2026 April Stewardship Advocate

2026 April Stewardship Advocate

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The article below titled "Residual Soil Nitrogen from Crop Year 2025: How Might It Affect 2026 Nitrogen Needs?" was authored by Amy Cooper, Mitchell Baum, Sotirios Archontoulis, Mike Castellano and Melissa Miller. It was published on March 24, 2026, in an Integrated Crop Management newsletter and distributed by Iowa State University and Outreach.

Residual Soil Nitrogen from Crop Year 2025: How Might It Affect 2026 Nitrogen Needs?

Optimum nitrogen (N) fertilizer rates for corn vary widely from field to field and from year to year. One reason is that the amount of nitrogen left in the soil after harvest is never the same. Measuring residual soil inorganic nitrogen-ammonium (NH₄⁺) and nitrate (NO₃⁻)-can help indicate whether fertilizer needs for the upcoming crop may be higher or lower than usual.

As part of the Iowa Nitrogen Initiative (INI), a partnership between Iowa State University and Iowa farmers, we measured fall soil inorganic N levels in 34 on-farm N fertilizer trials in 2025. Across these fields, the average residual nitrogen in the top 12 inches (30 cm) of soil was 22 lbs. N/acre. This is three times higher than levels measured in 2024, but similar to those observed in 2023 (Figure 1). In addition to higher average levels, we also observed substantial variability among fields. Residual soil N ranged from 5 to 80 lbs. N/acre in the top foot of soil. This range is roughly twice as large as what we observed in 2024, when residual N ranged from 1 to 37 lbs. N/acre.

Figure 1. Residual soil inorganic nitrogen (NH₄⁺ + NO₃⁻) measured in the top 12 inches (30 cm) of soil from Iowa Nitrogen Initiative on-farm trials. Soil samples were collected after harvest and before fall nitrogen application. We sampled 41 fields in 2023, 28 fields in 2024, and 34 fields in 2025.

Why were residual N levels higher in 2025?

Two factors likely contributed to both the higher residual N levels and the greater variability in 2025. First, much of Iowa experienced record-high nitrogen mineralization during the growing season (see: FACTS). In a typical year, soil moisture limits microbial activity and soil nitrogen mineralization during June and July, but in 2025 warm and wet conditions throughout this period allowed more organic nitrogen to be converted into plant-available forms.

Second, optimum fertilizer N rates in 2025 were approximately 24% lower than in 2024. Lower fertilizer needs combined with high mineralization likely also contributed to the higher levels of residual soil N observed after harvest.

Table 1. Results from Iowa Nitrogen Initiative On-Farm Nitrogen Fertilizer Rate Trials 

Across the 34 fields where soil was sampled in 2025 (a subset of the 420 fields in Table 1):

• 60% had less than 20 lbs. N/acre

• 35% had 20-70 lbs. N/acre

• 5% had more than 70 lbs. N/acre 

For reference, 20 lbs. N/acre is the lowest residual soil nitrogen level used in the N-FACT decision support system. In conventional soil test terms, such as the Late Spring Nitrate Test (LSNT), 20 lbs. N/acre of nitrate-N in the top 12 inches corresponds to roughly 6 ppm NO₃-N.

Both Iowa Nitrogen Initiative field data and N-FACT simulations show that residual soil nitrogen strongly influences optimum fertilizer N rates. Fields with higher residual soil N typically require lower fertilizer N rates to achieve maximum yield. 

What does this mean for 2026?

Given the recent increase in nitrogen prices with already tight margins, we encourage farmers and agronomists to consider measuring residual soil inorganic N to help refine nitrogen rate decisions going into the 2026 crop year. A general protocol for nitrate sampling is available in the publication "Use of the Late-Spring Nitrate Test in Iowa Corn Production." These measurements can be incorporated into the N-FACT tool to estimate how existing soil nitrogen may affect fertilizer requirements for the upcoming season.

Note: Historically, nitrogen rate recommendations were calculated as 1.0-1.2 pounds of nitrogen per bushel of corn. More recently, modern corn hybrids are more efficient in extracting nitrogen from applied fertilizer and nitrogen made available through mineralization. The fourth column of Table 1 shows that significantly less nitrogen is needed to produce a bushel of corn than in the past.

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In a World Where All Corn & Soybean Acres Are Strip-Tilled …

Caleb Hamer farms in Black Hawk and Tama counties with two business partners and his dad Ted on a diversified corn, soybean and seed operation. Caleb came back to the farm in 2008 to farm full-time. Caleb and a business partner started B & H Ag which is a custom operation business. Caleb says, "B & H Ag became our vehicle of succession planning, through it we bought out my family's operating unit, Hamer Farms Inc, and eventually brought my business partner's family operation into the business as well."

Legacy is important to Caleb as he thinks about the future, saying, "While we do not put the expectation on any of our children to farm, we wish to make it a viable option to have room for them to step into that role one day if they choose. This means maintaining now and growing when and where it makes sense."

Caleb plants 105-117 day corn at a population of 34,000-36,000 seeds per acre. They variable-rate their nitrogen, phosphorus and potassium and split apply nitrogen on half of their corn acres. In the fall of 2025, Caleb began working with a new crop fertility company and went to variable-rate applying all his macronutrients. By going to variable-rate applications, Caleb says, "We have been able to reduce use of synthetic phosphorus and sourced more manure sources for our crops' fertility needs."

Caleb's tillage system varies according to the crops and the fields. They use a two-pass tillage approach on their continuous corn. On other fields, Caleb notes, "We have a few farms in continuous no-till, but for the most part, we no-till soybeans into corn stalks and make a single field cultivator pass in front of the corn planter on soybean stubble."

As a seed grower, cover crops are an important component of Caleb seed operation. He says, "We use cover crops on our seed corn rotation farms. We have found cereal rye mixed with potash spun on behind the seed corn harvest and vertical tilled in gives us our best and most consistent stand establishment. We then will no-till soybeans into the rye and use herbicides to terminate (see figure 3). We also have tinkered with aerial seeding and different species mixes and have had mixed results. Dry falls have made cover stand establishment especially challenging."

Water quality and sustainable production is important to Caleb as demonstrated by his participation in a Middle Cedar River watershed project which will include the installation of a saturated buffer and other water quality monitoring and control structures.

When asked about what he wants other farmers to know about his operation, Caleb says, "We don't have all the answers, but we hope to partner with people who can help us find them."

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