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The most important first step in your fertilizer manage ment program is to take a soil sample. Your fertilizer and lime decisions will be based on its results. Take the sample as soon as possible after harvest of the previous crop. Refer to Extension publication Lime and Fertilizer Recommendations (AGR-1) for specific recommendations from soil tests.
Figure 5-1. Nitrogen uptake during the growth of winter wheat.
Source: Alley et al. 1989.
Wheat requires a small but important amount of N in the fall. This requirement can often be met with the residual soil N following a soybean crop, a corn crop with a high amount of N carryover, or 20 to 40 pounds per acre of nitrogen added at planting. Fall N fertilization becomes more important with late plantings (after the first week of November) and light stands (less than 25 plants per square foot). Sufficient fall N stimulates early tillering, which is important for high yields.
Nitrogen applied in late winter (mid February) should be in the range of 30 to 50 pounds per acre and should be paired with another application in March. Nitrogen applied in late winter encourages further tillering and maintains current tillers. Fields with thin stands or little fall tillering should receive higher amounts of N, while those with high tiller counts (above 70 tillers per square foot) should receive the lower amount of N (Figure 5-2). Excessive N applied in late winter can increase the potential for lodging, disease, and damage from late spring freezes.
Figure 5-2. Tillering influences on relative yield of wheat (average of three varieties in two tillage systems over
three years).
Nitrogen applied in spring is the most effective. The best time is at Feekes 5 (usually mid March), just before jointing, when wheat starts growing rapidly. This rapid growth causes a large demand for N uptake. Applications earlier than this increase the chances of N loss during extended wet periods. In most cases, apply enough N to bring the total amount of N applied in February and March into the range of 60 to 90 pounds per acre. In fields with higher yield potentials (greater than 70 bushels per acre), apply a total of 80 to 100 pounds per acre of N. Nitrogen rates higher than this increase the chances of lodging and do not increase yields, unless specific conditions that require more N are identified.
When comparing the single March application to the February and March split application at the same rate, research indicates that, with the split, yields are increased an average of 3 bushels per acre (although it varies from year to year) and that splitting reduces lodging potential. Split spring N applications are recommended when possible, but equipment and logistic problems cause some growers to make a single application. The single application is recommended at Feekes 5 (mid March). The rate should be in the range of 60 to 90 pounds per acre, with the higher rates being used on fields with greater yield potential (greater than 70 bushels per acre). A single application in February is recommended only when the stand or tillers in the field are low. Early single applications increase the chance of spring freeze damage because they encourage earlier heading and increase the chance of high N losses during extended wet, rainy periods. Single applications made too early generally result in lower yields and encourage the growth of succulent plants with lush canopies susceptible to diseases like powdery mildew.
Nitrogen must be applied in a timely manner for maximum yield potential. Delaying N application to an N-deficient crop after Feekes 6 (jointing) will result in decreased yield. As plant development advances past this stage, the yield response to added nitrogen progressively declines. After Feekes 9 (flag leaf extended), there is little yield return to the added nitrogen. However, N applied after Feekes 9 will increase the protein content of the grain.
No-till wheat requires about 30 pounds more N per acre than wheat planted with tillage because N recovery is often less efficient with no-tillage. No-till wheat usually requires 90 to 120 pounds per acre of N in the spring. If a split application is made, the February application should be in the range of 40 to 60 pounds per acre.
Since the difference between enough and too much N is small, distribution in the field is important. The best distribution will be achieved with liquid N or an airflow delivery truck for solid N forms. Spinner systems delivering solid materials are less accurate. You can improve distribution of a solid N material that has a lot of fine material by double spreading (reducing the distance between passes by half and spreading half the desired rate on each pass). If evenly distributed, all nitrogen sources perform equally well in February and March.
Leaf burn can be a concern with liquid N, but you can eliminate this concern by streaming the liquid N or reduce it by using flood nozzles, mixing with half water, applying less than 60 pounds of N per acre per application, and avoiding applications on cold, windy days. Although fertilizer burning of wheat is visually disturbing, research indicates no reduction in yield.
