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Use the nitrogen on your farm – then fertilize if needed

Alan J. Franzluebbers for Progressive Forage Published on 28 September 2017
robert Shoemaker and Jay marshall collecting soil

Have you ever wondered if the nitrogen (N) fertilizer recommendation you’ve been receiving has been accurate for your farm and for your management style?

Have you tested whether a higher application rate might enhance yield or if a lower rate might not change yield but end up saving you money? Did you know N recommendations are typically not based on soil testing? What if there was a soil test that could help refine how much N you should apply to match the goals and management style on your farm? Would you use it?

These are the questions I, as a researcher at the USDA-Agricultural Research Service in Raleigh, North Carolina, am posing to guide a practically oriented research program on soil biological activity. I have recently worked with farmers and agency personnel in North Carolina, Virginia and surrounding states to collect soil samples and analyze them for potential soil biological activity.

Smriti Pehim Limbu and Molly Pershing prepare to run an analysis of soil biological activity.

The goal is to characterize how much N the soils are releasing to crops. This process of N release to crops is dominated by “mineralization,” i.e., the conversion of organically bound N to plant-available forms of ammonium and nitrate. The research shows clearly some soils have a much larger capacity to mineralize N than other soils.

Which soils mineralize more N?

The answer to this question is somewhat based on soil type (like sandy or clayey soils or even soil series name, such as Cecil sandy loam or Norfolk loamy sand), but not entirely. The answer is more related to how one has managed a particular parcel of land for the past few years to a decade or more.

Growing crops with yearly disturbance of soil with tillage will cause breakdown of crop residues and soil organic matter. This breakdown (or decomposition) rapidly releases nutrients into an inorganic form, which are taken up by plants, leach through the soil profile, run off the land with heavy rainfall events or are transformed into gaseous byproducts and released to the atmosphere.

On the other hand, growing crops and forages with minimal soil disturbance (e.g., direct drilling or no-tillage planting), using cover crops whenever possible, rotating a variety of crops with forages included and applying organic sources of nutrients such as dairy or beef manure or chicken litter will often lead to an accumulation of organically bound nutrients that can be mineralized slowly over time.

These crop management approaches, along with well-managed pastures, are part of a conservation agricultural systems approach that leads to a larger reservoir of potentially mineralizable N; therefore, soil management is vital to how soil N mineralization changes.

How is N mineralization determined and can it be determined on my farm?

Nitrogen mineralization is often assessed by collecting soils from the field and incubating them under ideal temperature and moisture conditions in the lab for several weeks. Inorganic N (i.e., ammonium and nitrate) that accumulates in the soil solution at the end of incubation is considered plant-available and can be used to adjust N fertilizer recommendations from a target N requirement specific to a crop and its yield goal.

If it’s that simple, then why doesn’t soil testing offer this service?

The laboratory procedure for N mineralization requires a minimum of three weeks, and some scientists would argue that it should take even longer to get a reliable estimate. The time required is a limitation for soil testing services and farmers, both of whom desire rapid turnaround time of results.

Another issue is: Soil type and management matter tremendously in the estimate that will be obtained, and variability among soil types and management can be large. This means, essentially, each field would have to be analyzed separately and, therefore, no simple recommendation can be made without detailed soil sampling. Cost-to-return ratio is too high.

Is there another way?

Researchers have been learning carbon and N mineralization are highly related but also entangled in a complex web of growth and decay in the soil. What they’ve discovered is: A reasonable approximation of the quantity of N available to annual crops and perennial forages can be estimated from the short-term release of carbon dioxide from a soil sample that has been initially dried and then re-wetted.

Our lab, in collaboration with North Carolina State University, has been documenting the strong association between the flush of CO2 during three days following re-wetting of dried soil and the consistent accumulation of inorganic N during longer time periods.

The greater the flush of CO2, the greater quantity of N accumulated in soil. Several greenhouse growth trials have confirmed soils with high biological activity (i.e., high flush of CO2) lead to larger plants with greater uptake of N from unamended soil. The soil test requires some drying time, sieving and incubation in the lab for three days to complete. Therefore, soil test results with this new approach can be reasonably delivered within a week.

Producer trials

Our team of scientific collaborators (including graduate student Smriti Pehim Limbu), field extension agents and interested farmers have since taken the approach to the field to see if N fertilizer applications could be successfully altered based on determination of soil test biological activity (i.e., the flush of CO2).

