Organically sourced nutrients can be used as a substitute for some of the purchased fertilizer needs on a farm. Organically sourced nutrients lower purchased inputs and provide economic value when hauling manure to more distant fields. In regards to phosphorus and potassium, organically sourced nutrients are similar in availability to commercial fertilizer. In regards to nitrogen, a variety of factors affect the final amount of organically sourced nutrients that are available for crop production. A manure test is a basic tool in evaluating potential nitrogen availability. A presidedress nitrogen test (PSNT) can be used in the growing season to further refine recommendations. Other tools such as the Normalized Difference Vegetation Index (NDVI) can be used to evaluate plant nitrogen needs
and limit over- or under-applications. Over-applications result in excessive soil nitrogen at the end of the growing season, which will be lost to the environment through tile in the non-growing season.
Corn and wheat require additions of nitrogen to maximize yields and provide appropriate economic returns. Nitrogen in manure is in either an organic or ammonium form and is subject to the same conversions in soil as either of these forms from commercial fertilizer. The ammonium content is directly equivalent to manufactured nitrogen fertilizer. The organic fraction will become available through mineralization. A manure test is a valuable tool for estimating the amount of nitrogen available for crop production. The amount of nitrogen available depends on several factors:
• application timing in relation to crop uptake
• manure storage facilities
• application placement
• animal type
Corn grows in the warmer season, with frequent rainfall events. This leads to varying amounts of nitrogen annually mineralized from the soil. The current tool to recommend nitrogen for corn in Ohio is a combined yield/economic return model found at cnrc.agron.iastate.edu. The tool was developed from statewide yield trials and adjusted by nitrogen price and corn price. To better manage corn nitrogen needs, in-field information can be used to develop site-specific recommendations.
Because wheat grows in the cooler and more predictable rain patterns of the spring, Ohio uses a yield goal to set nitrogen rates (Table 3). Manure application rates should be carefully considered to avoid excessive nitrogen rates. Over-application of nitrogen often results in increased wheat lodging, and yield losses can be experienced.
Any farm receiving manure can take advantage of organic or ammonium N that is applied in their crop production system. Manure can be used on corn, wheat and other grass or N requiring crops. Strategies can be used to preserve the maximum amount of N possible when timing, placement and rate are considered.
The N cost of a corn enterprise budget makes up 15-20% of the variable cost of production. Offsetting purchased N cost can provide economic incentives to cover needed equipment or products that will enhance N available for the crop. The cost of moving manure further distances to avoid over application close to livestock facilities can also be covered. Reduce nutrient loss by matching available nutrients to crop need avoiding excess supply of nutrient. Thus an environmental gain can be made in reducing N losses through tile.
From an agronomic standpoint, a PSNT is an effective tool for understanding available nitrogen. Producers have significantly lowered, and in many cases replaced, the need for purchased nitrogen by using a PSNT when making management decisions. A caution in PSNT use is weather that is cooler through the early part of the growing season might result in lower than expected values due to decreased soil temperature and soil microbial activity. Application of NDVI technology has not been as widely used due to the need for high-clearance application equipment. As high-clearance equipment has become available, NDVI technology has been used more widely, but not often in manured fields. The potential to reduce nitrogen applications makes this application of the technology something to explore.
Direct water quality data has not been attained for projects where a PSNT or an NDVI method was used, but any reduction in nitrogen rate to better match the crop need would be expected to reduce nitrogen losses through tile.
Use a manure to test to evaluate nitrogen and phosphorus content, along with forms of nitrogen in the manure. When the nitrogen need of the crop is targeted as the manure application rate, phosphorus can be applied in excess and can result in elevated STP levels over time. This causes environmental concerns. If the manure application rate needed to supply nitrogen results in phosphorus applied at too high of a level, then a commercial fertilizer nitrogen source can be used to supplement. The forms of nitrogen in the manure can be used to predict the nitrogen available at
the time of the crop need.
A PSNT can be used to determine soil-available nitrogen that is available to the crop, and it can predict future need. This test is most useful in fields where manure has been applied regularly and mineralization from organic matter is expected, or a preplant (fall or spring fertilizer application) has been made. Take the PSNT sample as close as practical to when the results can be used to adjust nitrogen sidedress rates. To take the sample, use a random pattern, take 10–15 standard, 1-inch soil cores at 0–12 inches deep. Then, take a subsample of the soil collected for analysis. When working with a lab, it is often possible to get results back within 24–36 hours. Purdue University has provided a scale of additional nitrogen needs based on PSNT results (Table 4). This scale can also be used by Ohio producers.
The NDVI provides an evaluation of the comparative greenness of a crop canopy, which can be used to evaluate the nitrogen status of the crop. This has been used in a variety of crops in different parts of the world. For Ohio, the primary work has been done with corn. The best protocol for using the NDVI is to establish a nitrogen-rich strip across the field using a rate of nitrogen that would not be limiting. Placement across a field’s soil types, along with other factors, will maximize the information from the field. At a growth stage where the ground is mostly covered by the crop’s canopy, the meter can be used in the field to compare the nitrogen-rich strip to other areas of the field to determine if nitrogen is deficient. In corn, the stage of growth where the meter provides good data is V8 to V12. With this stage of growth, high-clearance application equipment is needed to make nitrogen applications. Various companies produce the NDVI meter in both hand-held and mountable versions. The mountable version can be placed on a tool bar. The NDVI values can also be obtained from remote-sensing. Determining nitrogen rates will vary depending on the equipment used. Most mounted equipment can be programmed to provide on-the-go nitrogen-rate adjustments. Numbers from handheld units can be entered into the following website: sensor based n rate calculator. Select the equation for corn developed by Ohio State researchers.
A manure test will cost $45-55 depending upon the actual nutrients included in the analysis.
PSNT costs range from $15-25 per sample plus cost of collecting the sample. Growers sending in these samples should work with the lab for the quickest turnaround of results.
The cost of obtaining an NDVI value will vary widely. A handheld meter can be purchased for around $450 to remote sensing service that can be purchased on a per acre cost. Additionally, cost for application need to be considered to provide understanding of all cost involved.
Manure contains both organic and plant available sources of N primarily NH4. The proportions and concentration ultimately available for crop utilization will depend upon storage, handling and application timing. The NH4 portion is readily available and manures with a higher level of this N component can directly replace purchased fertilizer. The N tied up in the organic portion will only become available through mineralization. Applying manure shortly before planting or into a growing crop can result in the highest utilization of N from manure. Applications that incorporate manure into the soil result in lower volatilization loses and preserve more N for crop utilization.
Consult with agronomists at OSU Extension, or other agronomists, for sampling criteria and current data to determine nitrogen rate recommendations prior to making final nitrogen applications.