Deciding or Picking a Nitrogen Rate

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By Dairyland Seed Agronomy Team

As harvest is starting to wind down, and hunting seasons are in full swing in some areas, asking an agronomist to pick a Nitrogen (N) rate for your corn crop is akin to asking a hunter what his or her favorite weapon to hunt with is. The answer in both instances is probably “well it depends.”

As an agronomist before recommending a Nitrogen rate, I typically will ask a few questions. It has been suggested that the number of potential questions that can be asked before suggesting an N rate is in close proximity to the number of inquiries new parents ask a physician about their child.

Some of the background information or questions that I have asked prior to suggesting an N rate are:

Cropping History: How many years has it been since you planted corn on this field, or is it corn-on-corn or wheat? How long since it has been in alfalfa, soybeans, or other legumes? These questions are to find out if we have the potential for any residual N from legumes or if we need to apply additional N to assist in breaking down the stalks, leaves, stems of a grass crop. Cover crops also come into play as they are terminated as they, in time, release nutrients back to the soil.

Manure History: Anytime animal manure is applied to a field you are adding to one degree or another Organic Matter (OM), bacteria, and biological products. This is in addition to N,P,K,S, Zn, and other nutrients that come with manure. The more recent the manure application, the more apt you are to receive a larger benefit of that manure.

Weather Conditions from Previous Year(s): If conditions were hot and dry in the previous growing season(s) we may have Nitrogen as well as other nutrients available from Organic Matter breaking down and releasing it. If we have additional Nitrogen available from OM breaking down that can be utilized, we potentially need to apply less N. Conversely, if the previous year(s) have been wet and we had standing water in areas, odds are we will have Nitrogen in the form of Nitrates that have been moved or leached through the soil profile. This leaching will increase the amount of N that will need to be applied.

Weather Conditions Present/Anticipated: If our crystal ball tells us that every day during the growing season temperatures will be from 65°F to 85°F and we will receive a slow gentle 1 inch soaking rain every 5-7 days, the need to apply additional N in season is diminished.

Soil Nitrate Test: If the field has a history of manure or warm dry weather the previous growing season, we increase the likelihood of having soil nitrates available in the soil. By taking a soil Nitrate test you find out what the levels are and can potentially reduce the amount of N applied.

Soil Type/Texture: Soil types that are sandy will not “hold” Nitrogen and smaller and more frequent amounts work the best. Soil types that have a high clay content will “hold” more Nitrogen but will be less apt to give it up and more prone to leaching if water starts to pond.

Organic Matter (OM): Soils that have a higher OM percentage will release N as it starts to break down. The rule of thumb is 20 units of N for every 1% of OM. So, a 5% OM soil would have 100 units of Nitrogen available during the growing season. In my experience the N released from OM is released later in the growing season.

Cation Exchange Capacity (CEC): The higher the CEC, the higher the amount of N the soil can hold on to. Typically, for every 1 unit of CEC you can hold 10 units of Nitrogen. A soil with a CEC of 18 would hold 180 units of N, and amounts greater than that would not be held. In soils with a low CEC, multiple applications with reduced amounts of N are suggested so the soil can hold onto it.

Soil pH: In my experience, as soil pH become more acidic and below 6.0 we start to lose N efficiency and will need higher amounts of Nitrogen to obtain the same yield, compared to a soil with a more neutral soil pH closer to 6.8 to 7.0.

Soil Potassium (K): Historically, as our K numbers get closer or above 200PPM and/or as our base saturations K numbers are at or above 2.5 (I use 3.0) we gain Nitrogen efficiency. In these instances, we can see greater yield gains with the same or reduced amounts of N applied.

Sulfur (S): Sulfur is much like Nitrogen in that it is a mobile nutrient which can be leached or moved through the soil profile. Sulfur “helps” the Nitrogen find the place in the plant that it needs to be. In theory, every time an N product is applied, S should be applied with it as well. When utilizing an in season N application, I recommend that 10 to 20% of that application be S.

Soil Drainage: As we have seen in years with heavy and frequent rainfall, if we have too much water standing or flooding our fields or portions of fields those areas are more apt to be N deficient. This would be mainly due to leaching through the soil or denitrification caused by the lack of Oxygen (O).

Nitrogen Product and Date of Application: Was the Nitrogen product broadcast onto the soil or was it banded into the soil? Was a nitrogen inhibitor used during the application? The greater the time frame from the date of Nitrogen application to the planting date and growing season, the greater the chance of losing the N to leaching or denitrification.

Number of Nitrogen/Nutrient Applications: By applying all your Nitrogen at one time, you increase the likelihood of losing that N to leaching or denitrification. By putting the same total amount of N on over the growing season but in multiple applications, you can gain Nitrogen efficiency and/or increase yield.

Plant Population/Row Width: As plant populations increase and or row widths decrease, in theory, we may need to increase the amount of N applied. The thought behind it is, as we increase the number of guests at a banquet, we need to increase the supply of food, so all our guests are fed. In practicality this may or may not be a true statement in this instance. The amount of soil that a corn plants root system explores is not a high percentage, so as we add 500-1500 plants/seeds or decrease row widths, in theory, if your N rate was high enough at a lower plant population it should be higher at that higher rate. A caveat to this is, in a low OM soil and or Sandy or lighter soil types you may be running out of N at the lower population already. With that, we may need to increase or more than likely increase and then split apply our N so it is more available later in the plant’s life cycle. Or, in the instance we have a high yielding crop, and we are becoming N deficient early in the plant’s life cycle, we again may need to apply more N or split apply it.

Hybrid/Trait: We may all have corn hybrids that for a lack of better terms are “Nitrogen Hogs” and need another shot of N to reach their full yield potential. Or, on the flip, side corn hybrids that are pretty defensive and can yield well if available N is lacking later in its life cycle. Another aspect is that for our insect traits to be fully active in the plant and roots the plant cannot be nutrient deficient. This is especially critical when it comes to nitrogen as N is the building blocks for the proteins that make up traits.

As can be gleaned from this article as well additional unasked questions, the rate of Nitrogen being suggested for your corn crop can be slightly more complex than what one might think. As always if you have questions about this or other agronomy topics contact your Dairyland Seed Agronomist or DSM.