Soil and Nutrient Management for Spring Wheat

KEY POINTS

• Nitrogen is often the most limiting nutrient for spring wheat production. Nitrogen deficiency can cause lower yields, lower protein levels in the grain and leaf yellowing, starting with the older leaves.
• Spring wheat has a low phosphorus demand, but the lower requirement needs to be met.
• Soil tests can indicate the availability of potassium, but they are not a direct measurement of the amount of potassium in the soil.
• If needed, phosphate and potash fertilizer should be incorporated before seeding.
• Banding fertilizer can reduce weed densities compared to broadcast applications.
• Wheat grows best in well-drained, slightly acidic to neutral pH soils like loam or clay loam. Wheat can also grow in sandy soils if effectively managed and irrigated.
• Soil pH should be between 6.0 and 7.5 for optimal nutrient availability. Low soil pH is a frequent problem in no-till wheat production.

Introduction

Spring wheat is a crop that is primarily grown across the northern tier of the western United States and across the western Canadian provinces. Spring wheat is better adapted than winter wheat to these production areas because of the inability of winter wheat to reliably survive the harsh winter growing conditions. Timely planting is particularly important for reaching crop maturity before the hot and dry weather common in early summer forces the crop to finish which can reduce yield potential and lower test weights. Understanding the fertility requirement of the crop and timing fertilizer applications to maximize yield while reducing potential fertility losses depends on soil type and other factors that affect nutrient utilization.

Soil Fertility

Soil Testing

A soil testing program provides valuable information for the fertility requirements required for a high yielding spring wheat crop. Accurate soil testing requires representative samples, accurate laboratory testing, and meaningful interpretations.

Representative Samples

There are important soil sampling guidelines and procedures that need to be followed to ensure soil testing accuracy. These procedures are critical to get the results needed to make effective fertility management decisions. A good soil sampling guide can be found at Soil Testing 101.

Accurate Lab Analysis

Using a local soil testing lab can help accuracy as a local lab understands the soil characteristics native to the area and can use testing procedures to better predict the availability of nutrients important for plant growth.

Meaningful Interpretations

Soil test reports and fertilizer recommendations from different labs are often confusing. That is why it is important to use the fertility recommendations from the soil testing lab used, for the crop indicated, and at the desired yield goal. Nitrogen (N) is often the most limiting nutrient and should be the first nutrient to address. The soil test will determine the amount of N that is in the soil sample and subtract it from the total required N. Any N credits from a previous crop or manure applications should be deducted from the total N required to reach the yield goal. Except for N recommendations, most soil testing labs report the soil test results as parts per million for each nutrient. Results for nutrients other than N will often be listed as a very low, low, medium, high, or very high levels. Soil testing labs use years of fertility studies to correlate the lab results, to expected crop response and to various fertilizer application rates. Recommendations for fertilizer application amounts for each nutrient are provided for the crop to be grown at the yield goal desired.

Fertilizer Management for Spring Wheat

As indicated earlier, N is often the most limiting crop nutrient for spring wheat production. Fertilizer calculators are available to help producers develop a N fertilizer plan. Washington State University developed a spring wheat N fertilizer calculator, called the Dryland Wheat Nitrogen Fertilizer Calculator², to help make N fertilizer recommendations for the producers in the Pacific Northwest. For the Northern Great Plains, a calculator from North Dakota State University may be helpful: Crop Nitrogen Calculator.

Split Application of Nitrogen on a Spring Wheat Crop

There are several reasons why splitting N applications in spring wheat can be advantageous. Spring wheat yield potential has increased substantially, leading to an increase in N requirements. Additionally, higher protein levels in spring wheat are important to the end user and can be increased by a late application of N around the flag leaf growth stage, increasing protein levels and generating a premium at the grain elevator. Lastly, by using a split N application strategy, it allows the producer to better manage the crop to respond to the yield potential of the crop while limiting the potential for N losses due to leaching or denitrification.

