Nitrogen (N) Recommendation
The most important concept for managing nitrogen recommendations is to understand that soil nitrogen management is a process; it is not a simple answer for how much N a crop needs. It means managing a variety of potential N sources, including soil N and applied N sources, over time within a complex and dynamic N cycle to meet the overall crop need but also to meet the peak demand for N by the crop.
Nitrogen management process
The following figure illustrates the process for managing N, which starts with a recommendation, but the ultimate outcome is the result of an on-going repeated process of management decisions, observation, testing, analyzing, and adjustments based on an understanding of the N cycle, which must be a continuous activity of the manager.
Reference: Figure 1.2-5, The Agronomy Guide
Recommendations are based on estimates of crop requirements for N as determined by extensive crop response research under Pennsylvania conditions. Most recommendations are based on the information you provided about the crop to be grown and the expected yield. The recommendations are given as pounds of N required per acre for each crop. A summary of the N recommendations for agronomic crops is given in the following table.
Nitrogen recommendations for agronomic crops
These recommendations should only be considered as the starting point for N management.
*Note:These are base recommendations and should be adjusted for previous crop, previous manure history, and planned manure applications (see “Manure Nutrient Management” section).
Adjustments should be made to the basic recommendations for the N contributions from legumes and manure. Legumes are able to fix the N they need from the atmosphere and thus do not require additional N. Also, growing a legume in a rotation preceding an N-requiring crop may result in a high level of residual N in the soil that can be utilized by the following crop. The N recommendations must be adjusted to take into account this residual N. Recommended credits for this residual N are provided with the N recommendation on the soil tests. These credits also are summarized in the following table for residual nitrogen contribution from legumes.
Residual nitrogen contribution from legumes
Adjust the basic N recommendation accordingly. Although the N recommendation for a legume is 0, and no yield response will result if N is applied, legumes will use some applied nitrogen, such as that from manure applications. However, N applied to legumes can increase weed and grass competition and thus shorten the life of a forage legume stand.
|Previous crop1||Percent Stand||High-productivity fields||Moderate-productivity fields||Low-productivity fields|
|(Soil productivity group 1)²
||(Soil productivity groups 2 & 3)²
||(Soil productivity groups 4 & 5)²
|Nitrogen credit (lb/A)|
|First year after alfalfa||>50
|First year after clover or trefoil||>50
|First year after soybeans harvested for grain.||---- 1 lb N/bu soybeans ----|
Approximate amounts of N that will be removed by legumes are given in the following table, Nitrogen removal by legumes.
Nitrogen removal by legumes
|Pounds of N
|Alfalfa (ton/A)||50||Although legumes will use N from manure and other sources, applying N may increase the competition from weeds and grasses. If you apply manure, limit it to an application rate that balances the crop’s P requirement.|
||Although legumes will use N from manure and other sources, applying N may increase the competition from weeds and grasses. If you apply manure, limit it to an application rate that balances the crop’s P requirement.|
||Although legumes will use N from manure and other sources, applying N
may increase the competition from weeds and grasses. If you apply
manure, limit it to an application rate that balances the crop’s P
|Soybeans (40 bu/A)||3.2||Although legumes will use N from manure and other sources, applying N may increase the competition from weeds and grasses. If you apply manure, limit it to an application rate that balances the crop’s P requirement.|
More information on manure application on legumes is provided under “Manure Nutrient Management” in The Agronomy Guide.
Nitrogen supplied by manure should also be considered in determining the N management for a crop. Residual N from past manure applications may reduce the amount of N required for the current crop. Also, the N in manure applied for the current crop must be accounted for. Manure N availability varies depending on how it is handled and applied. Manure analysis is available from the Agricultural Analytical Services Lab at Penn State. More detailed information on manure management is provided under “Manure Nutrient Management” in this section of The Penn State Agronomy Guide.
