Tag: Nitrogen

When it Comes to Nitrogen Leaching, Not All Cover Crop Practices Are the Same

Ian Goralczyk, Nathan Sedghi, and Ray Weil
University of Maryland, Department of Environmental Science & Technology

Cover crops are subsidized by taxpayers for use on more than 600,000 acres of agricultural fields in Maryland as part of an initiative to protect water quality and the Chesapeake Bay. As cover crops grow and take up nutrients, the water leaching from fields is cleaned up, especially with regard to nitrogen. However, the way that cover crops are typically managed may not be optimal for improving water quality. The Weil lab’s previous work has shown that the effectiveness of cover crops in reducing N leaching during the winter is dramatically affected by how early the cover crops are established, with cover crops planted in mid-October having little impact on N leaching compared to those planted a month earlier. The challenge is to find ways of getting cover crops established in early September, a time frame usually not possible with the typical practice of drilling cover crop seed after harvesting the corn or soybean cash crop. For this reason we studied a mixed species cover crop (radish, rye, and crimson clover) that was interseeded into standing soybeans canopies as compared to the standard practice of post-harvest drilling, and a no-cover crop control. We conducted the replicated experiment on two coastal plain fields with soils of contrasting textures formed in silty/clayey sediments, and in sandy sediments.

igure 1. Undergraduate researcher in the Weil Lab, Ian Goralczyk, installing a suction lysimeter for collecting soil porewater samples.
Figure 1. Undergraduate researcher in the Weil Lab, Ian Goralczyk, installing a suction lysimeter for collecting soil porewater samples.

This experiment was established at the Beltsville Facility of the Central Maryland Research and Education Center, with funding from Shore Rivers, LLC and the Maryland Soybean Board. The early planted cover was planted by broadcasting seed into a standing soybean canopy at leaf yellowing using a hiboy air-seeder on September 11, 2017. In each field, suction lysimeters were installed (Figure 1) to one-meter depth and samples were collected using a 85 kPa vacuum approximately every two weeks between December 17, 2017 and May 7, 2018. Soil pore water samples were filtered to remove particulate matter and frozen until they were analyzed for NO3-N and NH3-N on a LaChat® Flow Injection Analyzer.

One field had a silt loam surface texture and a clay loam subsoil (Russet-Christiana Complex). The other field had a loamy sand surface texture and sandy loam subsoil (Evesboro-Downer Complex). By utilizing fields of contrasting soil textural classes we can determine the effectiveness of these cover cropping methods with a range of soil conditions in order to broaden the scope of this study.

Cover crop use made a major difference in nitrate concentrations measured in the porewater collected at 1 m depth (Figure 2). Nitrate concentrations were reduced most where cover crops were established the earliest. As expected, the nitrate concentrations in the leaching water, as well as the impact of early cover crop establishment, were greatest on the sandy soil site.

While there were some individual samples that exceeded the EPA safe drinking water standard for nitrate-N (10.0 ppm), the average of all individual treatments was below this standard, and nitrate concentrations were consistently lower for the early interseeded cover crop treatment. A major reason why lower nitrate concentrations at one meter depth were observed for cover cropped plots is that the nitrate was taken up by cover crops roots and largely translocated to the aboveground plant tissue. This process captures the N before it leaves the potential rooting zone and recycles it to the surface soil where it may be released for use by future crops. This release could lead to decreased need for fertilizer nitrogen application to the following corn crop. Our data suggest that if similar cover crop interseeding practices (using aerial or ground-based methods) were applied on a large scale on commercial farms, the reduction in nitrogen loading to the Chesapeake Bay could be substantial. We can also conclude that early-planted cover crops are effective for reducing nitrate leaching on soils with a range of textural classes.

While these results are promising, it is important to note that they represent only one year out of a three year project, and that more data will be collected on different fields and with different cover cropping methods. We hope to provide farmers with guidance on optimizing cover crop species mixtures, planting dates and methods in order to enhance the impact of cover crops on nitrogen pollution while also improving soil health and farm profitability.

