KIMBERLY, Idaho — Applying 50 tons of manure per acre for one year is a heavy application. Repeating that rate for eight years is a lot of manure—and a lot of nitrogen.
Amber Moore, University of Idaho extension soils specialist, has calculated that some plots in a long-term manure application study have already received 2,746 pounds of nitrogen in the last two years alone.
“Most of that organic nitrogen is from manure, but some is from fertilizer,” she told a crowd of about 50 who attended a nutrient management field day last week. She expected to see nutrient leaching from the top foot of soil as plants were unable to utilize that much nitrogen. So far, that hasn’t been an issue.
Or, at least, not in potatoes.
Even where soil samples showed high nitrogen levels, those plots received additional nitrogen from commercial fertilizer. Still, soil sampling indicates the nutrient isn’t moving below the root zone. “We haven’t seen hardly any nitrogen in the second foot—yet,” she said.
She can’t say the same for the sugarbeet plots that were planted following barley. Those plots are adjacent to the long-term wheat-and-potato plots. Moore is starting to see soil nitrates up to 30 parts per million in the heavily manured plots.
“We are starting to see nitrogen leaching at the highest manure rate,” Moore said “Barley is not as much of a nitrogen scavenger as wheat.”
Soil sampling shows nitrate of 12 parts per million in the first 12 inches of soil in wheat-and-potato plots that received either no nitrogen from manure or fertilizer and those that received just commercial fertilizer. Plots that received the lowest manure rate have a similar nitrate levels.
Where Moore is starting to see a difference is in the plots that received the medium and high manure application rates. Sampling shows nitrate of 30 ppm and 35 ppm, respectively, for those treatments.
She admits to being a bit nervous about that much nitrogen in the soil, but petiole sampling indicates the plants are using that much nitrogen. Interestingly, the petiole nitrate samples were best in the biennial plots where manure is applied every other year before wheat is planted and then urea is applied for potato production.
Even though this is only the second year of the study, Moore is also noticing that salt levels are lowest in the treatments that receive the least manure. Biennial manure applications, even at higher rates, have lower salt than plots receiving annual manure applications.
While taking a break from manure application may help salt accumulation, phosphorus—another key nutrient—continues to accumulate. “It doesn’t seem to matter if you take a year off,” Moore said. “It looks like there is a linear relationship between the amount of manure applied and phosphorus in the soil.”
Increasing phosphorus levels in the soil are a key reason dairies are required to have nutrient management plans to determine how much manure can be applied to fields. This long-term study is designed to mimic applications that may happen close to dairies where it is convenient to put a little more on and fields farther away where transportation costs will limit how much manure is applied.
Moore has been somewhat surprised at how quickly manure can build organic matter levels in soil. She assumed it would take longer to see a response, but is already measuring organic matter of around 2 percent in the high manure application treatments. That’s about double the amount of organic matter typically seen in southern Idaho soils.
Greenhouse Gas Emissions
Organic matter is associated with better water-holding capacity and warmer soil temperatures, characteristics that are good for crop production but may also be influencing gas emissions from fields.
April Leytem, a soil chemist with the USDA’s Agricultural Research Service in Kimberly, is measuring greenhouse gas emissions from the plots. She is studying how manure application rate and irrigation affect nitrous oxide losses from soils.
In the first year, with wheat, she measured a huge spike in nitrous oxide following the first irrigation of the season and very little for the rest of the season. With potatoes, there is a spike with every irrigation although the amount measured decreases with each irrigation.
But overall, her study indicates only a half to one pound of nitrogen per season is being lost to the atmosphere through denitrification. She is also noticing that the low and moderate manure application rates have similar nitrous oxide losses, but the high manure rate is significantly greater. She’s not sure if the higher rate is the catalyst, or if higher organic matter levels are providing ideal conditions for denitrification. She’s hoping the next six years of the study will provide more clues.
Agronomists and producers are also curious. Dennis Crawford has worked with both dairies and crop producers in the Magic Valley for several decades. Moore and Leytem’s findings confirm much of what Crawford has observed over the years. “It’s nice to know you were seeing what you thought you’ve been seeing,” he said.
Source: Twin Falls Times-News