Last week we shared an article from the Soil Science Society of America that pointed out the benefits of using manure to fertilize and improve soils. The results are not surprising. Countless trials have described the beneficial results of applying manure. So why do we still use inorganic, manmade fertilizers? Andrew McGuire’s answer, after doing the math, is that there’s just not enough manure in the world to produce the amount of food people on the planet require.
Here’s a summary of his findings. You can read the two articles this is drawn from (along with all his citations) here and here.
How Much Manure Can We Produce?
The process of making manure starts with plants that are eaten by livestock who then give us the “end” product. McGuire started by looking at the losses of organic matter in this process:
• 57-81% between feed and fresh excreted manure (ASABE, 2005) which goes to the animal’s growth and maintenance
• 4-8% lost in collection and transport (10-40% of excreted manure, NRCS 1995)
Using these loss figures, and average yields of one acre of corn or silage fed to beef in feedlots and dairy cows in confined operations, he found that beef cattle could produce 1.6 tons manure per acre (low scenario), and dairy cattle 5.7 tons of manure per acre (high scenario). (See McGuire’s figures in his original post.)
“Surprised?” he writes. “I was. This is so little that it would be near impossible to spread evenly over an acre. And if you did, you would barely be able to see it.” (To give you an idea of what different rates of manure spread look like in a corn field, McGuire shares this link where you can guess the tons/acre. It’s pretty eye-opening.)
How Much Manure Do We Need to Supply Nutrients to Crops?
To supply an average yielding 174 bu. per acre corn crop with the nitrogen it needs, we would need to apply 22.2 tons of manure per acre. To supply it with adequate phosphorous takes only 3.8 tons of manure per acre. Unfortunately, our manure production falls short of both in the low scenario and can only supply the field with phosphorous under the high scenario.
Based on his figures, which are based on a review of the literature on this topic, McGuire concludes that we don’t have enough manure to supply nitrogen and phosphorous in the amounts needed by agriculture.
Can Manure Provide the Soil Organic Matter We Need?
But manure does more than just provide nitrogen and phosphorous. It can also provide organic material, a small but crucial part of soil. When we increase organic matter in soil, we improve soil health and function. So McGuire’s next question was, do we have enough manure to do this? In his second article on this topic, he set out to find out how much manure we need to maintain soil organic matter.
Let’s Start With How Much SOM We Lose Each Year
Organic matter breaks down over time with rates varying depending on temperature, moisture, soil texture (sandy vs clay) and oxygen levels. Organic matter in warm wet soils breaks down more quickly than in soils at that are extremely hot or cold or dry or wet. Tillage that increases oxygen to the soil also increases rates of loss.
Based on what decades of research tell us, here’s McGuire’s table showing how many pounds of dry organic matter we lose on average per acre. What it tells us is that if you have 6% soil organic matter, and you’re losing 1% per year, you’re losing 1200 pounds per year.
How Much Applied Manure Ends Up as Soil Organic Matter?
When you apply manure to the soil, one of the things you’re doing is feeding soil microbes. They do us the service of incorporating the organic matter into the soil, but as they eat, some of the biomass we wanted for our organic matter is lost as carbon dioxide, CO2. So that’s a loss for our organic matter replacement. The size of the loss depends on the types of soils, but on average only of the applied manure actually becomes soil organic matter.
How Much Manure Do We Need to Apply to Maintain Soil Organic Matter Levels?
“We will assume, optimistically, that 21% of our applied manure ends up as soil organic matter,” writes McGuire. We take our loss numbers from the table above and divide it by .21 to get the amount we need to apply. As his table below shows, if you have 6% SOM and a loss rate of 1%, you’ll need to apply 2.9 tons of dry matter per acre to maintain your 6% SOM.
For typical manure from a feed lot with 33% added moisture, McGuire’s table looks like this:
These are all just maintenance figures. If you’d like to increase your soil organic matter, you’d have to apply more manure.
McGuire also notes that, “These numbers will be close for both cattle (earth lot) and broiler manure. Multiply by 2.2 for dairy manure (at 72% moisture) or 2.8 for swine (slurry at 91% moisture).”
Next – How Many Crop Acres Does It Take to Produce the Manure We Need to Maintain Soil Organic Matter?
McGuire produced the table below using the average of the best and worst manure production rates (above) of 3.6 tons per acre divided into the manure needed to maintain one acre. These numbers seem to be reasonable based on other research.
As you look at the red numbers, you’re probably starting to conclude, as McGuire did, that in spite of the piles of manure that accumulate at cattle feedlots, dairies, and chicken and pig farms, we still don’t have enough to provide for all our fertilizer and soil health needs. Here’s what McGuire says:
“For soils with higher Soil Organic Matter (SOM) levels or loss rates greater than 1%, manure, by itself, will not sustain the soil. But manure is not the only source of SOM; crop residues can also be a significant source of organic material that ends up as SOM. High residue crops like corn (for grain) and wheat contribute the most while low residue crops like most vegetables, or crops where all the residue is removed at harvest, like corn or other silage, and hay crops, have little or no effect on SOM. Therefore, combining manure with a high residue crop and minimal tillage will expand the range of soils where SOM levels are maintained, pushing the red area of Table 4 to the right.”
Here are his final conclusions:
“First, let me be clear that manure is beneficial. Soils receiving manure can have higher nutrient and soil organic matter levels than soils without manure. Even better for those that can do it, applying manure can overcome the detrimental effects of tillage and production of low residue crops. However, we can now see that sustaining SOM levels in most situations, requires manure rates that are higher than production rates. The concentration of manure in one location does not change this. Even in locations with abundant manure, where we can apply enough to sustain soils, this imported manure is a loss to those distant fields that produced it. Relying on imported manure sustains some fields at the expense of others; we rob Peter to pay Paul.
“Those researchers that have taken a critical look at the use of imported organic amendments have concluded the same, and not just for manure, but for compost too. Magdoff and Weil (2004) state “Application of organic amendments that originated elsewhere can be expected to involve degradation of the soil from which the C and nutrients were originally harvested, making the practice questionable from an overall sustainability viewpoint.” This concentration of manure from many fields onto one field is not sustainable.
“Manure is a scarce resource.
“When faced with piles of manure and limited area to spread it on, it is easy to forget that manure is connected to crop production. Like other products that are used far from their source (e.g. tropical hardwoods, seafood), manure’s real scarcity or abundance is hidden.
“Some farmers are in the enviable position of having access to lots of manure, or of being able to pay for it to be transported to their farm. Their unique situation does not change the fact that there is not enough manure to go around, nor does it make them more sustainable. As we have found, manure rates that can supply a crop with nutrients or increase soil organic matter levels are often only possible because of the degradation of the fertility or SOM of other fields. It is not possible for all farmers to use manure to maintain their soil’s fertility or organic matter level.”
Note from Kathy: On Pasture readers, I confess to not being a math whiz, nor am I conversant with all the literature on this topic. So if you have questions about this I urge you to visit Andrew McGuire’s original posts. You can post your questions with him and I’m sure he’ll provide good feedback.
Can Manure Supply Nitrogen and Phosphorus to Agriculture?