Figure 2. Nitrate-N concentrations in porewater from 1 m depth in fields of contrasting soil texture. Average of all sample dates during the 2017-18 winter-spring leaching season (N=33). Error bars are one standard error.
Figure 2. Nitrate-N concentrations in porewater from 1 m depth in fields of contrasting soil texture. Average of all sample dates during the 2017-18 winter-spring leaching season (N=33). Error bars are one standard error.

Fixing Soybean’s Need For Nitrogen

Article from the American Society of Agronomy

Soybean is rich in protein, which is great for the humans and animals eating it. But this high protein content comes at a cost.

To make protein, soybean plants need a lot of nitrogen. The plants get some of the nitrogen they need by working with specialized bacteria in the soil. These bacteria live in root nodules. They pull nitrogen from the atmosphere and convert it to a form the plants can use.

soybean roots with nodules
A soybean root with nodules. These nodules house bacteria that “fix,” or extract, nitrogen from the atmosphere for the plant’s use. Photo credit Luiz G. Moretti.

But this process–biological nitrogen fixation–may not provide all the nitrogen soybean crops need. Farmers may have to apply nitrogen fertilizer as well.

A new study, however, shows it’s possible to increase the number of soybean root nodules—and the bacteria that live there–to increase crop yields. This could remove the need to apply additional nitrogen fertilizers.

“That opens the possibility of achieving higher yields of soybean based exclusively on biological fixation,” says Mariangela Hungria, a researcher at Embrapa Soja, Brazil.

Hungria, lead author of the study, and her colleagues coated soybean seeds with the bacteria (the usual method used by growers). They supplied additional bacteria by spraying it on the plants during other stages of growth. Soybean plants that received the additional spray inoculation developed more root nodules. And more nodules led to higher yields.

In fact, adding bacteria to seeds increased yields by 27% and 28%. Spraying bacteria on the soy fields during growth pushed up yields even further.

The increase in root nodules after additional spray inoculation surprised Hungria and her colleagues. Previous research indicated that each nodule makes it more difficult for soybean plants to develop subsequent ones. But in this study, soybean plants were able to form new nodules when researchers provided more bacteria.

“To discover that nodules aren’t regulated as strictly as previously thought is an important finding,” says Hungria. “The limitation happens particularly at the beginning of soybean growth when the first nodules appear.” After that initial stage, more nodule growth is possible.

More biological nitrogen fixation, and less nitrogen through fertilizer, can also increase sustainability. First, it reduces carbon emissions. Nitrogen fertilizers are usually produced using fossil fuels. “For every pound of nitrogen fertilizer manufactured, at least 10 pounds of carbon dioxide may be released,” Hungria states.

The second improvement in sustainability is on the field. Excess nitrogen fertilizers from the field can flow into bodies of water. Too much in an aquatic ecosystem can cause algal blooms. These deplete the water of oxygen and lead to “dead zones” devoid of life. Biological fixation using bacteria, however, means more of the nitrogen is used by the crop.

Less fertilizer use also has an economic impact. Nitrogen fertilizer costs can add up quickly, both for farmers and for countries. Brazil imports about 70% of the nitrogen fertilizers used in the country.

Several farms in Brazil began using the study’s strategy in October 2016 (the summer crop in Brazil). Initial results have been promising, says Hungria. The higher soybean yields seen in the study are sustained on these larger scales.

Hungria thinks these results will extend beyond Brazil as well. “But they have to be verified because the genetic background of soybean is different in each country,” she says. Collaborations with Kansas State University, to verify if the results can be extended to the U.S., have just started.

Inoculating soybeans with rhizobia
Inoculating–adding helpful bacteria to soybean seeds–usually occurs at sowing time. However, in this study, soybean crops at various stages of growth were also inoculated by spraying the plants with bacteria. Photo credit Luiz G. Moretti.

