a yield value. I then used Appendix A from the Iowa Manure Management Plan form to get estimated yields of corn and estimated corn silage yield (Assumed 70% moisture and a harvest index of 0.5). The Nitrogen Use Factor for corn (weighted based on the county being considered), corn silage, and hay were obtained. Only three counties, Lyon, Washington, and Clark, couldn’t use all the manure produced on the corn acres available in their count, but each had sufficient acres if land was considered.

Several factors could be contributing to this:

1. I could be making flawed assumptions about the types of manure systems used (more lagoon systems instead of deep pits, for example).

2. Some manure is being applied to pasture land, which I didn’t consider in the analysis.

3. Some manure is going to hay or soybean.

These three considerations alone would take care of any issues. However, it could also be that some of the manure within these counties is exported from the county.

The final piece of the puzzle is understanding how farmers are valuing the manure and trying to breakdown the commercial fertilizer and manure at a county level. Unfortunately, at least for today I’m out of space, and as of yet, having trouble finding county level nitrogen fertilizer data.

People, Pigs, and Poop

 

Recently, there was a little exercise for how much swine poop there is in Iowa and turning it into a pyramid. In that exercise, they (Raygun – I didn’t fall out of my chair or roll my eyes. I was excited, conversations about manure are welcomed) calculated about 85 billion pounds of pig poop per year – I won’t dispute that number though I calculate a slightly higher amount. I’ll even add in cattle and poultry and estimate 85 million tons of livestock poop annually.

But how much human poop is there? Iowa has about 3.2 million people in it. A person poops about 175 grams per day or 0.38 lbs. This is about what was assumed when they compared humans to pigs, but unfortunately, that’s not the pig number they were using; the pig number includes urine and wash water, too. As it should, because, in the name of water quality and manure management, we would also manage that component. Humans generally make about 0.37 gallons of urine daily, so another 3 pounds of material. So, humans are up to 3.4 lbs of “manure” a day, not that 0.38 lbs.

I mean, if we are going to talk “poop,” we probably want an apples-to-apples, or a poop-to-poop comparison, don’t we? But here is where it gets complicated – for my livestock manure numbers, I include wash water volumes – because we manage it like manure, and we should view this wash water like manure. I’m glad we do. But does that mean for humans we should include our “wash water” as well? That would include the water you use when you flush a toilet, shower, or do dishes. In developed countries like the US, the average person generates about 80-100 gallons daily. Let’s go with 90 gallons a day, or 750 pounds a day. Extrapolate this to a year, and you get 438 million tons of human wastewater! Or about five times what we generate from livestock production.

So, let’s play the game of how much poop is there?

So, for a pig, we have about 10 pounds a day, about 10% of this is solids material, and I’m going to say that fecal material is about 50% moisture, so a pig excretes about 2 pounds of feces a day, 8 pounds of urine. Throw in wash water used at the site, and we are at 10.8 lb/day. So, what’s the comparison now?

Table 1. Comparison of human and pig related “manure” and wastewater generation.

	Human	Pig
Feces	0.38	2	lb/day
Poop (urine + feces)	3.4	10	lb/day
Wastewater	750	10.8	lb/day
Population	3,200,000	30,500,000	million
Feces	221,920	11,132,500	tons/yr
Poop (urine + feces)	1,985,600	55,662,500	tons/yr
Wastewater	438,000,000	60,115,500	tons/yr

 How we choose to manage wastewater greatly influences the question of what characteristics are important for me to know about that wastewater. Alternatively, the characteristics of the wastewater greatly affect how I’d choose to manage the wastewater. All that to say, a simple volume comparison isn’t enough; we have to dig deeper. What does this tell us? Humans send more “manure” to wastewater treatment systems in Iowa than livestock would, but the animal manure would have more feces in it. But this, at its heart, is why we choose to manage human and livestock manures so differently. If you have a lot of water, not much stuff in it, and are far from cropland, treatment and discharge makes sense. If you are managing volume to be smaller and get higher nutrient concentrations in it, then making decisions to use that material to replace fertilizer makes more sense.

So, how should we think about wastewater and characterize it? There is more to it than this, but if we want to keep it simple, we should start with four parameters.

Total volume, chemical oxygen demand (COD), nitrogen, and phosphorus. Why these four? Because, at their core, they tell me a lot about how poop could impact the environment. How much are we dealing with, what’s the immediate impact to water (chemical oxygen demand), and what is the potential for eutrophication (nitrogen and phosphorus).

