Information in this fertilizer guide applies to winter wheat produced after no-till fallow or tillage fallow. This guide is one of a set of publications that address the nutritional requirements of non-irrigated cereal crops in northcentral and Eastern Oregon.
Nitrogen
Calculate nitrogen (N) application rates by subtracting soil test nitrogen from crop demand for nitrogen. Adjust for excessive straw and soil sampling in the spring of the summer-fallow year. Evaluate application rates by reviewing the protein content of harvested grain.
Crop demand for nitrogen
Multiply expected yield by the nitrogen requirement to get crop demand for nitrogen. The nitrogen requirement, which is the amount of nitrogen required to produce 1 bushel of wheat, is based on a grain protein goal (Table 1).
|
Grain protein goal (%) |
Average nitrogen requirement (lb N/bu) |
Range of nitrogen requirement (lb N/bu) |
|---|---|---|
|
9 |
2.2 | 2.0–2.4 |
|
10 |
2.4 | 2.2–2.6 |
|
11 |
2.7 | 2.4–2.9 |
|
12 |
3.0 | 2.6–3.2 |
|
13 |
3.3 | 2.8–3.5 |
Average nitrogen requirements are suitable for most situations. The ranges provided in Table 1 can be used to compensate for growing conditions or varieties that are genetically predisposed to lower or higher grain protein contents.
A grain protein content of 10 % is optimum for soft white wheat. Desired grain protein concentrations for hard wheat range from 11% to 13%.
Nitrogen requirements for high-protein hard wheat are greater than those for low-protein soft wheat. The extra protein in hard wheat accumulates in grain when plant uptake of nitrogen exceeds that required for maximum yield (Figure 1).
Subtract soil test nitrogen
Laboratory methods are used to test soil samples for plant-available nitrogen (soil test nitrogen).
Collect samples from the effective root zone (usually 4 feet) in 1-foot increments and have them analyzed for nitrate nitrogen (NO3-N). Samples from the surface foot also should be analyzed for ammonium nitrogen (NH4-N). Add reported values for all depths to get total soil test nitrogen (Table 2).
|
Soil depth (inches) |
Ammonium nitrogen (NH4-N) (lb/acre) |
Nitrate nitrogen (NO3-N) (lb/acre) |
Total soil test nitrogen (NH4-N + NO3-N) (lb/acre) |
Amount to subtract (lb/acre) |
|---|---|---|---|---|
|
0–12 |
5 | 30 | 35 | 35 |
|
13–24 |
— | 25 | 25 | 25 |
|
25–36 |
— | 15 | 15 | 15 |
|
37–48 |
— | 5 | 5 | 5 |
|
Profile* |
5 |
75 |
80 |
80 |
|
49–60** |
— | 12 | 12 | — |
|
61–72 ** |
— | 10 | 10 | — |
* Calculation of the nitrogen application rate should be based on soil test results from the top 4 feet or the effective root zone.
** Nitrogen in the fifth and sixth foot usually does not contribute to yield, but may increase grain protein.
Periodic assessment of nitrate concentration in the fifth and sixth foot can be used to fine-tune nitrogen management. If nitrate concentrations are high or increase over time, consider adjusting the application rate or the time of application. Split applications may improve nitrogen use efficiency.
Adjust for excessive straw
Nitrogen “tie-up” in crop residue (immobilization) temporarily reduces the amount of available nitrogen in the soil; immobilization can be a problem when greater-than-average quantities of straw are present in the field.
Grain yield can be used to estimate the quantity of straw. Straw loads increase by approximately 100 pounds per acre for each bushel increase in yield. Adjust the calculated nitrogen application rate as shown in Table 3 if grain yield from the previous wheat crop exceeded the long-term field average by 10 bushels per acre or more.
|
Greater-than-average wheat yield (previous crop) (bu/acre) |
Corresponding increase in straw production (lb/acre) |
Increase application rate by(lb N/acre) |
|---|---|---|
|
10 |
1,000 | 10 |
|
20 |
2,000 | 20 |
|
30 |
3,000 | 30 |
Adjust for soil sampling (spring of summer-fallow year)
Decrease the calculated application rate by 15 to 25 lb N/acre if fields are sampled in the spring of the summer-fallow year. This adjustment will compensate for mineralization. Mineralization is a biological process that increases the supply of available nitrogen in the soil; it is favored by moist or wet soils and warm temperatures.
Mineralization may increase the supply of available nitrogen by more than 15 or 25 lb/acre when average or above-average precipitation follows a prolonged drought. Adjustment of the nitrogen application rate is not necessary if fields are sampled within six weeks of fall seeding.
Review protein content of harvested grain
A postharvest review of grain protein can be a good way to evaluate application rates. Higher-than-desired protein indicates overfertilization, if growing conditions were normal or about average. High protein also can be caused by unusually dry conditions or nitrogen that is positioned deep in the soil profile.