Researching bacteria and nitrogen fixation may just be the beginning. “I think microorganisms can be the ‘stars’ of a new era of agriculture, in which we consider not only food security but also sustainability,” she says.

Read more about Hungria’s research in Agronomy Journal. The research in Brazil was funded by Universidade Estadual Paulista, Fundação Agrisus, Embrapa, and Total Biotecnologia.

 

Using the PSNT and Tissue Testing for Nitrogen

Kelly Nichols, Agriculture Agent Associate
University of Maryland Extension, Frederick County
kellyn@umd.edu

With all of the rain that we have had earlier this spring in some areas, nitrogen deficiency is likely to show up, if it hasn’t already. Nitrate (NO3–) is the main form of nitrogen that plants take up. When soils are saturated, the bacteria in the soil do not have access to oxygen in the air, so they use the oxygen in nitrate instead. During this process, called denitrification, nitrate is converted to a gaseous form of nitrogen. The nitrogen gas then is lost to the atmosphere and is no longer available to plants.

The symptoms of nitrogen deficiency will show up on the lowest leaves of the plant first. Nitrogen is mobile in the plant, so if it runs out of nitrogen, it will pull nitrogen from the lower leaves and send it to the newer leaves. The nitrogen-deficient leaves will be yellow; this yellowing will start at the tip of the leaves and move along the middle of the leaf.

If your plants are starting to show nitrogen deficiency, don’t panic! There is still time to correct this by adding additional nitrogen. There are two tools that farmers can use to determine if and how much additional nitrogen should be added.

The first tool is a tissue test. This test can be used for not only nitrogen deficiencies, but also to detect other nutrient deficiencies. A majority of the soil testing labs in the region also offer plant tissue analysis. After choosing which lab you will send your samples to, make sure you find the tissue sampling instructions on the lab’s website. Samples will need to be taken from a specific part of the plant at a specific growth stage of the plant. The lab has calibrated their tests to those plant parts and growth stages; a different method of sampling will result in a useless report.

Take two samples – one from the area that is deficient, and one from an area that is not deficient. Both areas should have similar management practices. Once you have the results, you can compare the two areas and determine practices to mitigate any nutrient deficiencies. If additional fertilizer is needed and you have a nutrient management plan, work with your plan writer to ensure that you are in compliance with applying the additional fertilizer.

The second tool is the Pre-Sidedress Nitrogen Test (PSNT). This test is offered by University of Maryland Extension Nutrient Management Advisors and can be used on corn. The PSNT is a soil test that can be taken when the corn is 6-12 inches tall. This is just prior to when the corn is about to rapidly take up nitrogen. The results of the PSNT will tell if you there is enough nitrogen in the soil to get the corn through its next growth spurt.

The PSNT can be used in fields that have a previous crop of a legume and/or have had manure or biosolids applied. The PSNT cannot be used in fields that have received more than 50 lbs/A of commercial fertilizer nitrogen, or in fields that have a history of receiving commercial fertilizer as the only nutrient source.

Contact your county’s nutrient management advisor if you would like to schedule an appointment for a PSNT. Below are the steps for taking a soil sample in order to prepare for a PSNT. Remember that the soil sample needs to be taken when the corn is 6-12 inches tall.

Soil samples should be taken 12 inches deep (or as deep as you can get them) midway between the rows. Thirty to forty cores should be taken. Put the samples in a bucket, break up any clumps of soil, remove any rocks or other foreign material, and mix the soil. Spread the soil out on newspaper to dry. The samples should be dried immediately after they are taken.

A few soil testing labs around the region also offer the PSNT. If you choose to use one of these labs, check their sampling procedure on when and how to take samples, as it may differ. Also check with your nutrient management plan writer once you have the results to ensure that you are in compliance with your nutrient management plan if you sidedress nitrogen.

