by Sally Colby
Farmers know that manure is an important aspect of the crop cycle, but it’s easy to forget that value when manure is stored and winter work takes precedence on the farm.
Dr. Rick Koelsch, department of biological systems at the University of Nebraska, says we’ve always recognized the value of fertility from manure, but sometimes forget how manure enhances soil quality.
Koelsch quotes an Iowa State University article from 1907 that lauds manure as ‘one of the most effective means at the disposal of the farmer to permanently improve his soil … its value to the soil can scarcely be measured, for no other substance has equal power in maintaining permanent fertility.’
“It has always boggled my mind that we’ve ever equated the term ‘manure’ with ‘waste’ and use them in the same context,” said Koelsch. “It’s such a tremendous disservice to manure.”
Dr. Charles Wortmann, soil scientist at the University of Nebraska-Lincoln, addresses the ways in which manure affects soil physical properties. “Soil is mostly inanimate,” he said, “99.5 percent on a dry weight basis. But it does provide habitat for much plant and microbial life, and also a lot of chemistry.”
Wortmann says some soil physical properties, such as landscape position, texture, aggregation and susceptibility to erosion can affect crop response to manure. Properties such as aggregation and soil/water infiltration can also be influenced by manure.
Understanding soil associations in the landscape is important to good soil management. “Soil texture is the result of sand, silt and clay content of the soil,” said Wortmann. “Soil texture is fundamental to other properties and interactions. We might expect greater manure effect on soils that are relatively high in sand content or high in clay content.”
The concept of aggregation is important. Soil particles are microscopic and require an electron microscope for examination. “If we didn’t have soil aggregation, soil would be a massive, inert block,” said Wortmann. “The size and strength of the aggregates are affected by other soil properties such as soil organic matter (SOM) and by management practices including manure application.”
Susceptibility to erosion is another property that influences soil response to manure. “We’re most concerned with sheet erosion — the gradual movement of fine particles along the surface,” said Wortmann. “Water erosion tends to be downslope. Some (erosion) occurs due to raindrop splash effect. Sheet erosion is the most erosive process, but it’s often not noticed. Much more visible to us are rill and gully erosion, which occur with concentrated flow.” Small aggregates are easily carried by wind and water, while weak aggregates break down for easy transport and sealing of soil, which leads to crusting and reduced water infiltration.
Wortmann cites a study during which compost was surface applied, and in the first three years, runoff was reduced by about 60 percent. The following four years, there was additional runoff reduction due to the residual effect. Sediment loss was also reduced over the total seven-year period. Wortmann says there’s approximately a 2 percent reduction in runoff and erosion per ton, per acre per year of dry weight of manure application.
During the study, there were no effects on runoff or losses of sediment or phosphorus comparing winter and spring application without any incorporation. “If you have one or two moderate rainfall events that do not cause runoff before you have a runoff event,” said Wortmann, “there is enough reaction of the soil with manure that there is not an increase in phosphorus loss with surface application as compared to where manure was applied.”
Manure application reduces sediment and runoff, but increases runoff phosphorus concentration. The quantity of phosphorus loss may be increased or decreased, depending on ‘does the reduction in runoff and sediment loss fully compensate for increased runoff P concentration?’
Linda Schott, graduate research assistant and PhD candidate at University of Nebraska-Lincoln, says there are many studies on the effect of cover cropping and crop rotations on soil health, but not as much focus on how manure affects soil health. “In general, practices that increase soil organic matter (SOM) increase soil quality,” she said. “Manure increases soil organic matter, so it increases soil quality.”
Schott’s research focused on summarizing short and long term studies reporting the effects of livestock manure and municipal biosolids on soil physical, chemical and biological properties, and indirect indicators of soil quality such as climate resilience, crop yield and crop quality.
When soil organic carbon is increased through organic amendments such as manure, a lot happens. Water holding capacity increases, nutrient retention increases, and structure and biological qualities improve. “All of these lead to a increase in soil organic matter because as biology breaks down and consumes carbon, it stabilizes and eventually becomes organic matter,” said Schott. “Manure increases soil organic carbon 10 to 100+ percent. It’s variable because it depends on a lot of factors such as climate and soil type.” Schott also found that about 20 percent of carbon in manure persists after the first year. The carbon that remains in the soil contributes to the buildup of soil organic matter.
In reviewing soil physical properties over time, Schott says manure helped increase aggregate stability and soil infiltration. Water holding capacity was variable – sometimes it increased, sometimes it decreased; depending on soil type.
Soil biological properties are greatly enhanced by manure. Schott says an abundance of bacteria and microbes influence soil biology. “Included in abundance are bacteria and fungi, earthworms, nematodes and microarthropods,” she said, adding that soil is also affected by microbial diversity and the activity of bacteria and microbes. “In general, manure significantly increased bacteria and fungi abundance in soil compared to inorganic fertilizer.”
Schott says this is important because soil is 95 to 99 percent mineral particles, and depending on location, the remainder is comprised of valuable soil organic matter. Soil organic matter includes stable humus, decomposable plant matter, bacteria, fungi and other biological components such as yeast, algae, protozoa and nematodes; all of which are important for nutrient cycling and plant health and growth.
The bottom line doesn’t change. “Manure significantly increases soil organic matter, which allows bacterial and fungal populations to increase,” said Schott. “In comparison to inorganic fertilizer, manure increases these populations 10 to over 100 percent. Nematodes that feed on fungi and bacteria increased, and earth worm populations also significantly increase.”
As composted manure piles, manure pits, pumps and spreaders remain idle until the ground is fit and regulations allow spreading, remember how manure is always ready to contribute to building better soil.