When the amount of N to apply is questionable, a plant sample collected at Feekes 5 might be helpful in determining tissue N. Cut a handful of wheat about ½ inch above the ground at 20 to 30 places in the field, and place a subsample of the total in a paper bag. Send the sample to a commercial lab with a quick turnaround time so N application will not be delayed. Table 5-1 shows guidelines for N use.
| Table 5-1. Guidelines for nitrogen use. | |
| Plant N | N recommendation |
| % | lb/ac |
| 2.3 | 100 |
| 2.7 | 80 |
| 3.2 | 60 |
| 3.6 | 40 |
| 4.0 | 20 |
| Murdock (unpublished data) | |
A chlorophyll meter is a hand-held, non-destructive, field diagnostic tool that actually measures plant leaf greenness. Chlorophyll measurements can provide additional information to help predict the amount of N fertilizer that needs to be added at Feekes 5 or 6 (March). Large amounts of N (150 pounds per acre) are added to two or three small areas or strips in the field in early to mid February. At Feekes 5 (March), chlorophyll readings are taken on 10 to 20 plants in the high N areas and then on 20 to 30 plants in the rest of the field. The measurements are taken on the first fully expanded leaf (leaf with a leaf collar) from the top of the plant. Measurements are made about halfway between the tip and base of the leaf. The following formula is used to make the March N recommendations:
N = 6 + (7 x D), where N = N (lb/ac) needed for optimum growth at Feekes 5 (March), and D = differential chlorophyll reading (difference between chlorophyll reading in the field and the small areas with high N rates added in February)
Example:
1. Small areas or strips with high N (150 lbs/ac) added at Feekes 3 read an average of 52 at Feekes 5.
2. Field reads an average of 45.
3. 52-45 = 7.
4. 6 + (7 x 7) = 55 lbs/ac N recommended.
A soil nitrate (NO3) test usually is not helpful unless there are unusual conditions that might cause high N levels in the soil, such as high N carryover due to a poor corn crop or heavy manure usage. Take samples to a depth of 3 feet in February, and place them on brown paper for drying. The NO3-N measured in the samples will be reported in ppm, which should be multiplied by 12 to get pounds per acre. If 120 pounds per acre of nitrate-N are found, no N needs to be added.
Phosphorus (P) is essential for root development, tillering, early heading, grain fill, early maturity, and resistance to winterkill. Wheat takes up about 0.67 pounds of P2O5 for each bushel produced, and 80 percent of this is in the grain. A soil test is necessary to determine the proper rate of phosphorus fertilizer. Apply phosphorus fertilizer in the fall, prior to seeding, for best results.
Potassium (K) helps to lower the incidence of some diseases and increases straw strength, which helps reduce lodging. Wheat takes up about 2 pounds of K2O for each bushel produced, and 80 percent of this is in the stem. A soil test is necessary to determine the proper rate of potash fertilizer. Potassium fertilizer should be applied in the fall but can be applied in the spring if necessary.
Calcium (Ca), magnesium (Mg), and sulfur (S) deficiencies have not been observed on wheat in Kentucky. Calcium will be adequate if the proper soil pH is maintained. Magnesium should be added only if the soil test is below 120 pounds per acre. Sulfur deficiencies are best determined by tissue tests at Feekes 5. If the N-to-S ratio is greater than 15-to-1, S should be added at the rate of 20 to 40 pounds per acre. Only soluble sources that contain S as sulfate should be used at this stage of growth.
Micronutrient deficiencies have not been found in wheat in Kentucky. The best way to determine micronutrient needs is through plant tissue testing. Micronutrient deficiencies generally occur when the soil pH is too high or too low.
A soil pH range of 6.0 to 7.0 with a target pH of 6.4 should provide excellent conditions for growth. Lime should
be applied prior to planting, in the fall.