In a series of on-farm research trials, primarily in North Carolina and Virginia, reasonably strong association between the level of soil test biological activity and likelihood of response to N fertilizer has now been established for stockpiled tall fescue (Figure 1).

Exonomically optimum nitrogen rate of tall fescue to meet threshold yield

These results suggest mature pastures that often accumulate soil organic matter and mineralizable sources of carbon and N (on the right side of the figure) will have low likelihood of yield response to N fertilizer.

On the other side (left side of the figure), new pastures and frequently hayed land without sufficient carbon sources to feed a thriving soil biological community will have a much larger likelihood of responding to N fertilizer. Therefore, soil test biological activity tells us the potential amount of N that can be mineralized and subtracted from a standard maximum fertilizer recommendation, since mineralization is a function of soil microbial activity.

Another interesting result from conducting this research on farms in the region is the relatively low incidence of fields responding to N fertilizer application overall. Many of the pastures selected for this research had high soil test biological activity, suggesting tall fescue pastures in the region are at sufficient levels for available N to not requiring frequent N inputs from fertilizer.

Grazed perennial pastures are known to cycle nutrients effectively from soil to plants to grazing animals and back to soil again through dung and urine. A more certain way to know how much N can be mineralized from your soil is to conduct a soil test.  end mark

PHOTO 1: Robert Shoemaker and Jay Marshall collect soil in August 2015 prior to establishing a nitrogen rate trial on fall-stockpiled, tall fescue pasture in Loudoun County, Virginia.

PHOTO 2: Smriti Pehim Limbu and Molly Pershing prepare to run an analysis of soil biological activity. Photos provided by Alan Franzluebbers.

Alan J. Franzluebbers is with the USDA – ARS, North Carolina State University. Email Alan J. Franzluebbers.

Talking with Robert Shewmaker

Robert Shoemaker owns a 100-head cow-calf operation in Fauquier County, Virginia, and works as a nutrient management specialist with the Virginia Department of Conservation and Recreation. He has also been active in the American Forage and Grassland Council and Virginia Forage and Grassland Council.

Shoemaker says, “As a farmer and nutrient management specialist, we have been involved with Dr. Franzluebbers’ work for several years. Overall, on corn and fescue, we are seeing a general trend as soil CO2 test levels increase, which is a measure of potential soil nitrogen release. We see the crop response to added nitrogen tend to decrease.”

Generally, on corn in Virginia, Shoemaker says, “We start out with 1 pound of nitrogen per bushel of expected yield and decrease total nitrogen rates based on crop history and manure applications. The CO2 test gives growers the opportunity to get a field-specific soil test analysis done at or before planting, which is quite revolutionary.

This technology, if proven, may be coupled with other nitrogen prediction tools such as the nutrient management plan or a soil nitrate test taken at side-dress to portray a more accurate picture of the correct nitrogen rate on a given field.”

Shoemaker says the test may also be useful to determine economic rates of nitrogen on fescue pastures being used to stockpile for winter grazing. The standard nitrogen recommendation in this case is to apply 60 to 80 pounds of nitrogen per acre in August.

However, oftentimes the CO2 test will indicate a field with no nitrogen may yield as much as 80 to 90 percent of the dry matter accumulated with a full dose of nitrogen. Optimally, a slight increase in yield does not outweigh the cost of the added nitrogen or additional lime needed to overcome the drop in soil pH.

Shoemaker says, “We are seeing the higher CO2 tests and subsequent soil nitrogen release that is already there in situations where we have long-standing, well-managed rotationally grazed pastures.”

Progressive Forage Editor Lynn Jaynes

Soil biological testing

The procedure used to test for N availability in our lab has been modified with a commercially available test by Solvita (Solvita - soil). This was possible based on the inputs and advice of Rick Haney with USDA-Agricultural Research Service in Temple, Texas, who also has worked with various commercial labs in the past few years to offer soil biological activity as part of the Haney Soil Health Test.

Various commercial soil testing labs now offer soil biological testing, so check around with labs in your area to see if available. More thorough documentation of the rationale and scientific basis for testing of soil biological activity can be found online (Acsess DL articles/1/1/150009).

—Alan Franzluebbers

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