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Thinking of Top Dressing Nitrogen? Read This First

High fertilizer costs, dry field conditions, or excess soil moisture: how do you effectively get nitrogen (N) in place in-crop? Areas that were dry last year with poorer crop yields have the potential for high residual rates of N. The dry conditions this spring caused some growers to reduce rates of fertilizer to minimize the risk of seed damage and manage risk on inputs. On the other hand, if you farm in an area with saturated soils, you may have cut rates to minimize nutrient runoff and help plant establishment or used a broadcast seeding approach to get the crop in with the intent of applying N later. In many situations, the conversation now leads to an N top-up due to the recent improvement in growing conditions.

Case Study on Residual N #

John Heard, Soil Fertility Extension Specialist with Manitoba Agriculture, took soil samples to measure soil nitrogen changes from fall of 2021 to spring of 2022. Clay soils in Manitoba lost 30 lb/ac of N, whereas sandy soils lost upwards of 65 lb/ac of N due to leaching this spring.

Soil sampling well after crop emergence might not provide much value other than for corn crops, where recommendations for spoon-feeding the crop can be made from the results.  However, for crops that are just emerging, and where you are uncertain about how much residual N was carried over from last year’s crop then soil testing can be useful. Particularly if you think N moved down the soil profile and you need to bridge a deficiency with a top-up of N..  Another practice to determine if crops are needing a top-up of nitrogen is to scout. While checking fields, look for lower leaves that are turning yellow as this is often a visual sign of N deficiency.

Canola #

Canola takes up its highest rates of N during flowering through to the end pod formation. In-crop applications of nitrogen should occur before the 4- to 6-leaf stage to allow the fertilizer to move into the root zone. The Canola Council of Canada (CCC) mentions that the most common methods are broadcast spreading of urea (46-0-0) or surface dribble-banding UAN (28-0-0). 

Corn #

For corn, nitrogen can be banded between the rows to get the product as close to the root zone as possible.  Nitrogen top-dressing in corn is most effective between the V3-V7 growth stages prior to V8 where the highest rates of nitrogen are then taken up. It’s important to match nitrogen rates to yield potential and field conditions, as excessive nitrogen can cause reduced yield and increase moisture levels in the stalk at harvest.

Enhanced Efficiency Fertilizers and Additives #

Ray Dowbenko, a crop nutrition consultant in Calgary was asked about using specific fertilizer technologies and what value they bring to those broadcasting N in-crop. “Polymer coated urea is probably something you should not consider broadcasting in our shorter prairie growing season-.  The nitrogen release profile of a product like ESN for instance means that much of the post plant applied nitrogen will not be released from that granule until later in the growing season and well after the crop’s peak N demand.  However, if you are out broadcasting urea or UAN, you would likely benefit from using a urease-inhibitor. These products slow the conversion of urea to ammonium and allow more time for rain to fall before N losses from volatilization start occurring.” This highlights an important factor whether you’re broadcasting urea or dribble banding UAN, keep an eye on the weather forecast. Rainfall is needed to move the N down into the soil – without rain most of that N won’t be plant accessible. 

Dowbenko continues, “There are plenty of urease inhibitor products on the market for growers to consider but make sure you know what you’re buying – levels of active ingredient and costs vary so it’s important to know what you’re paying for.”. 

Do 4R Principles Still Apply when Plans Change? #

Your fertilizer plans may have changed but following 4R nutrient stewardship guidelines – questions around right rate, right source, right time, and right place still present strategies that will help your in-crop N applications be more effective.  Fertilize to your crop’s yield potential and field conditions. Later seeding of some crops has reduced the yield potential and with that a recommendation for lower N rates to achieve target yield. With the current high cost of N, you may also want to consider a urease inhibitor, and whenever possible, time N top-dressing operations when rain is imminent or with a forecast that is calling for rainfall.   

There are risks and rewards that come with additional N applications via top-dressing.  Contact your fertilizer retailer, crop consultant, or provincial extension specialist for further information on this topic.

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