The N requirements for a spring wheat crop roughly starts with two pounds of N for each bushel of wheat.³ A 60-bushel wheat crop requires 120 total pounds of N per acre. If the total N requirement is applied prior to planting, producers should consider potential for N losses under certain conditions. Sandy soils with excessive spring moisture can push N below the root zone of the growing wheat plants and be lost to that crop. In heavy clay soils, a second type of N loss might be seen. If there is excessive spring moisture, that early applied N can be lost by denitrification, as the N is converted to a gas in water saturated soils and lost into the atmosphere.

Split Nitrogen Application Program

A split N application program begins with soil test results along with any N credits to determine the total N required for the expected yield. The first N application is a preplant application of N that is often 30-50% of the total fertilizer N required. This N application helps the crop get off to a good start and help promote tillering. This strategy reduces the initial N applied which reduces the potential for N leaching during early crop growth.

The first top-dress N application helps drive yield. This application is made around the early jointing growth stage (Feekes 6) characterized by the appearance of the second node (joint) above the soil surface and rapid spike growth. An additional 20 to50% N can be applied at this stage. Yield potential should be evaluated prior to application and a higher rate of N applied if a higher-than-normal yield is expected. Lower rates can be applied on fields with lower-than-expected yield potential.

A second top-dress N application is applied at the flag leaf growth stage (Feekes 8). This supplies the last 20% of N required and helps increase the protein level of the grain. This late top-dress application will not increase the yield potential under most growing conditions.

When applying N to a growing wheat crop, there is a potential for leaf burn to be observed. There are several ways to help reduce leaf burn potential, including:

1. Applying liquid N with a steamer nozzle to minimize N application on green leaf tissue.
2. Applying granular N fertilizer with a dry fertilizer applicator.
3. Applying urea ammonium nitrate (UAN) through a center pivot irrigation system if available.
4. Applying a diluted 1:1 mixture of liquid N fertilizer and water to help reduce the concentration.
5. Applying N when the weather is cool, non-humid, and calm and avoid top-dressing N on hot, windy days.⁵

Additional Fertility Recommendations

Using soil test results and fertilizer recommendations from the lab, any additional fertilizer should be applied prior to spring wheat planting or added to an early N top-dress application, depending on the nutrient being applied.

Phosphorus (P) is essential for root development, tillering and resistance to winterkill. Since it is a very immobile nutrient in the soil it is important to place the P fertilizer in the soil at a depth where wheat roots can reach it. If soil test levels are extremely low, a band application near the seed placement may allow for a one-third reduced application rate but this reduced application rate will not increase soil test results over time.

Potassium (K) should also be applied before planting if soil test levels are low. Many soils have enough K levels in the soil to support a high yielding spring wheat crop. The exception to this general observation is when the crop is grown on sandy soils or fields that have a history of many years of continuous soybean production.⁴

Sulfur (S) is the third major crop nutrient that may need to be included in a nutrient management plan for spring wheat. This is one nutrient that a soil test may not be a good predictor for a crop response to a deficiency. S is very mobile in the soil, and if the crop is being grown on a sandy or sandy loam soil with less than 3% organic matter, this along with above normal precipitation, can reveal a S deficiency. Sulfur deficiencies often show up during early spring growth as stunted plants, yellow growth, and thin spindly stems, which can result in delayed maturity. A crop exhibiting these symptoms as it is breaking dormancy and greening up, can benefit from a base application of ten pounds of S in an early top-dress fertilizer application. Ammonium thiosulfate (ATS) fertilizer is a liquid formulation of S that is highly plant-available but can cause leaf burn like some liquid N sources and must be managed to address these concerns.

Copper (Cu) is a nutrient that has seen only sporadic yield response and only then on a low-organic matter, sandy soil. If Cu is applied as a copper sulfate application, a rate of five pounds of Cu per acre pre-plant can help correct this deficiency. This amount of Cu fertilizer applied can last for multiple years.