Alright, let’s look at chemical oxygen demand. For untreated municipal wastewater the COD/five-day biochemical oxygen demand (BOD5) ratio is about 2. Why am I using this ratio? BOD5 is a much more common measure of wastewater strength (great history to this measurement, and it comes from London and the Thames River – basically because it took five days for the sewage they dumped in the river to make it to the ocean). So, what’s the BOD5 of municipal wastewater? It depends, but a good average number is around 220 mg/L. I’ve also included N and P in human wastewater and what I estimate is excreted by a pig for comparison (Table 2).

Table 2. Estimated COD, N, and P in human and swine wastewaters.

	Human	Pig
Wastewater	750	10.8	lb/day
COD Concentration	440	84,500	mg/L
N Concentration	40	8400	mg/L
P Concentration	8	1360	mg/L
COD Mass	192,165	5,065,142	tons/yr
N Mass	17,470	503,517	tons/yr
P Mass	3,494	81,522	tons/yr

 How do we try to turn this into water quality impacts? Quantifying impacts is difficult, it requires us to make assumptions about how treatment and utilization impacts COD, N, and P movement and losses to water quality. With municipal wastewater, we typically treat and then discharge. To quantify what may be making it to a stream, we have to estimate the percent removal with treatment and then quantify where it ends up. For COD, hopefully, around 90% will be removed, and this will be mostly converting material into CO2 (70%) and municipal solids (20%).  The municipal solids would then be land applied. However, land application is highly effective at COD removal and preventing it from entering water, so we’ll say 0.05% is lost from the land applied fraction.

In terms of nutrients, it gets a little more complicated and depends on the treatment system being used. For phosphorus, hopefully 50% of the P ends up in the municipal solids (which are land applied) and 50% are discharged after treatment. Of those land applied, again it depends on the management practices used, but assuming good phosphorus management, probably only 0.5% of the P land applied moves with water from the land application area. For animal manures, we will use the 0.5% for all phosphorus as it should all be land applied. In the case of nitrogen, ultimate fate is again harder because it is very much dependent on if the wastewater treatment method employed. Still, for a working version of what is happening, we’ll go with 30% is denitrified, 40% is nitrified and discharged, and 30% is recovered in the wastewater sludge and land applied (assume 20% of N is lost during storage before land application). Assuming that it is land applied as a fertilizer, we’ll go with 20% of the nitrogen is lost after land application. With manure, I’m going to assume 20% is volatilized during manure storage and lost to the environment and that, again, 20% of the nitrogen that is land applied is lost. I’m providing these results in Table 3 to show an estimated N loss.

Table 3. Estimated impact on the environment from human and pig manure after treatment for human wastewater and land application as a fertilizer for pig manure.

	Human	Pig
COD	19,236	2,533	ton/yr
N	8,875	181,266	ton/yr
P	1,756	408	ton/yr

 Where does that leave us? Swine manure probably is having more impact on the environment than human wastewater in Iowa. At least in part this is due to the vast differences in populations of pigs and people. I’ll give you one more table, COD, N, and P estimated to be released to the environment, but on a per person and per pig basis.

Table 4. Estimated impact on the environment per person or per pig after treatment for human wastewater and after land application as a fertilizer for pig manure.

	Human	Pig
COD	12	0.2	lb/person(pig space)-year
N	6	12	lb/person(pig space)-year
P	1	0.03	lb/person(pig space)-year

 I want to do this one more time (table 5). What happens if we say that where we were applying manure would have received fertilizer anyway. Well, assuming the manure is being managed like a fertilizer, nitrogen and phosphorus losses from that acre would be similar. That is, the losses are driven by land use, and not directly by manure (and we can, and should in the future have a discussion on if manure is being managed as well as commercial fertilizer, and how to continue to improve our management of both).

Table 4. Estimated impact on the environment per person or per pig after treatment for human wastewater and after land application as a fertilizer for pig manure.

	Human	Pig
COD	12	0.2	lb/person(pig space)-year
N	5	12	lb/person(pig space)-year
P	1	0.03	lb/person(pig space)-year

 Each method, municipal treatment for human wastewater and storage and land application of manure for livestock, has its pros and cons. If we were to treat pig manure like human waste, does water quality get better? For COD and P, I don’t think so; in fact, we probably add more of each to Iowa water ways using this method. If we treat N like human waste – it’s complicated and depends greatly on the amount of N that goes into denitrification, but unless there was a land use change associated with no longer having manure as fertilizer, we’d still get some of the losses with the use of commercial fertilizer on those crop acres that we get right now when we use manure.

The system is complicated. We need to continue to innovate to reduce N volatilization losses from storage. Specifically, these volatilization losses, are what make nitrogen losses from manure greater per pig than per person. We need to continue to develop improved nitrogen utilization practices and nitrogen fertilizer recommendations tailored to each year, location, and growing season so we can do better utilizing manure nutrients and lessen impact on water quality. The conversation is difficult, and hopefully, that comes through, that it is more than a pyramid of poop.