Deciding or Choosing a Nitrogen Rate

As we are in or just getting done with many hunting seasons, asking an Agronomist to pick a Nitrogen (N) rate for your corn crop is akin to a 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 ask several questions. It’s often said that the number of questions I might ask is comparable to the number of inquiries new parents make when consulting a physician about their child.

Before suggesting an N rate, I have considered the following background information and questions:

Cropping History: How many years has it been since corn was last planted in this field? Is it a corn-on-corn rotation, or has it been wheat? How long has it been since the field was planted with alfalfa, soybeans, or other legumes? These questions are to find out if we have the potential for any residual N from legumes. Or do 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 in time they 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 the 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 have had standing water in areas, odds are we will have Nitrogen and 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 will be from 65°F to 85°F and we will receive a slow gentle one-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, 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 higher OM percentage will release N as it starts to break down. The rule of thumb is 20 units of N for every 1 percent of OM. So, a 5 percent 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 low CEC additional applications, 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 loss 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 200 PPM 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 of reduced amounts of N applied.

Sulfur (S): Sulfur is much like Nitrogen in that it is a mobile nutrient which can be leached through the soil. Sulfur “helps” the Nitrogen find its pIace 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 15 percent 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 in 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 farther away the date of Nitrogen application was from 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 idea is that as the number of guests at a banquet increases, the food supply must also grow to ensure everyone is adequately fed. In this instance, this may or may not be a true statement. The amount of soil that a corn plant’s 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 that in low OM soils and/or sandy or lighter soil types, nitrogen may already be depleted at lower population levels. With that, we may need to increase our rate of N. Additionally, we may need to increase or 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 so the plant is not deficient. Alternatively, if a deficiency does occur, it happens later in the plant's life cycle, closer to the R6 physiological maturity stage (black layer).

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 more defensive can yield well if available N is lacking later in its life cycle. Another consideration is that for the insect traits to be fully effective in the plant and its roots, the plant must not suffer from nutrient deficiencies. This is especially critical when it comes to nitrogen as N is the building block for the proteins that make up traits.

As highlighted in this article, along with other potential unanswered questions, determining the appropriate nitrogen rate for your corn crop can be a bit more complex than it might initially seem.

If you have questions about this or other Agronomy topics contact your Dairyland Seed Agronomist or DSM.