One last note – there are a couple of other nutrient deficiencies that have symptoms similar to nitrogen deficiency. Potassium deficiency looks like a yellowing or burning also on the lower leaves. However, it will appear on the edges of the leaves (as opposed to nitrogen which appears in the middle of the leaves). Sulfur deficiency also results in yellow leaves. However, sulfur is not a mobile nutrient in the plant. When the plant runs out of sulfur, the top leaves will become yellow. (Nitrogen deficiency starts in the bottom leaves.)

 

Nitrogen Loss After Heavy Rainfall

By Dr. Gurpal Toor, Associate Professor of Nutrient Management and Water Quality; and Dr. Bob Kratochvil, Extension Agronomist
University of Maryland, College Park

INTRODUCTION: The last few days have kept many of us indoors at the expense of promise of springtime weather! However, there is nothing unusual in these weather patterns; rather this is another reminder about the uncertainty and unpredictability of weather. As the below map shows, in one week (May 12–19), various counties in Maryland received from 3 to 10 inches of rainfall. Several counties received several inches of rainfall in a day. This excess rainfall has not only affected the timely planting of corn but has the potential to affect the corn that is already planted.  

IMPACT: The biggest impact of this rain will be in the fields that received all nitrogen fertilizer prior to or at planting. This is a good time to revisit the implications of adding all nitrogen fertilizers at pre-plant and why applying all nitrogen in one-time application is NOT A GOOD IDEA. Plants are like us who need food in the form of nutrients several times a day. So, why do we apply all nitrogen fertilizer at pre-plant? It’s convenient for sure, but not the best farming decision in terms of plant needs. This is why University of Maryland Extension does not recommend applying all nitrogen fertilizers prior to or at planting. Consider that early in the season, corn needs ~1 pound of nitrogen per day per acre, which during later vegetative stages increases to ~3 pounds per day per acre. If you applied 200 pounds of nitrogen prior to or at planting, the corn may use only about 30 pounds in the first 30 days. You are hoping the rest of the nitrogen (200–30 =170 pounds) will remain in the soil for use by the plants later in the season. However, if weather is not favorable, like what occurred during May 12–19, some of this nitrogen will be lost via three ways, as shown below in the image.

There is no way to know how much nitrogen was lost from fields as this requires precise measurement because it will vary depending upon soil type, location, and amount of rainfall received. However, some general conclusions can be drawn that some nitrogen was lost via leaching, surface runoff, and gas loss. Consider that 1 inch of rain adds 27,154 gallons of water to an acre and weighs about 113 tons. This 1 inch of rain will penetrate 6 to 15 inches in soil (depending on soil texture). This rain water will flush (leach) the soluble and highly mobile nitrate form of nitrogen (most common form of nitrogen) deeper into the soil profile. Several inches of rain like we received in May will leach nitrate below the plant roots. Another way nitrate will be lost is via surface runoff from the fields. If that’s not enough, ponded water in the fields will convert nitrate to nitrogen gas, which will escape to the atmosphere.

RECOMMEDATIONS: As the saying goes “you can’t change the past” but you can take steps to minimize nitrogen losses in future years. First and foremost–do not apply all of nitrogen fertilizers prior to or at planting. This is because there is higher risk of nitrogen loss in spring as uptake of nitrogen by corn is minimal until approximately 35 days after emergence. At that point, corn enters into rapid growth phase and nitrogen uptake occurs. Thus, apply a small amount (~20–30 percent prior to or at planting) and add the remainder (~70–80 percent) as a sidedress when the corn is 10 to 20 inches tall. In the fields where all of nitrogen fertilizer was applied at pre-plant, consider doing a pre sidedress nitrate test (PSNT) to determine if there is a need to add more fertilizer. Contact Maryland Department of Agriculture to assure that this application will be in compliance with your nutrient management plans. Kudos to those farmers who only applied a part of nitrogen fertilizer at pre-plant and are planning to apply the remainder as sidedress; you saved yourself some fertilizer and money that otherwise would have been lost!