Miller updates Ag Week visitors on soil fertility, cover crops, precision ag

Jarrod Miller, Extension agronomist, has used drones in several aspects of his research, including comparing crop nutrient sensing tools on a drone versus pulled by a tractor. (Photo courtesy Delaware Cooperative Extension)

HARRINGTON, Del. — At the agronomy session held during Delaware Ag Week last month, Dr. Jarrod Miller, Extension agronomist at the University of Delaware, gave a report on recent research on soil fertility, cover crops and precision agriculture.
In a study involving corn ear leaf tissue testing, samples were taken from several sites, including Marydel and Georgetown in Delaware, and at five of the University of Maryland’s Extension research centers. Research was sponsored by the Maryland Grain Producers Associaiton.
“There are (nutrient) values — critical values — that if you drop below that level, you’ve lost 10 percent of yield,” Miller said.
For example, the nitrogen level at tasseling should be 3 percent. Miller said farmers should follow instructions for when to sample and where on the plant to take the sample.
Showing slides of yields in the various locations, Miller said late maturity hybrids had the highest yields in 2019, while there were no differences between early, middle and late hybrids in 2018.
Marydel, with 255 bushels per acre, and Clarksville, with 251 bushels per acre, had the highest yields. Marydel had the highest yield in 2018 also.
Plots in Salisbury, where the yield was 199 bushels per acre, were irrigated. At nearby Poplar Hill, which is not irrigated, the yield was 150 bushels per acre.
Poplar Hill also had the lowest organic matter, cation-exchange capacity, calcium, manganese and other micronutrients. Marydel had the highest soil phosphorus, magnesium, sulfur, zinc, copper and iron. Clarksville had the highest soil potassium levels.
“Nutrients in the soil are not all the answer. Soil fertility is only part of the equation. Soil type also matters,” Miller continued.
Of the critical nutrients tested in 2019:
• For nitrogen, 26 of 42 samples were above 3 percent in the ear leaf. In 2018, only 2 of 32 samples were above 3 percent. All Salisbury samples in 2019 were below 3 percent.
• For phosphorus, 41 of 42 samples were above a critical value of 0.25 percent. In 2018, 31 of 32 samples were above.
• For potassium, 39 of 42 samples were above the 2 percent critical value. In 2018, only 23 of 32 were above the critical value.
“In 2018, fields were swamps. We had too much rain, and nitrogen leached out,” Miller said.
He told farmers, “If you’re not sampling, you don’t know when nitrogen is not getting into the plant.”
Most samples were above the critical value for phosphorus, also for potassium, but fewer samples were taken in 2018.
Marydel had highest nitrogen content and the highest yield.
Miller said nitrogen was put out on all the sites when needed, but some still had low yields. He added that 45 percent of the yield can be attributed to nitrogen.
“Correlation does not mean causation,” he said. “It’s not just the soil levels (that matters) but the amount taken up.”
Marydel, for example, did not have the highest soil potassium levels, but did have greater yields and tissue potassium level.
As magnesium levels increased, yields decreased. Magnesium does not necessarily contribute to yield, Miller said.
Almost all the studies were meeting critical value.
With sulfur, all samples were above critical thresholds. “We did see strong positive trends above critical value. The trend says keep sulfur in the soil,” Miller said.
Results for micronutrients were all over the place. Only boron did not reach critical value in any of the samples.
In 2018 there was an excellent trend with boron: as boron increased in tissues, yields increased. “But in 2019, we saw the opposite results,” he said. “Poplar Hill had the highest boron, but the lowest yield.”
Micronutrient starter was applied at two different rates, but did not increase yield. No effect was seen adding starter.
Yields were more related to how wet the year was, Miller said. By University of Delaware recommendations, none of the soils were lacking in any nutrient.
Delaware Soybean Board paid for similar research in soybean fields. During ag week, soil and issue tests still had to be analyzed.
Results were similar, but plot variability may have masked differences, Miller said.
Switching to wheat, Miller gave seeding rate recommendations of four states:
• Penn State plants 1.5 to 1.7 million seeds per acre to achieve 1.3 to 1.5 million plants per acre.
• Michigan State plants 1.2 to 2.2 million seeds, recommending an increase in the amount the later you plant.
• Nebraska plants 600,000 to 2.5 million seeds.
• North Carolina State plants 1.1 to 1.5 million seeds per acre, to produce similar yields.
Miller said Delaware follows what others are seeing, but maybe a little higher. “If you plant less than 1.5 million, you get less yield. We recommend planting more than 1.5 million and less than 2 million seeds per acre.”
He then asked, “Has anyone (done a) drone tiller count? Can we use drones to do the count? Counting tillers in spring helps determine split nitrogen applications.”
Another study compared drone measurements of NDVI (normalized difference vegetation index) to that of a Greenseeker crop sensor, which can detect areas of higher plant mass.
“Can a drone give NDVI equivalent to Greenseeker?” Miller asked. “It gives a very good correlation; however, a drone is consistently higher in the air, so high up that it doesn’t see all the soil that a tractor sees.”
The rise and fall are about the same rate, Miller said.
This means it is possible to take a drone photo and create an application (with specialized software) that shows the high and low parts of field for nitrogen.
From there, the data would inform variable nitrogen application.
“Drone results correlate better to yield, but not by much,” he continued. “A drone does find the poor spots.”
He added, “A drone gives us quick evidence of what we know happens. Now we have the numbers to prove it.”
Miller then turned his discussion to cover crops and studies funded by USDA’s Natural Resources Conservation Service in Delaware.
For one study on planting green into living cover crops a farmer agreed to burn down a couple of strips in 2018, but that happened to be a low area. The grower planted corn into a vetch rye mix, then it rained.
Where the crop had been burned down early, there was a better stand, but moisture skewed the test. Miller urged farmers to check seed to soil contact repeatedly during planting.
A student helped with a study on cover crops and nitrogen cycling. Four treatments were tried with two crops, corn and soybeans. There were three questions: How much nitrogen will clover or vetch add to the soil when mixed with rye? How does later termination increase the plant-available nitrogen? Does later termination decrease emergence of corn and soybeans?
With corn in Georgetown, in 2019, there was no effect of any of the cover crops on yield. Later termination decreased yield by 16 bushels.
With the Middletown corn crop in 2019, the study showed any cover crop is better than none, by 12 to 15 bushels. Later termination increased yield, but not by numbers that were statistically significant.
With soybean crops in 2019 at the Georgetown site, vetch had a 17-bushel advantage over no cover crop or rye cover crops. In Middletown, a clover mix or rye alone increased yields by 5 bushels. There was no termination effect in either location. The study will be repeated this year and in 2021.