When should we think about phosphorus availability?

The other day I had the opportunity to provide a guest lecture to a class of juniors and seniors in Agricultural Systems Technology here at Iowa State. The class is about animal production systems and I was asked to provide a lecture on “manure.” It is always interesting when you get general requests like this as there are literally hundreds of issues about manure and it seems like you could pick almost any of them. However, in cases like this I usually talk about methods for determining appropriate manure application or in a case like the how to fill out an Iowa Manure Management Plan. There are lots of skills that I think would be useful, but when it comes down to it, making sure they understand the steps in determining manure application rates is the one I’d really like them to know.

As we talk about it, we use the check book analogy, i.e., there are two things you need to keep track of, the ins and the outs and we want to try to keep those in balance. When I do this for manure the two things we want to know are the demand side (how much of this nutrient do we need) and the supply side (how much of that nutrient is our manure going to provide). We do this first for nitrogen and talk about how losses might occur during application and how this might cause you to adjust your rate and what nitrogen availability means. Then we do this for phosphorus, talking about situations when your manure application rate might be limited by the amount of phosphorus you put on (based on the Phosphorus Index here in Iowa). Normally I do nitrogen first as its more complicated due to volatilization losses and having to worry about availability that it makes phosphorus seem easy.

However, the other day I got a good question… it was, “why don’t you worry about phosphorus availability when you do this?” I thought for a second, it seemed I done these calculations so often and usually just glossed over the phosphorus availability assumption that it was second nature to me, but now with an interested student in my office asking why I’d get a chance to explain it. So I answered, well usually we think of the fields getting manure as one that get repeated applications year after year, and since typically our manures supply more phosphorus than the crop needs we have built up phosphorus levels in the soil so we aren’t really concerned about availability since there will be enough there to satisfy our crop.

I’ve always thought this approach was fair, and was quick to accept it, but the student’s question got me thinking… is this still true? Recently we’ve seen lots higher N:P ratio’s in swine manure; this has occurred for numerous reasons including improved availability of phosphorus in DDGS as compared to more traditional ingredients as well as the inclusion phytase into the swine diet to improve phosphorus digestibility, but it might mean that we start to care more about phosphorus availability in swine manures. And just the other day I was writing about how far you could move manure if you were getting it to fields were there was a need for this phosphorus. What about these cases, i.e., fields with low soil test phosphorus, what does phosphorus availability mean in these instances?

Well, in general, a large fraction of the phosphorus in manure is considered to be plant available immediately after application (often 60-90%). The fraction that isn’t immediately plant available is often an organic fraction that will become available as microbes in the soil break apart those organic molecules and free up the phosphorus. The other part of phosphorus is very much soil dependent, there are lots of things (iron, aluminum, calcium) that can react with the phosphorus and make it more difficult for our crops to obtain. This is why soil sampling is so important, it gives us an estimate of the potential of seeing a yield effect with phosphorus application. Fields in the high or very high category see little to no response in yield with additional phosphorus, so considering phosphorus availability has no effect. In fields with very low or low soil phosphorus concentrations though we have much greater potential for a yield impact, and in these cases it would be important to make sure we are applying sufficient available phosphorus to supply what the crops need the first year or two. So if you are applying your manure to fields with low soil test phosphorus, you just might want to look at some of those first year phosphorus availability numbers to make sure your crops have what the need. For more information on phosphorus recommendations in Iowa see PM 1688: A General Guide for Crop Nutrient and Limestone Recommendations in Iowa.

Protect your soil to reduce erosion

As I’m sure we all recognize, soils are an important part of our agricultural systems, but they to far more than just provide a place for plants to grow roots and anchor themselves. Soils are really an ideal growth medium, they maintain spaces that are filled with oxygen to help, others that are filled with water, they hold nutrients in a place where plants can get them, and they insulate roots from drastic daily changes in temperature. As such, it acts as an ideal growth medium. That means protecting it an making sure it remains in the field for years to come is vital for maintaining our agricultural productivity.

Have you seen this video of raindrops hitting a sand surface that was captured a team of scientists in the Chemical Engineering and Material Science department at the University of Minnesota. Their video shows the impact of water drops on the surface of sand particle. The high-speed photography reveals the detailed liquid-drop impact dynamics at various impact velocities. As you watch the video look at how the soil particles are struck and scatter when the drop hits; drops can be up to 6 mm in diameter and hit the soil at up to 20 miles per hour. As you can see this force from the raindrops can dislodge soil particles and splash them up to three feet away. If there is no residue coverage left, then our soil is exposed to these pounding forces that breaks apart our soil aggregates and facilitate the formation of a crust that seals the soil from taking in additional moisture as quickly as it otherwise would. However, if we keep crop residue on the surface or grow cover crops, we can reduce the impact this raindrop impact has. Residue provides a cushioning effect that absorbs the impact of the rainfall, and helps protect your soil.

So what are some ways that you can help protect your soil resources by leaving more residue? Here are three tips:

1. Try to follow a crop rotation sequence where a high-residue producing crop is grown every couple years. Or if you have need for a forage, consider planting a perennial like alfalfa that will maintain surface residue coverage for several years.