Lower-than-desired protein may be due to an insufficient application rate. Low protein also can be a problem when late-season rainfall results in an above-average yield or when nitrogen losses occur during or after application. Examples of nitrogen losses include “escape” of anhydrous ammonia from dry soil or an unsealed soil surface, volatilization of surface-applied urea, and nitrate leaching below the root zone.
|
Grain protein (%) |
Grain nitrogen removed from field during harvest (lb N/acre) |
|||
|---|---|---|---|---|
|
(50 bu/acre yield) |
(60 bu/acre yield) |
(70 bu/acre yield) |
80 bu/acre yield) |
|
|
9 |
41 | 49 | 57 | 66 |
|
10 |
46 | 55 | 64 | 73 |
|
11 |
50 | 60 | 70 | 80 |
|
12 |
55 | 66 | 77 | 87 |
|
13 |
59 | 71 | 83 | 95 |
Phosphorus
Application of 20 to 30 lb P2O5/acre should increase yield if soil test phosphorus (P) levels are 5 ppm or less (Table 5). A phosphorus application is not recommended when soil test values are greater than 15 ppm.
Phosphorus response in fields with soil test values of 6 to 15 ppm is highly variable. Yield increases from fertilization seem to be associated with:
- High yield potentials.
- Late seeding dates.
- Root diseases that limit plant growth and development.
In fields with soil test levels between 6 and 15 ppm, effects of fertilization are best evaluated through on-farm experiments.
|
Soil test phosphorus (P) (ppm)* |
Plant-availableindex |
Amount of phosphate (P2O5) to apply (lb/acre)** |
|---|---|---|
|
0-5 |
Very low | 20-30 |
|
6-10 |
Low | 10-20 |
|
11-15*** |
Moderate | 0-10 |
|
Over 15 |
High | 0 |
* Plant-available index is correlated to sodium bicarbonate-extractable phosphorus only and does not apply to other test methods.
** Recommended application rates apply to banded or subsurface shank applications.
*** Phosphorus response in fields with soil test value between 11 and 15 ppm is highly variable.
Banding phosphorus helps achieve optimum efficiency. Placement of either liquid or dry material with the seed, below the seed, or below and to the side of seed is recommended. Subsurface shank applications are also effective. Broadcast applications are not recommended.
Potassium
Soil potassium (K) concentrations in the region generally are high or very high (greater than 100 ppm extractable K). Fertilizer applications are not recommended.
Sulfur
Sulfur (S) is one of the most limiting nutrients for wheat production — second only to nitrogen in importance. The sulfur requirement of the wheat plant is about one-tenth the nitrogen requirement.
Sulfur is necessary for optimum yield and high-quality baking flour. Sulfur deficiencies in wheat are fairly common in spring after a wet winter. Above-average precipitation moves sulfate-sulfur (SO4-S), the form of sulfur available to plants, below the root zone. Deficiency symptoms often disappear later in the season as root growth extends to deeper layers of the soil profile.
The soil sulfur (SO4-S) test is not definitive. Low or moderate soil test values (Table 6) are a first indication that fertilization might be warranted. Other factors need to be considered.
Yield responses are more likely if one or more of the following situations apply:
- Winter wheat is seeded late in the fall
- More than five years have passed since the last application of sulfur
- Greater-than-average quantities of straw are present in the field.
Field experience, observation, and on-farm experimentation provide valuable information about the need for sulfur.
|
Soil test sulfate-sulfur (SO4-S)(ppm) |
Plant available index |
Amount of sulfur (S) to apply(lb/acre)* |
|---|---|---|
|
0–5 |
Low | 10–15 |
|
6–10 |
Moderate | 5 |
|
Over 10 |
High | 0 |
Banding sulfur is most efficient. Place either liquid or dry material with the seed, below the seed or below and to the side of seed.
Subsurface shank applications are also effective.
Ammonium thiosulfate liquid (Thiosul, 12-0-0-26) is an effective source of sulfur, but it can injure or kill seedlings when placed with the seed. Avoid this problem by placing the product below or below and to the side of seed.
Use elemental sulfur with caution because it is not immediately plant-available. Microorganisms oxidize elemental sulfur to plant-available sulfate, but conversion occurs slowly and is regulated by the moisture status and temperature of the soil. Most of the elemental sulfur will not be available until two to three years after application. Rates of 100 lb elemental S/acre may be necessary to ensure that adequate sulfate is available during the first growing season.
Chloride
Research shows that application of chloride (Cl) may increase grain yield, test weight and kernel size. Note, however, that these responses occur only some of the time.
