Potash can be applied broadcast or with row fertilizer. Total potash plus nitrogen should not exceed 60–70 pounds per acre when applied 2 inches from the seed in the row fertilizer. Potash can be broadcast on the surface in no-till systems. Incorporation by plowing down and secondary tillage are the preferred methods under tilled systems.
Phosphate can be applied with the potash or as row fertilizer. Apply potash and phosphate any time in the fall or spring, but avoid broadcasting on frozen ground. The efficiency of phosphate uptake is improved greatly by band applications (see “Using a starter fertilizer,” following page).
An effective nitrogen management strategy is one that minimizes the potential for nitrogen losses occurring from (1) volatilization of urea, (2) leaching of soil nitrate, or (3) denitrification (the conversion of nitrate to N2 gas in the soil). Volatilization can be minimized by incorporating fertilizers that contain urea or by dribbling liquid nitrogen solutions (UAN).
Leaching and denitrification losses can be minimized by delaying nitrogen application until corn is 10 to 16 inches tall (sidedressing). This is especially critical on soils with a high potential for leaching or denitrification. On these soils, responses to sidedressing are relatively consistent. Leaching is most serious on well-drained, sandy, and gravelly soils, but it also can occur on well-drained upland limestone and shale soils. Denitrification is most serious on soils rated as somewhat poorly drained or poorly drained, but it also can occur on any soil that is saturated with water.
All of the commercially available nitrogen sources give adequate performance, provided the properties of the material are taken into consideration at application. Below is a listing of commonly ap- plied nitrogen sources for corn:
- Incorporate (knife in) anhydrous ammonia 6 to 8 inches deep. Soil should be moist. Sidedressing corn at 10 to 16 inches tall and at planting are the preferred times of application. Fall applications are not recommended.
- Incorporate urea within 24 to 36 hours to avoid nitrogen loss from volatilization. Mechanical incorporation by plowing down, disking in, cultivating in as a sidedress, and timing the application just before a rainfall all are effective methods. One-half inch of rain is sufficient. Broadcasting urea on corn plants causes some leaf injury, and although this is not a widespread practice, it has been used by some farmers and in research trials with satisfactory results. Sidedressing that is incorporated is preferred over topdressing.
- UAN solutions are approximately one-half urea. Therefore, observe the above application methods for urea, but do not apply UAN as a topdress, because such application would cause severe leaf injury. Sidedressing 50 to 90 percent of nitrogen requirements using UAN and dribble tubes is preferred; it increases nitrogen efficiency over broadcast spraying solution nitrogen. This is especially important in no-till or mulch-covered fields. Incorporating or knifing-in the UAN improves efficiency slightly over dribble applications.
- Ammonium nitrate, ammonium sulfate, and the ammoniated phosphates can be surface-applied without significant volatilization losses.
The efficiency of recovering applied nitrogen is improved by sidedressing. This practice applies the nitrogen nearest to the time when the corn crop’s need for nitrogen is approaching its peak—during rapid vegetation growth. Sidedressing also shortens the time during which the nitrogen is subject to possible field losses from leaching or denitrification. Figure 1.4-2 illustrates how delayed nitrogen application reduces nitrogen vulnerability. Corn yields do not always respond to delayed applications, but under some conditions—for example, a wet year—yield differences can be sizeable, as Table 1.4-5 illustrates. In 23 Penn State studies conducted from 1982 to 1995, the yield response to delaying at least half of the N until sidedressing averaged 5 bushels per acre.
Fifty to ninety percent of the nitrogen requirement can be applied at sidedressing. In fields that receive periodic manure applications and where legumes are grown in a relatively short rotation, applying all but the starter nitrogen as a sidedress is the best approach. In continuous corn where manure is not applied, applying more nitrogen than just the usual starter amount at planting time may be advantageous. This usually is done by using UAN as a carrier for the herbicide. Remember that spraying UAN on the soil surface can result in significant nitrogen loss. Additional nitrogen can be applied safely in the starter fertilizer, as long as the nitrogen source is not urea, and as long as the rate and analysis of the starter fertilizer are such that the total amount of nitrogen plus potash is kept below 60 to 70 pounds per acre.
As noted in the discussion of nitrogen materials, these delayed applications can be done in a number of ways. The best way probably is to dribble UAN solution between the rows of corn using a sprayer equipped with drop tubes. Anhydrous ammonia also is an effective sidedress material, providing that soil conditions are sufficient to entrap the gaseous ammonia in the soil. Urea, too, can be used as a topdressing material, but it may result in significant nitrogen loss and some leaf damage.
Nitrogen and Soil pH
Applications of nitrogen increase soil acidity (lower the pH). As a general rule, for each pound of nitrogen applied as ammonium sulfate, 6 pounds of limestone are required; 3 pounds of limestone are required for each pound of other nitrogen sources, including manure nitrogen.
Under normal growing conditions, fertilizers applied at recommended rates do not create nitrate-nutritional problems in livestock. Excessive accumulation of nitrate in the corn plant may be caused by any factor that produces plant stress, such as prolonged drought and defoliation of the plant by disease, insects, or hail.
In general, the greatest risk of high nitrate levels has been in drought-stunted fields that have received excessive manure or nitrogen fertilizer. The risk of high nitrate levels is highest immediately following a drought-ending rain. Nitrates accumulate in the lower portion of the plant, so harvesting higher under these conditions can help you avoid high nitrate concentrations. Normally, the ensiling process removes about one-half the nitrates present in the fresh corn silage. If you suspect unsafe levels of nitrates in corn silage, have the silage tested at a reputable forage-testing lab.