by Deborah Jeanne Sergeant
Animal-based soil amendments can help producers grow more; however, they carry greater risk than other amendments. M. Charles Gould, Extension educator at Michigan State University, presented “Safe Use of Animal-Based Soil Amendments” as a recent webinar.
Gould began with a definition of soil amendments: “organic and inorganic materials applied on and mixed into agricultural soils to change the physical and chemical characteristics of the soil.” Managing amendments properly can help producers grow more food safely while preserving soil health.
Inorganic materials are those that are mined or man-made. These don’t provide a direct pathogen. Organic, on the other hand, come from nature and aren’t processed.
Gould added that using organic amendments in a cropping system improves physical properties of the soil, aeration, water- and nutrient-holding capacity, water infiltration, nutrient/mineral values for plant growth, cation exchange capacity, pH and the suppression of soil microbes that could harm plants.
“The benefits of soil amendments translate into higher yields and greater profits,” Gould said.
He stated that the environmental and management effects on nutrient release in a cropping system include soil moisture, soil temperature, cover crops, soil organic matter, crop rotation and soil pH.
“Amendments are most effective when the soil is moist,” Gould said. “Ultimately, using amendments must be sustainable. That’s a really key point. We want to be profitable.”
That’s why he believes that using soil amendments should result in achieving specific outcomes. Soil amendments cannot take the place of crop rotation, however.
“If we’re talking a green manure and we’re going to till it in, there’s going to be nutrients in that cover crop tilled in,” Gould said. “As it breaks down, those nutrients are released. We have some fertilizer benefit there.”
Operators need to apply manure correctly. When that happens, “it is a product that can increase yield and has benefits for the soil. It’s all in the management of it. Based on the percent of solids, it shows the type of manure that’s there and how it can be used,” he said. “Manure supplies macro- and micro-nutrients that are essential for crop growth.”
Manure functions like a slow-release fertilizer, helping build soil organic matter and fertility, though the nutrient content is different depending upon the species producing the manure, the animals’ diet and, if applicable, any bedding content.
“Using manure means farmers need to adjust their commercial fertilizer application rates,” Gould said. “That’s gotten us in trouble over the years. We haven’t accounted for the nutrients in manure and we just assume they’re not there, when in reality it provides a lot of N, P and K.”
In addition to nitrogen, phosphorous and potassium, various other nutrients abound in various manures. Gould cited poultry and hog manure as higher in P than other livestock manure and that horse manure is relatively low in N because of the bedding it usually contains.
Properly storing manure is vital to its safety in use with produce, including the minimum distance.
“Minimum distance depends on a lot of factors, including runoff controls, wind spread, slope, lay of the land and the quantity and how it’s contained,” Gould said. Manure storage and treatment sites should be located as far from produce production and handing areas as possible. Barriers or physical containment securing manure storage or treatment areas from runoff, leaching, animal, foot and/or equipment traffic or wind spread are good ideas.
Gould noted, “Tractors that come in contact with untreated manure can cause contamination, as can equipment.” Anything used to handle manure could transmit contamination to other parts of the farm, so using dedicated equipment or cleaning it is necessary. He said applying manure in the field, rather than storing it, is preferred.
“However, we just can’t go out and willy-nilly apply manure,” Gould added. “We need to match the nutrients of the manure with the nutrient requirements of the crop and apply so it doesn’t run off into groundwater.”
He encouraged farmers to document their application rates, times of application and types of manure.
“It’s worth your time to provide that documentation to show you’re following your manure management systems plan,” he said. “We want to make sure pathogens don’t come in contact with fruits and vegetables. We also need to make sure we’re not polluting because we’re putting nutrients where they shouldn’t be.”
The USDA states that applications of raw manure should happen no later than two weeks prior to planting, a minimum of 120 days prior to harvest for crops in contact with the soil and a minimum of 90 days prior to harvest for crops not in contact with the soil.
“Treatments that may reduce pathogen levels may be divided into two kinds: passive treatments and active treatments,” Gould said. Passive treatment relies on the passage of time in conjunction with environmental factors that help reduce pathogens, such as UV, moisture and temperature fluctuations. “Passive lets Mother Nature do her thing.”
Active treatments require more management and inputs, such as pasteurization, heat drying, anaerobic digestion, alkali stabilization, aerobic digestion or a combination thereof.
“Farmers need to consider how manure may enter a field from other sources, like runoff or wildlife,” Gould said. “Adjacent field practices can have an affect on your produce.”
Farmers should also create corrective action plans for problems, according to Gould. They should include what to do if the manure has contact with harvestable parts of a crop (such as using alternative markets for that produce) and a “kill step” (“doing what’s necessary to kill the pathogen that the crop may be exposed to,” Gould said).
Gould also spoke on compost, a topic he feels is often misunderstood. A lot of farms call manure compost, but that’s not correct.
“Manure is a feedstock that goes into making compost,” he said. “Compost is controlled, not left in the back 40 to decompose over time.”
Gould offered a recipe for compost: one part “green” materials, like grass clippings, coffee grounds and manure, with two to three parts “brown” materials, including dried leaves, chopped straw or hay, sawdust or wood shavings. In stage one of making compost, which takes one to two days, the mixed materials are the same temperature as the air. Gould said that psychrophilic bacteria (natural to the environment) are active at this stage.
“Once temperatures get above 50º, the microbes become active and make the nutrients plant-available,” he said. “If you have a lot of organic matter in the soil and nitrogen out there, the microbes will scavenge for that so there might not be sufficient nitrogen for the crop you want to grow. The carbon-to-nitrogen ratio is critical.”
During stage two (about three to five days), a well-mixed pile will increase in heat to 100º – 140º F.
“Mesophilic bacteria, active between 50º and 120º Fahrenheit, are the most abundant and do the most decomposition,” Gould said. “Mesophilic are really the key. If your compost recipe is correct, you’ll see your compost heat up.”
Oxygen and moisture are key at this stage. “Keep tabs on it to make sure you turn it to introduce oxygen to the pile and add moisture as you need to,” Gould advised.
During stage two, thermophilic bacteria (active from 110º – 160º F) are active. This is when the fastest breakdown happens due to heat. The carbon-to-nitrogen ratio decreases because of carbon loss and the pH rises above neutral.
“You know you’re doing it right if you see your volume reduce,” Gould said. Producers should see a volume reduction of 50 – 60% and weight reduction of 40 – 80%.
“It’s at this stage that it’s really critical for the pathogens, weed seed and fly larvae to be destroyed,” he said. “Composting is both an art and a science.”
At stage three (about six days and for weeks or months), the pile cools. Gould said turning the pile can jumpstart mesophilic bacteria, reheating the pile.
During stage four, the compost returns to the ambient air temperature. It cures and the nutrients are available for plant use for up to a year.
“The pile can be colonized with actinomycetes, protozoa, rotifers, nematodes, fungi and numerous species of invertebrates,” Gould said. “If your compost heap is populated with those, you know you’re doing it right.”
In general, it takes time, temperature and turning for successful compost production.