Part 1, Section 2: Soil Fertility Management
Soil Fertility Management
SOIL TESTING
Recommendations
Nitrogen (N) recommendation
No soil analysis is used to make the N recommendations on the report. These 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 Table 1.2-6. Nitrogen recommendations also must take into account residual N from previous legumes in the crop rotation and from previous manure applications.
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 Table 1.2-7. Adjust the basic N recommendation accordingly. Although no yield response will result, 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. Approximate amounts of N that will be removed by legumes are given in Table 1.2-8. More information on manure application on legumes is provided under “Manure Nutrient Management” in this section of the Agronomy Guide.
Nitrogen supplied by manure also should 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 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. An N soil test, the Pre-sidedress 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 PSU Agronomy Facts #17, Pre-Sidedress Soil Nitrate Test for Corn, for details.) A new 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 pre-sidedress test, run at the critical time just before the major N demand by the crop. However, with this test, the chlorophyll level in the corn leaves is measured directly with a hand-held 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 PSU Agronomy Facts #53, The Early-Season Chlorophyll Meter Test for Corn, for more details.)