Harvesting of soybeans and corn has just begun in South Dakota. Crop plans for next year will soon begin as producers choose hybrids, varieties, and determine crop nutrient inputs. Currently, the climate is trending to be very dry which could influence 2014 crop plans. Fertilizer purchased in the fall typically has been lower priced and therefore more attractive for crop budgets. There are other fall fertilization considerations as well.
The International Plant Nutrition Institute has developed the 4R approach to fertilizer management which is:
- The right fertilizer source.
- Applied at the right rate.
- Applied in the right place.
- Applied at the right time.
(For more information visit the 4R Nutrient Stewardship Portal). Depending on the nutrient to be applied there are several fertilizer sources. Common nitrogen sources are anhydrous ammonia (82-0-0), urea (46-0-0) and UAN (28 or 32-0-0) while MAP (11-52-0), DAP (18-46-0), and 10-34-0 (ammonium poly-phosphate) are the main sources for phosphorus and also contain some nitrogen. The primary potassium source is potash (0-0-60) which contains both potassium and chloride (45% Cl). Other fertilizer sources are available but not commonly used. Dry fertilizers are usually broadcast applied while liquids are generally banded. However, there are exceptions to both depending on the type of equipment used.
Nitrogen applied in the fall could be susceptible to several loss pathways. Losses may come from volitilization of urea containing fertilizers that are surface applied and not incorporated with tillage or rainfall. Nitrogen fertilizer in the nitrate form or converted to nitrate in the soil is susceptible to leaching losses and de-nitrification losses if soils become extremely wet and warm prior to crop uptake. In addition, gaseous loss can occur with anhydrous ammonia applied in extremely dry or very wet soil. Nitrogen probably shouldn’t be fall applied as several studies have indicated possible N losses from fall application. In one study at the Southeast Research farm near Beresford from 1990-2006 in a corn/soybean rotation, the late fall urea application yielded 10 bu/a less when compared to the split N application (½ in spring, ½ sidedress). If fall N applications are used, apply after soil temperatures fall below 50° F. In addition, urea fertilizers should not be surface applied without incorporation and fall applications of nitrate containing fertilizers (UAN) are not recommended. Avoid fall applications of N fertilizers on coarse textured soils or soils prone to spring flooding.
Applications of phosphorus, potassium, zinc and other non-mobile nutrients in the fall can be a good practice. If soil tests are low, planter starter band applications of these nutrients can provide good early plant growth, with the remainder of the recommended nutrients applied as broadcast applications. If soil tests are medium or higher, much if not all of the phosphorus can be applied as a starter seed band or broadcast. If fertilizer is applied with strip till, these bands should be at least 2 to 3 inches below the planting depth as the high concentrations of fertilizer salts can decrease germination. Therefore strip bands do not work well as a starter because of the deeper application depth.
Finally, determining the right nutrient and the right nutrient rate should be determined by a soil test and choosing a representable yield goal. For more information and determining a recommended nutrient rate see the SDSU Extension Fertilizer Recommendation guide.