Build Nitrogen Inventory Using the 4Rs
Nitrogen (N) management for your customers’ fields is about maximizing total input and output efficiencies, like an automobile assembly plant, describes Farm Journal Field Agronomist Ken Ferrie.
“Each morning the facility gets the exact number of parts needed to assemble the vehicles on the line that day,” Ferrie says. “After assembly, the vehicles are shipped out within a mere number of hours.”
The assembly process requires high-touch management and planning to ensure every step is in sync. The same is true for N programs.
“Don’t try to apply all of the N in the fall with the goal of making it until R5 in August,” Ferrie says. “Instead, think of an N program as ‘just-in-time inventory’ where your supply and demand are in sync.”
The Farm Journal Test Plots program in Illinois and Michigan has focused on N availablilty and nutrient management’s 4Rs—right product, rate, timing and placement.
Timing. The goal is to calculate daily N uptake need and match it to the growth stage. Several computer modeling programs do this. Ferrie says many are fairly accurate but need ground-truthing before being used in decisions.
To evaluate daily and cumulative N usage, the Illinois crew pulled soil tests to track nitrates, took tissue samples, documented plant uptake and grew corn hydroponically.
From emergence to V8, plants consume less than 2 lb. of N per acre per day. While N uptake isn’t much while plants are small, don’t underestimate this stage. It impacts overall yield potential, Ferrie notes.
From V10 to V18 (the rapid vegetative growth stage), the plant needs 5 lb. to 10 lb. of N per acre per day. Once it’s reached R1 and is pollinated, daily use backs off to 2 lb. per day.
“When corn hits R1, you still have 60 days of this stage remaining,” Ferrie says. “It’s critical you meet the demand, whether you or the soil supplies N, to reach maximum yield.”
Knowing uptake needs dictates how aggressively you apply N based on growth stage, Ferrie says.
For example, if nitrate samples pulled on May 10 (growth stage matters more than date) range medium to low, you need to react immediately. N usage will soon increase in the rapid vegetative stage. However, the same sample pulled on July 25, and a medium to low result is manageable–the plant only needs 2 lb. per day.
Ultimately, the more N the soil supplies, the less needed to apply. Fields with low-supplying power are more likely to respond to multiple N applications, even later in the season.
Before applying N late in the season, make sure the plant needs it.
For example, in 2015, central Illinois received 16" of rain in June. As a result, later N applications at tasseling created large yield responses because the corn didn’t have enough N to finish the season.
“You have to understand the risk of loss—just because the late-season application resulted in high yield responses in 2015 doesn’t mean the same results the next year,” Ferrie says. “In 2016, an N-friendly year with moderate rainfall in our area, we had consistent nitrate samples at 1' and 2' during the growing season.”
Ferrie and crew pulled weekly nitrate samples. The nitrate samples pulled after the in-season N application was made were higher in the top 1' rather than in the 2' sample. For six weeks, a similar response was observed. Ferrie says this is positive. The applied N stayed in the upper 1' of the soil where it is most accessible.
The week of June 13, heavy N uptake began, and the N at 1' continued to be higher than at 2'. In contrast, for most of the summer in 2015, the 2' samples had higher nitrates than at 1' because the 16" of rain pushed N farther down.
In southern Michigan, Farm Journal Associate Field Agronomist Missy Bauer found similar results. Over a three-year span, on the same irrigated corn-on-corn field, she studied four scenarios to evaluate N timing on the front end versus the back end: 95 lb. N upfront, 90 lb. sidedress; 110 lb. upfront, 75 lb. sidedress; 155 lb. upfront, 30 lb. sidedress; and 185 lb. upfront, 0 lb. sidedress. All treatments had 30 lb. N via fertigation at V10 and again at tassel.
In 2015, a N-deficient year, the corn showed a big yield increase, up to 36 bu. per acre, to the 90 lb. and 75 lb. applied on the back end. In 2014, a similar year, the response was 21 bu. However, in 2016, an N-friendly year, yield responses to the N applied on the back end were minimal.
“In 2016, there was little denitrification or leaching and mineralization was better, meaning it created its own back-end N,” Bauer says. “You have to tweak and tailor your N program on an annual basis based on the present environmental factors.”
Placement. In central Illinois, Ferrie and crew studied another field with four management zones, three soil types and a base N rate of 110 lb. (fall dry fertilizer, spring broadcast and planter applications). The field included three variable-rate N trials: All N sidedressed at V5 with a coulter (trial #1); all N sidedressed at V12 with 360 Y-drop (trial #2); and split N application at V5 and V12 (trial #3). (See chart on page 20.)
Before each application, soil nitrates were pulled to determine rate. At V5 (May 3), nitrate tests indicated the corn needed a variable rate from 90 lb. to 120 lb. In trial #1, a 90 lb. to 120 lb. variable N rate was sidedressed with a coulter. In trial #2, zero N was applied. In trial #3, a variable N rate of 45 lb. to 60 lb., a half rate, was applied.
When the corn reached V12 (June 14), nitrate tests were pulled on trials #2 and #3. In trial #2, the tests showed the corn needed a variable N rate from 120 lb. to 150 lb. Tests in trial #3 indicated the corn needed an additional 60 lb. to 75 lb. variable rate bringing trial #3’s total application to 105 lb. to 135 lb. N.
This plot showed little yield advantage across management zones from a double split N application, which means N was not the limiting factor.
However, in the sandier soils of Michigan, Bauer ran a similar study on three farms and found split N applications paid off (see chart on page 20). There was an average 7.4-bu. increase with split applications (half V3-V5 with a coulter and half V9-V10 with a Y-Drop) versus sidedressing at V3-V5 with a coulter. Delaying the in-season application to V9-V10 with the Y-Drop bumped yield 4 bu. compared with the V3-V5 coulter. When the total N rate was cut by 20 lb. with the split applications, yield dropped by almost 7 bu. per acre, showing N was a yield-limiting factor.
Product. In Illinois, Ferrie compared a 120 lb. preplant broadcast N application with and without an inhibitor (see table on this page). In this area, the natural soil N level ranges from 30 lb. to 60 lb., without any N applied, which is important to know when evaluating the inhibitors’ N rate. Any N above the application rate is from soil mineralization.
“The inhibitor made a difference in total N on May 18,” Ferrie says. “The field lost less N, and we could cut the sidedress rate in this case.”
Rate. To evaluate rate, Ferrie used a field with a base application of 160 lb. with an additional 90 lb. N sidedress (based off soil nitrates). Rate checks included 30 lb. above and below. The original 90 lb. program yielded the best across the field. At 120 lb., yields decreased in all but one management zone. (see chart on page 18)
“At harvest, the corn in every 120 lb. strip was starting to fall down,” Ferrie says. “That’s common—we can make corn fall when we overapply N.”
It’s essential to hit the right rate at the right time. Be careful to not overapply, especially on fields with a high N-supplying power.
In the same field, 26 lb. of sulfur was added to the three rates. (Note: 31 lb. of additional sulfur was applied via fall broadcast.) The purpose was to evaluate sulfur as an inhibitor, not yield response. Tissue samples were pulled in and out of the check to verify adequate sulfur.
At harvest, Ferrie observed the standability issues seen at the 120 lb. rate showed up in the 90 lb. rate. The 60 lb. N application with sulfur performed the same or slightly better than the 90 lb. N without sulfur.
“By adding sulfur we protected the N and made 60 lb. act more like 90 lb.,” Ferrie says.