2. Wait until spring for tillage operations.

3. Try planting a cover crop, especially in fields where low residue crops where grown. For example, focus on fields where your corn was harvested as corn silage and lots of the residue removed.

Assessing Hazards in Manure Storages & Handling Systems

As I was installing a new circuit in my garage the other day, it got me thinking about where I learned this useful skill. To no surprise, my first experiences with wiring were at the farm with my dad. One day he decided it was time to replace all the lights in a barn (no doubt due to some university research saying better light encourages cows to give more milk). As with most farm projects, it of course goes better with a little help, and as a result I was drafted into service as the “flashlight holder” and the “where did I set the (insert tool name here) locater.” Now this might not appear to be an important job, but let me tell you it really is, for both the flashlight holder and the guy trying to get the wiring done. Any time you’re wiring the power is off and you are in the dark, so someone to hold the flashlight for you gives you two hands to work with, and if you are the one holding the flashlight it’s your chance to learn a new skill from someone who knows what they are doing. So what’s that got to do with safety… well, although most projects went well, every now and then one went differently than what we expected and at the end we’d say, “that was lucky.” In those cases it wasn't that we didn't realize that there were things that could go wrong in what we were doing, it was that we didn't take the time to think about what those things were and the best ways to minimize those risks.

So, now the fun part, where I say something that is supposed to inspire you to take the next step, walk around your farm, and identify the risks associated with your manure management system. Then of course, I finish by trying to provide you with a tool, tip, or technique to help you in the process. This all sounds like a simple process, 1, 2, 3, and it all comes together and your safety issues are solved. However, in trying to come up with my story about why safety is important only one thing came to mind… family. So I hope you take the time this winter to think about the safety program at your farm, and help develop a culture of doing things correctly and safely so your family has happy holidays for years to come.

In terms of developing a safety program, I think there are about four steps to work through, with your manure system being no different. 1. Select the job to be analyzed, 2. Break that job into a sequence of steps, 3. Identify potential hazards associated with each step, and 4. Determine preventative measures to address that hazards identified. Before we get stated, just two thing to remember, Safety is not optional and Safety is more than signs!

That are lots of activities that go into your manure system and even more into your farm, but since this is a manure blog we’re only going to focus on the manure portion of safety. Any manure system really accomplishes five tasks these are manure collection, transport to treatment/storage, storage, transport to land application area, and land application. It’s important that you consider all five steps, understanding that in many ways the steps that happen infrequently (transport to the land application area and the process of land application) might present more risk not only from the activity itself, but also because these tasks are so rarely performed.

So, here I’m going to focus on the manure storage. What safety hazards are there? The risks differ for each type of manure handling systems and the activity you are performing. Enclosed structures are commonly associated with asphyxiation hazards related to gases from manure decomposition, while open structures are often associated with drowning risks. In the case of open manure structures, one way to reduce the risk of drowning is by purchasing and installing a fence around the storage. This can work really well at keeping children and animal out and just as importantly identifies to people entering that there is a risk present. Though this is a great start, and focuses on just the general day to day risks, there still may be times when we may have to enter inside the fence, for example if we are collecting a manure sample or getting ready to agitate and remove the manure. Having a safety harness available that can be attached to anchor points on the storage might be one to reduce the risk of this activity of manure sampling.

Now that we've  talked about safety , I’m going to try to help show you a way to organize the process. In the table below I've identified a specific item, the day-to-day operation and maintenance of a manure storage. The next step is to identify examples of potential hazards. In this case it might include the risk of falling in (that drowning hazard) or the risk of hazardous gasses. The final step is then to think about what we could do to reduce or manage that risk, so for example placing a fence around the storage to help prevent entry or in the case of under building storages, making sure pump-out covers are in place.

How is the storage operated and maintained.	Examples of Potential Hazard	Example Preventative Measures
If the storage takes place in an open or closed tank…	Falling into the tank leading to: possible trauma, drowning, and potential life-threatening situations Such tanks or pits are often confined spaces and would be expected to have hazards: oxygen deficiency, hydrogen sulfide toxicity, methane exposition potential; as well as others, especially under certain conditions like the presence of foam or during times of manure agitation	Avoiding entry into the manure storage area. Place a fence around the storage or in case of under barn pits, make sure the access points have sturdy covers.

If you are interested in learning manure about manure storage safety ASABE has a standard EP 470: Manure storage safety that can be a nice resource. It details several safety risks and some things to consider in managing those risks.

How far can I afford to haul my manure?

When debating the economics of manure there are many things that can be considered, whether it be costs associated with manure collection in the barn, building or maintaining the manure storage, or moving the manure to the field and land applying it. How these costs are accounted for can make a big difference in whether we think the manure is providing a cost effective fertilizer resource, or if its use doesn’t pencil out compared to commercial fertilizers.

