Tag: Nitrogen

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!