Nitrogen testing is not possible as part of a routine soil testing program. N is very dynamic in the soil plant system. Consequently, the available N changes throughout the season. For N testing to be valid, it must be conducted very near to the time when the crop has the most demand for N and thus used to make adjustment to N management during the season. An N soil test, the Presidedress Soil Nitrate Test (PSNT), has been developed for corn. This test is an in-season, presidedress analysis for nitrate-N in the surface foot of soil. The test is very good for determining if the soil contains adequate N to produce the current corn crop, and it can give some guidance for adjusting sidedress N recommendations. This test is especially useful where manure is expected to contribute significantly to the N needs of the crop. This test is available only for corn (see Agronomy Facts 17: Presidedress Soil Nitrate Test for Corn for details). Another test using a chlorophyll meter to estimate the N status of the corn based on the color of the corn leaves has been developed. This test is also a presidedress test, run at the critical time just before the major N demand by the crop. However, with this test the greenness of the corn leaves is measured with a handheld meter. Thus, no samples need to be collected and analyzed, and the results are available immediately. The recommended use and accuracy of this test is similar to the PSNT (see Agronomy Facts 53: The Early Season Chlorophyll Meter Test for Corn for more details). With both of these tests, the procedures must be followed exactly for the results to be valid.
Along with an understanding of the N cycle, it is also necessary to understand that an N recommendation must not just consider the total amount of N recommended for the crop but also the uptake dynamics of the crops being grown.
The following figure shows the N uptake curve for corn as an example. Nitrogen uptake by corn and seasonal weather patterns that must be considered in designing a nitrogen application strategy. If fall, winter, or early spring applications are necessary, especially with manure, a cover crop will improve the efficiency of this application dramatically.
Reference: Figure 1.2-6, The Agronomy Guide
The objective of managing N for this corn crop is to supply adequate N but, more important, in the right amounts and at the right time to meet the uptake demand of the crop and avoid conditions when there is a high loss potential. In the case of corn, the amount and rate of uptake are low early in the season, but about 6 weeks after planting there is a dramatic growth period and the N uptake requirement of the crop becomes very high. This demand may be as much as 10 lb N/A/day. If a large amount of N is applied long before planting, there is little crop demand but temperatures are high enough for microbial activity to mineralize N into available forms, and higher rainfall during this time is increasing the potential for N loss, the efficiency of use of this N by the crop will be very low. The strategy needs to be managing the N so that the availability peaks when the crop demand is highest. Thus, applying manure or compost near to planting time will provide a small but significant amount of available N from the mineral N in the manure to meet the early crop needs. This should reduce the risk from the early spring high N loss potential when there is low crop demand. Then, if there is good soil quality for optimal conditions in the soil in sync with more ideal weather conditions to promote microbial activity, the organic N will mineralize to supply the high N demand of the crop during the rapid growth phase. If N must be applied well before the season—for example, fall or early spring manure applications—cover crops are vital for capturing the N and holding this against loss. Then when the cover crops are killed at planting time, the decomposition of the residue will release the N closer to the time of crop need.
The source of N and the method of application will also influence the nitrogen use efficiency of the crop. Nitrogen sources are discussed later in this chapter.
Finally, experience and observation are critical to successful N management. Keeping records of N management activities and good weather and observing the results are important parts of this process. A relatively new tool to assist with assessing the performance of your N management program is the Late Season Cornstalk Nitrate test. For this test, samples of pieces of the lower stalk of corn plants near to maturity are analyzed for their nitrate level. This analysis provides a very good assessment of whether the corn crop was deficient for N, had adequate N, or had and excess of N. This, along with the management records for the growing season, can be very helpful in assessing the success of the management and/or suggesting improvement for future years (see Agronomy Facts 70: Late Season Cornstalk Nitrate Test for more details).
This sounds simple, but it is incredibly complex due to unknowns related to the weather, the quality of the soil, the characteristics of the N source, and so forth. The key is the N management process. Use all information available to make decisions. Carefully observe the conditions in the soil, the crop, and the weather. Analyze these observations relative to the N cycle and crop uptake dynamics. Make adjustments where necessary and possible. Sometimes adjustments in management can be made in real time; other times, these experiences must be saved for use in making adjustments in following years. (See Agronomy Facts 76: Nutrient Management to Improve Nitrogen Use Efficiency and Reduce Environmental Losses for more details).