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It is important not to skimp on your corn fertilization program. You fertilize to provide essential nutrients for corn growth and development, and to replenish what has been removed from the soil. Like other inputs, it can be difficult to know where to cut back on fertility when you may need to cut costs.
Soil testing determines nutrient levels available in the soil and provides locally calibrated fertilizer recommendations. A soil test can provide a measure of nitrogen (N), phosphorus (P), potassium (K), as well as other secondary and micronutrients. It can also provide values for organic matter, soil pH, cation exchange capacity (CEC), soluble salts, soil salinity, and soil physical characteristics.
Test results typically include a fertilizer recommendation that is based upon your yield goals. A realistic yield goal should not exceed 10-20% of the average yield from the last 3-5 years. This is because yield variations from year to year can be caused by yield limiting factors other than fertilizer application rates, so applying more fertilizer likely will not improve yield above 10-20%.
It is important to understand that soil tests do not measure inputs of N and other nutrients from the mineralization (breakdown and release of nutrients) of organic matter. Therefore, you need to take into account your crop rotations and manure applications in regards to N availability. Stover from continuous corn or wheat residue can result in immobilization where your N gets tied up because soil bacteria need the available N to break down the residue. Your corn crop can be starved for N until it is made available again through mineralization.
Maintaining proper soil pH is the best way to promote the availability of nutrients. Liming may be needed if the soil pH gets down to 5.5 or lower. High pH problems for corn production generally start at a pH > 7.8, which are typically saline (salty) or sodic soils. Salts in soil are measured using an electrical conductivity (EC) test. Soils with high EC readings require good water management.
Fields with a history of manure application generally have sufficient N, P, and K for corn silage production, especially when manure has been applied annually for more than 5 years. Starter fertilizer should be considered to increase early corn growth if a field has not received manure for more than 3 years.
The pre-plant soil N test measures the readily available N in soil and is mainly used to determine if a starter fertilizer application is needed. The test is reasonably effective for identifying fields that are not likely to need fertilizer N, especially in non-manured soil. While the test reflects the readily available N at the time of sampling, it may not reflect the N that will be mineralized and made available during the season. Therefore, the test tends to over-estimate the fertilizer required in manured fields, or other fields with high N mineralization potential.
The pre-sidedress soil nitrate-N test (PSNT), conducted when corn has five or six leaves, measures N during the growing season, after some N has become plant-available but before the crop’s time of greatest need. The test measures the carry-over N from the previous season and the N mineralized from the soils organic fraction. The PSNT is the more appropriate soil test for manured soils, as it better reflects N derived from manure and more accurately identifies fields likely to be N sufficient. The test can be used to adjust the current-season sidedress N fertilizer rate for fields that have or have not been manured.
Split applications of fertilizer between pre-plant broadcast and post-emergence sidedressings or applications through sprinkler lines should be considered to better supply nutrients when the crop needs it. Modern corn products may not only have higher N requirements, they may also take up a larger portion of their N requirements after silking than older corn products. This provides more incentive to consider split applications where feasible.
Brown, B., Hart, J., Horneck, D., and Moore, A. 2010. Nutrient management for field corn silage and grain in the inland Pacific Northwest. University of Idaho Pacific Northwest extension publication PNW 615. http://www.cals.uidaho.edu.
Hart, J., Sullivan, D., Gamroth, M., Downing, T., and Peters, A. 2009. Silage corn nutrient management guide for Western Oregon. Oregon State University extension publication EM 8978-E. http://ir.library.oregonstate.edu.
Building proper nutrient levels based on soil tests. Asgrow® and DEKALB® agKnowledge Spotlight. www.aganytime.com.
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