Chloride (Cl) fertility response for spring wheat and durum is well documented across the region. When a Cl deficiency is corrected, a yield increase of two to five bushels per acre has been observed. This yield increase is often attributed to resistance to certain leaf and root diseases which shows up as an increase in kernel size.

Other crop nutrients such as zinc, iron, manganese, or boron have not commonly shown a yield response when added to a spring wheat fertility program. It is thought that most western soils supply enough of these nutrients and supplemental application is unnecessary.

Soil Management

Spring wheat can be grown successfully on most soil types. A fertile, deep, loam soil with good internal drainage is often considered the best soil to grow a spring wheat crop. However, many other soil types that include various amounts of sand, silt, or clay, and different fertility and management requirements, can successfully grow a spring wheat crop if managed correctly. There is one soil type, classified as a peat soil, which can have high levels of iron, sodium, and manganese, has been identified as unfavorable for spring wheat production.⁶

Soil pH Requirements for Spring Wheat Production

The optimum soil pH for spring wheat production is between 5.5 and 7.5.⁶ Soil test results can establish the pH of the soil sampled and determine the amount of lime required to correct a low pH soil or acidic soil condition. Growing spring wheat in acidic soils with a pH of less than 5.5, can lead to increased solubility of aluminum, manganese, and iron which can be toxic to the plant and reduce plant growth and yield potential. A low pH soil can be adjusted to recommended values by liming the soil in the fall prior to planting with a limestone (calcium carbonate) or magnesium carbonate application. The amount of lime required depends on the initial soil pH, soil type, and the buffering capacity of the soil along with the quality of the agricultural lime source (how finely the lime has been ground). Spring wheat produced on alkaline soils, with a pH of more than 7.5, can cause a reduced availability of certain micronutrients such as iron, zinc, and manganese. Also, the availability of phosphorus can vary as the pH of the soil increases.

Salinity Concerns

Saline soils is a problem that can show up when the crop is irrigated with a water source that has a high salt load and the soil has poor internal drainage. Higher levels of soil salinity lead to decreased moisture absorption from the soil into the plant, impeding plant growth. Soil salinity is measured by the electrical conductivity (EC) of the soil solution water in the soil. Spring wheat has above average soil salinity tolerance. This means that a soil with an EC of 6.0 will not have a yield effect on spring wheat crop, while that same 6.0 soil EC will have a 50% relative yield decrease for a corn crop. When growing a spring wheat crop in a soil with a salinity of 13.0, a relative yield decrease of 50% can be expected.⁷

Sources:

¹ Curell, C., and Charles, C. 2022. A field guide to soil sampling. Michigan State University. MSU Extension Soil Health. https://www.canr.msu.edu/resources/a-field-guide-to-soil-sampling.

² Esser, A. 2023. Using the spring wheat nitrogen fertilizer calculator. Washington State University. WSU Dryland Cropping Systems Team. https://smallgrains.wsu.edu/using-the-spring-wheat-nitrogen-fertilizer-calculator/.

³ 2023. Soil fertility and plant nutrition. Servi-Tech. Crop Files. Agronomic Commodity Crops. https://cropfile.servitech.com/crop-files/1-00-soil-fertility-plant-nutrition.

⁴ Franzen, D.W. 2022. Fertilizing hard red spring wheat and durum. North Dakota State University. https://www.ndsu.edu/agriculture/ag-hub/publications/fertilizing-hard-red-spring-wheat-and-durum.

⁵ De Olivera Silva, A. 2021. Time to top dress wheat. Oklahoma State University. https://osuwheat.com/2021/01/15/time-to-topdress-wheat-2/.

⁶ Cherlink, V. 2023. How to grow wheat efficiently on a large farm. ESO Data Analytics. Crop Cultivation. https://eos.com/blog/growing-wheat/.

⁷ Cardon, G.E., Davis, J.G., Bauder, T.A., and Waskom, R.M. 2014. Managing Saline Soils. Colorado State University Extension. https://extension.colostate.edu/topic-areas/agriculture/managing-saline-soils-0-503/.

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