As fertilizer prices climb, particularly nitrogen and phosphorus, the economic benefits gained from manure-applied nutrients escalate. The ability to reduce fertilizer inputs to the crop enterprise provides an incentive to move manure further and invest in equipment to complete the task in new application windows. Moving manure to new fields also adds micronutrients and organic matter, which improves soil health.

While we know the value that phosphorus and potassium from manure have for soil nutrient management, it is the ability to capture nitrogen to support crop needs that pays the bills for transport and application equipment.

A worthy substitute

Manure-sourced phosphorus and potassium is a pound-for-pound equivalent to commercial fertilizer, and we apply it based on soil test levels. We can target higher application rates (within environmental restrictions) to build low soil test fields or use crop removal rates to maintain soil nutrient concentrations.

One knock on manure is the inconsistency of nutrient levels during application. We can manage rate inconsistency so that crop yield is not at risk by using grid or zone soil sampling strategies and a sampling interval of at least once per rotation (every two to four years).

The ability to utilize manure nitrogen to replace commercial fertilizer is the most economically beneficial use of manure nutrients. The Ohio State University Extension corn and wheat enterprise budgets indicate that the nitrogen cost for corn will be 17% ($97 per acre) and 22% for wheat ($56 per acre) of the variable production cost for the 2026 crop. If we can substitute manure nitrogen into the system, we can redirect this expense to the manure application.

In nutrient use efficiency, the “four R’s” — right rate, right placement, right timing, and right source — are used to focus on plant uptake efficiency. The amount of crop-recoverable nitrogen from manure application depends on the timing and placement of the application. The three common application timings of late summer and fall, spring, and during the growing season all impact the amount of manure nitrogen that can replace commercial fertilizer.

Nitrogen applied in liquid manure is in the ammonium (NH4+) form. Ammonium is fairly soil stable, but soil bacteria convert ammonium into nitrate forms, which are subject to leaching and denitrification loss. Because the conversion requires soil bacteria activity, soil temperature can be used to manipulate manure nitrogen retention. At soil temperatures of less than 50°F, bacterial activity slows. Thus, the amount of crop available nitrogen for corn or wheat from summer and fall applications improves when based on how quickly soil temperatures drop to less than 50°F. The closer the application is to soil temperatures below 50°F, the more nitrogen is maintained in the soil.

In the field

A two-year project to quantify spring available soil nitrogen and reduced nitrogen sidedress rates with fall-applied swine manure was conducted at our Northwest Agricultural Research Station. Rates of 5,000 and 8,000 gallons of swine manure were applied on Oct. 7, 2020, and Dec. 8, 2021, and then the acres were planted to corn in the following growing season. The date when the 2-inch soil temperature declined to less than 50°F was Nov. 12 in both years. Soil samples were collected in June of each year and analyzed for nitrate-nitrogen (N) content. A control treatment of 185 pounds of nitrogen with no manure was used for comparison.

Figure 1 shows the June soil nitrate-N concentrations and the resulting corn grain yield. In both years, the soil nitrate-N concentration was significantly higher than the unmanured control. The fall manure applications were sidedressed with 135 and 95 pounds of nitrogen for the 5,000- and 8,000-gallon rates, respectively. A nitrogen rate of 185 pounds per acre was applied to the control field.

The resulting corn yield was not significantly different between the three treatments in 2021; however, these yields were achieved at a reduced commercial fertilizer nitrogen rate. In 2022, the 8,000 gallons of manure and 95 pounds of applied nitrogen were significantly higher yielding than the 5,000-gallon fall manure rate and control treatment. Like 2021, the manure treatments produced the same or higher yields with less commercial fertilizer nitrogen applied.

Evaluating the soil

A pre-sidedress nitrogen test (PSNT) is a useful tool for evaluating soil nitrogen in the spring. Previous Ohio research has identified that nitrate-N test result of 10 parts per million (ppm) or less is a background level and a full nitrogen rate is required to meet corn nitrogen needs. When nitrate-N is greater than 25 ppm, no additional nitrogen is required. Between 10 and 25 parts ppm, a reduced nitrogen rate is justified.

Current recommendations reduce the nitrogen rate by 30 pounds when nitrate-N soil concentrations are between 11 and 15 ppm, 50 pounds between 16 and 20 ppm, and 90 pounds between 21 and 25 ppm. The study shown in Figure 1 indicates a rate reduction of at least the recommended numbers, and doubling the reduction for each PSNT category could be justified.

Finding its place

Another factor in preserving manure- applied nitrogen for crop use is to consider placement. Surface applications result in volatilization losses that reduce the crop available soil nitrogen. In a one-year trial in 2016 where a 5,000-gallon-per-acre rate of swine manure was applied before planting, both in a surface and incorporated placement at the same rate, soil nitrate-N and final corn yields were reduced.

The nitrate-N soil concentration was 13 ppm for the surface application and 37 ppm for the incorporated treatment. Corn yields were 135 bushels per acre compared to 162 bushels per acre for the surface and incorporated treatments, respectively. The lesson is that incorporation conserves nitrogen for crop uptake with spring-applied manure.

A study conducted in 2022 and 2023 provides some information for the question of what sidedress nitrogen rate is needed to maximize corn yield after fall-applied manure. The study used eight sites where fall manure was applied and eight sites with no manure application. At corn sidedress timing, a range of nitrogen rates (from 0 to 250 pounds of nitrogen) was applied. Figure 2 shows the average corn yield for both treatments across all the trial sites.

We define the nitrogen rate that maximizes crop yield as the agronomically optimal nitrogen rate (AONR). In the fall-applied manure treatment, corn yield was maximized at an AONR of 120 pounds of nitrogen per acre with a 208 bushel per acre yield. Where no fall manure was applied, the AONR increased to 160 pounds of nitrogen per acre with a 184 bushel per acre yield.

Working with Mother Nature

Applying manure to growing corn and wheat to provide the nitrogen needed from manure has become common in Ohio. In corn, swine manure is a source that can meet the nitrogen and phosphorus removal needs of a two-year crop rotation through the application of manure. Dairy manure requires additional nitrogen, so a manure test will come into play to determine what nutrients are needed to match the crop’s needs. However, the in-crop application hits the four R’s well and is the best use of homegrown nutrients.

The weather dependence of manure application logistics cannot be ignored. They limit what we can do in given windows. Planning to hit application timing and placement to maximize crop available nitrogen is the greatest agronomic benefit we can capture from manure-applied nutrients

This article appeared in the November 2025 issue of Journal of Nutrient Management on page 6 and 7.

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