In evaluating the costs of manure application, the first choice a farmer has to make is whether to hire a commercial manure applicator to apply the manure for them, or to perform the application themselves. There are numerous reasons to consider both options, whether it be the ease of allowing someone else to handle the application, bio-security issues of bringing different equipment onto your farm, control over when manure application occurs, costs, or availability of equipment and labor. Here I'm going to base my cost estimates on information on what our commercial applicators are charging and use this to get some rough estimates. This is meant to be illustrative only, not an analysis for any particular farm.

When using manure as a fertilizer one of the most common questions is, how far can I afford to haul it and still have it be a viable, cost-effective fertilizer? What this question usually means is how far can I transport the manure and still have it be cheaper to use than commercial fertilizer. Like most thing in life, it depends. Things like what the nutrient content of the manure is, which nutrients are we getting value from (nitrogen, phosphorus, potassium, trace minerals, or even organic matter), what sort of crop rotation are we using, or even whether we are applying it ourselves or hiring a custom applicator to apply it can all impact the results. In addition, current prices of commercial N, P, and K fertilizers can affect this distance as this sets the value on the nutrients in our manure.

Let’s look at an examples; we are going to work with corn-soybean rotation, set current nitrogen, phosphorus, and potassium prices at $0.44 per lb N, $0.49 per lb P2O5, and $0.40 per lb K2O, and focus on swine manure first. Current nutrients concentrations (average ± standard deviation) in deep-pit slurry from a swine finishing operation are around 58 ± 25 lbs N/1000 gallons, 18 ± 8 lbs P/1000 gallons, and 20 ± 7 lbs K/1000 gallons (average ± standard deviation). Our commercial manure haulers on average are charging  around $0.02 per gallon, plus a millage fee of $0.0035 per gallon per mile transported. These prices are only rough guidelines that may vary throughout the state depending on your manure, soil, and crop production characteristics.

If a farmer is getting value from the N, P, K than they can afford to haul their swine slurry manure about 12 miles and have it be as cost effective as commercial fertilizer. Transport distances of 5 to 20 miles are possible depending on the manure’s nutrient content. However, if only the nitrogen is of value then the hauling distance equivalent to the fertilizer value is only about 1 mile. This illustrates that moving the manure further from the farm may be justified if it provides the opportunity to better utilize all the nutrients in the manure and take advantage of the phosphorus and potassium it contains.

Similar analysis can be conducted for dairy slurry, beef manure solids, layer manure, and turkey litter. Results of all manure types are summarized in table 1. In that case of solid manure, application costs were set at $6 a ton plus an additional $2.20 for every mile the manure had to be hauled. Again, these prices are just a rough estimate and could vary considerably based on your manure, field, crop, and equipment characteristics.  If all nutrients (N, P, and K) are being valued, manures can be transported between 2 and 34 miles while still being as cost effective as commercial fertilizer; however, if only nitrogen is being valued than economically justifiable transport distances are typically only 0 to 2 miles. In general, these results suggest that solid manures could be more economically transported further distances than the liquid manures, and that finding ways to capture the value of the phosphorus and potassium can really increase how far we are able to move our manure.

So what does this mean to you? Current market conditions seem to indicate that crop prices over the next year or two might be lower than what we experienced in the past couple years. Finding ways to reduce expenditures as we try to get the crop in the field and to harvest might help in reducing our risk. Manure can play a role in this, if you have fields in need of phosphorus and others that don't strive to get your manure resources to the fields where you can take advantage of the fertility it has to offer.

Some of you may be wondering, if I can really economically justify moving manure this far, we don't we see more of it moving these kinds of distances. Well, there are really lots of reasons, but one of the most important is we generally don't have to move manure anywhere near this far to find fields that can benefit from its application, and if those fields can benefit from all those manure nutrients why not save a little money in the costs of hauling them. Another, important consideration is the amount of time it would take to apply the manure. As I'm sure most of us are aware, there are a limited number of days available in the fall and spring to get our manure application accomplished. Hauling further distances either requires us to get new, bigger equipment, to haul on more days, or to haul under some less than ideal conditions to get the job done. Thus like most things, finding the right balance so that you can take advantage of your manure resources and get the job done is critical.

As a note, in determining the value of the manure I only considered nitrogen that would be available for crop production in the first year. More information about the amount of nitrogen available in the manure can be found in the ISU publication "Using manure nutrients for crop production," which you can find at http://store.extension.iastate.edu/Product/Using-Manure-Nutrients-for-Crop-Production. 

Table 1. Break-even hauling distances for manures from different types of operations and different cases of how manure are valued. The first number represents the average manure, values within parenthesis represent how farm manure within 1 standard deviation of the average could be transported.

	Manure Type
	Swine Slurry	Dairy Slurry	Beef Solids	Layer Manure	Turkey Litter
Break-even Hauling Distance (mi) All Nutrients Value	12 (5 - 20)	8 (2 - 13)	14 (9 -20)	25 (16 - 34)	16 (13 - 19)
Break-even Hauling Distance (mi) Only Nitrogen Valued	1 (0 - 4)	0 (0)	0 (0)	1 (0 -3)	2 (1-2)
Break-even Hauling Distance (mi) P & K only (P-limited)	5 (0 - 10)	6 (2 - 11)	13 (8 - 13)	21 (13 - 28)	11 (9 - 13)