Chloride applications are known to increase yield of winter wheat suffering from “Take-all” root rot, and they reduce the severity of physiological leaf spot. Yield responses in the absence of disease also have been observed and may be a consequence of improved plant–water relations.
Consider applying chloride if soil test concentrations in the surface foot are less than 10 ppm. The recommended application rate for chloride is 10 to 30 lb/acre.
Benefits from fertilization may last for several years. Yield increases, when they occur, usually range from 2 to 5 bu/acre. Responses are most often associated with above-average yield.
Growers are advised to experiment with chloride on small acreages. Do not apply chloride with the seed; it is a soluble salt that can delay germination or injure or kill germinating seeds. Rain is required after application to move surface-broadcast chloride into the root zone. Potassium chloride (KCl) is the most readily available source of chloride.
Boron
Application of boron (B) fertilizer may be warranted if the surface-foot DTPA-Sorbitol soil test level is less than 0.3 ppm. Application of 1 lb B/acre is appropriate for broadcast treatments. Smaller amounts of boron (less than 0.1 lb B/acre) can be applied as a foliar spray or as a banded (away-from-the-seed) treatment.
Banded applications are best made with a wide opener that mixes fertilizer with a relatively large volume of soil. With-the-seed applications are an option if products contain a negligible quantity of boron.
Proper calibration of application equipment is crucial because the margin between an agronomically appropriate rate and a toxic rate is extremely narrow. Limit first-time applications of boron to small acreages.
Zinc
Soil test zinc (Zn) levels have declined over time. Zinc application may increase grain yield if the surface foot DTPA-extractable soil test level is less than 0.5 ppm. The potential for response increases when sodium bicarbonate-extractable soil test phosphorus levels exceed 25 ppm and the soil pH is greater than 7.0.
Application of 1 lb Zn/acre (or less) can be made as a with-the-seed or banded (away-from-the seed) treatment. With-the-seed applications of zinc may decrease the severity of Fusarium crown rot.
Foliar treatments are also an option. Foliar application rates are usually much lower than rates applied to soil, so repeat applications, in following years, may be warranted. Shallow (2-feet-deep or less) soils may be more prone to zinc deficiency.
Lime
The long-term, repeated use of ammonium-based nitrogen fertilizer causes acidity. Plant-available levels of phosphorus, calcium and magnesium are reduced in soils impacted by acidity. Aluminum toxicity, which can be a serious problem when soil pH levels drop to 5.3 or lower, will stunt root growth and reduce grain yield.
Lime (calcium carbonate) is the most effective amendment for increasing soil pH. Calcium carbonate reacts with water to neutralize acidity.
The amount of lime to apply can be determined from a buffer pH analysis of soil. Most soil testing labs will use the SMP (Shoemaker-McLean-Pratt) or Sikora (modified SMP) buffer pH methods. Both methods are effective for estimating the lime requirement of Oregon soils. Use the buffer pH value to adjust lime application rates (Table 7). For more information on soil acidity and lime materials, see the Eastern Oregon liming guide, EM 9060.
|
Buffer pH value(SMP or Sikora)* |
Lime application rate(tons/ac)** |
|
|---|---|---|
|
Tillage-based systems |
No-till systems |
|
|
Over 6.5 |
0 | 0 |
|
6.2 to 6.4 |
1–2 | 1 |
|
5.6 to 6.1 |
2–3 | 2 |
|
Below 5.5 |
3 | 2 |
* SMP and Sikora buffer values may be used interchangeably.
** Lime recommendations are based on 100-score lime and a 6-inch sampling depth.
References
Doolette, C.L, T.L. Read, C. Li, K.G. Scheckel, E. Donner, P.M. Kipittke, J.K. Schjoerring and E. Lombi. 2018. Foliar application of zinc sulphate and zinc EDTA to wheat leaves: differences in mobility, distribution, and speciation. Journal of Experimental Botany.
Horneck, D.A., D.M. Sullivan, J. Owen and J.M. Hart. 2019. Soil test interpretation guide, EC 1478.
Jones, G.B., A.D. Moore and E. Smith. 2024. Soil testing lab selection and recommended analytical methods for Oregon. EM 9423.
Koenig, R.T. 2013. Dryland Winter Wheat. Eastern Washington Nutrient Management Guide. EB 1987E.
Lutcher, L.K., C.H. Hagerty and D.R. Kroese. 2024. Zinc supply effects on wheat production in a low precipitation zone. Agrosystems, Geosciences, & Environment.
Lutcher, L.K., N.W. Christensen, W.F. Schillinger, D.J. Wysocki and S.B. Wuest. 2012. Phosphorus fertilization of late-planted winter wheat in no-till fallow, PNW 631.
Sullivan, D.M., D.A. Horneck and D.J. Wysocki. 2021. Eastern Oregon liming guide, EM 9060.
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