BOZEMAN — As winter wheat harvest in Montana comes to a close, producers may be thinking about fertilizer decisions for upcoming winter wheat seeding. Nitrogen, phosphorus, potassium, sulfur and chloride should all be considered in fertility programs, according to Clain Jones, soil fertility specialist with Montana State University Extension and professor in the Department of Land Resources and Environmental Sciences in the College of Agriculture.
Phosphorous and potassium
Phosphorus and potassium are immobile nutrients in the soil and tend to be relatively stable in a field over time, Jones said. Thus, the timing of soil testing for phosphorous and potassium is less important than for more mobile nutrients, like nitrogen.
“These nutrients are best applied below the soil surface near winter wheat seed because placing them in the root zone will maximize availability to the crop,” Jones said. Producers should base phosphorus and potassium fertilizer applications on fall soil tests in the top 6 inches for “Olsen P” and “exchangeable K,” two lab tests that estimate the amount of each nutrient available to plants.
According to Jones, phosphorus is important for healthy root growth and successful overwintering of winter wheat. If soil tests show Olsen P levels of less than 24 parts per million, then winter wheat can benefit from phosphorous fertilizer. Previous MSU research has found that because phosphorous can help winter wheat survive Montana’s harsh winters, phosphorous fertilizer is needed for winter wheat more than most other crops grown in the state, which need Olsen P levels to be approximately 16 parts per million.
Phosphorus fertilizers can be applied in fairly high amounts in the seed row as long as minimal nitrogen and potassium are applied with the seed, Jones said. Common phosphorous fertilizer sources are monoammonium phosphate (MAP; 11-52-0) and liquid ammonium phosphate (10-34-0).
Potassium deficiencies are generally rare in Montana but can occur in coarse-textured soils or in fields where straw is frequently removed. Soil potassium test levels above 250 parts per million are adequate for winter wheat production. However, winter wheat can respond to potassium fertilizer (potash; 0-0-60) at potassium test levels above 250 parts per million, especially in no-till soils that tend to be cooler than tilled soils in spring.
“Potash is best used with a subsurface-banded application to get the potassium in the root zone,” Jones said. “If applied in-furrow with the seed, limit the total amount of nitrogen and K2O, or potassium oxide, to less than 20 pounds per acre. Potash is also effective when broadcast at seeding.”
Chloride is a micronutrient that winter wheat producers should consider in fertility programs. Certain winter wheat varieties (Bobcat, CDC Falcon, Raptor, Peregrine and Promontory) are susceptible to physiological leaf spots in chloride-deficient soils, according to Uta McKelvy, plant pathologist with MSU Extension and assistant research professor in the Department of Plant Sciences and Plant Pathology.
“Physiological leaf spots closely resemble fungal leaf spots such as tan spot, but they are not caused by pathogens,” McKelvy said. She pointed out that “physiological leaf spots tend to develop most severely on the flag leaf, although it may be observed before flag leaf emergence. In contrast, fungal leaf spots are mostly found on older leaves lower in the canopy. Physiological leaf spots may also occur more uniformly across a field, while fungal leaf spots are generally associated with areas of high residue in the field.
“Distinguishing between physiological leaf spots and fungal leaf spots is difficult but important when deciding if a fungicide application is necessary,” McKelvy added. She recommended individuals contact their local Extension agent for help identifying physiological leaf spots. Whole plant samples can also be sent to MSU’s Schutter Diagnostic Lab to identify the cause of leaf spots.
“If planting a variety known to display physiological leaf spots, or if physiological leaf spots have been observed in a field before, producers should consider applying 10-20 pounds potash per acre, which contains 4.5-9 pounds chloride per acre, to mitigate chloride deficiency,” Jones said.
He added that chloride is very mobile, so it can be applied as a broadcast application if there are concerns with salt damage if applied in the seed row. To verify that cereals have chloride deficiency, a whole plant sample or just flag leaves can be analyzed for chloride. Tissue levels of chloride below 0.1% are likely deficient, although once the flag leaf has physiological leaf spots, even a rescue treatment of chloride will not reverse the physiological leaf spots. Jones pointed out that chloride tissue testing can still be useful to identify the need for chloride in future growing seasons.
Nitrogen and sulfur
Producers should wait to apply nitrogen and sulfur until spring, Jones said, as these nutrients can leach in shallow or coarse soils, and test levels can change dramatically over winter. MSU fertilizer guidelines for nitrogen are based on spring soil nitrate tests.
“This can require producers or their advisers to soil sample twice: once in the fall for phosphorous and potassium and again in the spring for nitrogen,” Jones said. “Since this is not very practical, fall nitrate levels can be adjusted based on typical differences between fall and spring nitrate on a farm.” He added that he has generally found that nitrate increases about 15 to 25 pounds per acre from late summer to early spring, but on shallow soils in wet years, nitrate levels can decrease over winter. Over- or under-applying nitrogen can affect the bottom line much more than the cost of soil sampling and analysis.
Soil sulfur tests often do not predict crop response to sulfur fertilizer very well, according to Jones. Therefore, producers should consider other factors to determine if sulfur fertilization is necessary. Sulfur deficiency is more likely in coarse, shallow soils and in soils with low organic matter. “If past yields or protein levels have been unexpectedly low and don’t respond to additional nitrogen fertilizer, sulfur may be deficient,” Jones said. Strip trials with gypsum or ammonium sulfate can determine if winter wheat is responsive to sulfur in a given field.
More resources available
MSU Extension has several resources for making fertilizer decisions available at store.msuextension.org, including Montana Wheat Production Guide, Fertilizer Guidelines for Montana Crops, Interpretation of Soil Test Reports for Agriculture and Developing Fertilizer Recommendations for Agriculture.
The MSU Fertilizer Recommendation tool (sarc.montana.edu/php/soiltest) and the Economic Nitrogen Calculator (msuextension.org/econtools/nitrogen/index.html) are online tools to help calculate fertilizer requirements. Questions about this or other soil fertility topics may be directed to Jones at firstname.lastname@example.org or 406-994-6076 or may be addressed by visiting the MSU Extension soil fertility website at landresources.montana.edu/soilfertility.
For questions on identifying physiological leaf spots and crop diseases, contact McKelvy at email@example.com or call 406-994-5572. Information on sample submission to the Schutter Diagnostic Lab can be found on the lab’s website at diagnostics.montana.edu/contact.html.
MSU News Service