Not Your Grandpa’s Manure Spreader

Another manure application season is arriving, so what better way to prepare than to talk a bit about manure spreader technology? At first, manure spreaders might seem pretty simple – they are used to get manure from point A (the barn or barnyard) to point B (the field). However, they have to do so much more than that.  Manure spreaders not only have to get manure from the barn to the field, but once they are in the field they have to evenly spread that manure over the field.

The early history on manure spreaders is a little shaky, but the general story is they were originally horse drawn and had to be manually unloaded using a pitchfork. Eventually Joseph Kemp, who is credited with developing the first automated spreader (in 1875) developed a new version that unloaded itself. This spreader essentially left all the manure right behind itself, not the most desirable pattern, but definitely better than the backbreaking work of pitching the manure off. Joseph Kemp then sold his design to International harvester in 1906.

 

 The next leap forward was by Joseph Oppenheim, who’s “New Idea” added paddled beaters at the back that shredded and flung the manure to the side. This created a “widespreading” pattern that more uniformly spread the manure over the soil surface. A lot has happened since then; we switched from horses to tractors for power, made bigger and bigger spreaders, and switched steel wheels to rubber. However, we still often deal with the same challenges - trying to get the right amount of manure applied to a field, getting the manure application as uniform as possible, and trying to do it all as quickly so we can start after our soils have cooled and before they are frozen.

The spreaders we have today have incorporated new techniques to help in hitting the application rate and controlling how much manure is applied, and that is the part I’m going to focus on. I’m going to look at slurry spreaders during this discussion. The rate control consists of 4 basic parts: a rate controller (the brains of the system), a flow meter (the sensing element that measures the variable we are trying to control), a GPS unit to map and sense tractor speed, and then a means of adjusting the flow (often a valve in the system or a hydraulically speed controlled pump). Below I have a schematic that that walks through the parts of the system and how they work with each other. The brown arrows represent manure flows and the black line represent information flows, which are handled electronically or hydraulically.

So the system starts with knowing what rate application rate we are trying to achieve, let’s say its 2500 gallons an acre. We need to enter this into the on-board controller. The controller also needs information on the size of our tool bar. Let’s say it is 8 injectors at 30” centers. This means with every pass we’d cover 20 feet. Finally, the GPS feeds the tractor and spreader speed into the controller. Let’s say it gets a reading of 7.5 mph (660 ft/min). The controller does a little math and says at this speed we are covering 13,200 square feet per minute (or about 0.303 acre/min), so to achieve our desired application rate we need a flowrate of 757.6 gallons per minute. As the system starts up the pump will push some flowrate through to the application toolbar, let’s say the flowmeter reads 1000 gpm. The controller than sends a signal to the actuator to slightly close the valve. It keeps doing this until the desired flow rate matches what we are trying to achieve. Pretty sophisticated controls, but necessary to make sure we are getting the most from our manure fertilizer.

So what’s this look like on a real manure spreader. The picture below shows the side view of a Houle spreader. It is not visible in the front in this picture, but the next one shows the pump in the front of the spreader. As we start applying this pump kicks on and starts manure moving through the pipe on the top of the tank. The grey portion in the pipe, that’s the flow meter, it’s constantly measuring the flow rate of manure and sending this information to the controller in the cab. The controller uses this information, along with the speed of the tractor, the desired application rate, and the width of the implement to determine whether to open or close the valve. You can see the valve and actuator; in this case it a hydraulically closed actuator that opens or closes the valve.

Profile view of of manure spreader.

PTO driven pump on the front of the manure spreader.

In cab controller that electronically opens and closes the valve to match that actual flow rate to the desired rate.

The science behind 50-degree soil and nitrogen application

Every year we hear a chorus of reminders to wait until soil temperatures at the 4-inch depth are 50°F and trending cooler before applying anhydrous ammonia, and those of us in the manure world tend to echo these comments. That is if you are applying an ammonia rich manure, liquid/slurry hog manure wait until soils start to cool before applying. So, what is the science behind this recommendation, especially for manures?

This recommendation is based on the potential for nitrogen loss. Remember, there are a few forms of nitrogen that can be applied or are found in soils these include ammonia/ammonium, nitrate, and organic nitrogen. Of these forms, all forms can be lost, but ammonia and nitrate tend to be the most mobile.

Ammonia is lost as a gas, so if we are using an ammonia/ammonium fertilizer (like swine manure) it’s important to get the fertilizer into the soil quickly where the ammonia will react with the soil particles and be held, rather than letting it sit on the surface where some of it can be lost to the air. This is why injection or immediate incorporation can be a great technique for getting the most from your manure, it makes sure that we aren’t immediately losing some portion of the nitrogen we are applying.

Nitrate on the other hand is lost with water, especially water moving through the soil to groundwater or tile drains. Nitrate is super soluble, so if water is moving and we have nitrate in our soil, it is probably moving with the water. We tend to have larger rains in the spring coupled with wetter soils from snow melt, this means that if the nitrogen we applied is in the nitrate form there is a high opportunity for it to be lost in the spring.

When it comes to manures, it’s pretty much nitrate free when we apply it, but microbes in the soil will process it and turn it to nitrate. The activity level of these microbes is controlled by how much ammonia is present, the amount of water and oxygen in the soil, and the soil temperature. Although all these variables are important, for now I’m going to focus just on temperature. A good rule of thumb is that microbial activity will double for every 18°F increase in temperature (so if our soil is at 68°F those microbes will be turning the ammonia in the manure into nitrate at about 2x the rate they would if our soil was at 50°F). Often times this means that not only will the microbes have more time to cause the conversion to nitrate, but they might be doing it much faster than if fertilizer application had waited.

So what’s this look like? I did a little exercise where I calculated a term like a degree-day, I’m going to call it activity-days. It is an index that takes into account how many days the microbes in the soil had access to the nitrogen fertilizer and how active they would have been on those days (based on the temperature of the soil). I then took the ratio of how much more nitrification you might expect if you applied at a certain day compared to the amount that you might expect if you applied when the soil reached 50°F. I did this for 12 sites (with a few of these sites having data from a couple of years) and plotted out the relative risk of nitrification compared to application date. What you can see pretty clearly is we start out with a steep slope (relative risk of nitrification decreasing quickly) until we high a relatively risk of around 1 (that would be the soil at a temperature of 50°F). Once with hit this point, generally around the first week of November the relative risk of nitrification decreases much more slowly.

Does nitrogen becoming nitrate mean we are going to lose it? No, it takes rainfall or snowmelt in the spring that will cause a leaching event, but it does increase the risk of loss. Certainly there is a balance between making sure we get our manure applied before the soil freezes  and applying two early, but hopefully the graph above illustrates a bit behind the science of the 50°F and cooling recommendation.

As a reminder, Iowa State University Extension and Outreach maintains a statewide real-time soil temperature data map on their website that ag retailers and farmers can use to determine when fall applications are appropriate. The website can be found at http://mesonet.agron.iastate.edu/data/soilt_day1.png.