Fertilizer Guidelines for Montana Crops

Nitrogen (N), phosphorus (P), potassium (K) and other fertilizers can increase crop yield and quality when soil analysis indicates deficiencies, soil nutrients are unavailable, past history would predict a response, and other agronomic practices are optimum. The following soil analysis guidelines (Tables 1-17 for N, 18 for P, 19 for K, and 20 for micronutrients) are primarily based on research conducted in Montana, but where current or any data is not available, we have used a compilation of research from surrounding states and provinces to develop the tables. These tables provide guidelines in terms of fertilizer rates for a range of yield potentials, available N and soil analysis values for P, K and micronutrients. Fertilizer guidelines assume that growth-limiting factors such as sodium and salts are not limiting growth.

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Nitrogen

The soil analysis for available N is for NO₃-N to a depth of 2 feet. Deeper soil samples for N or other soil mobile nutrients (sulfur, boron and chloride) will improve the reliability of N and other mobile nutrient fertilizer guidelines. When organic matter mineralizes it has the potential to release N into the soil for potential plant uptake. The Montana N fertilizer guidelines assume an average organic matter level of 2%. This is directly incorporated into our recommendations on available N requirements. For soils that have organic matter (O.M.) levels that exceed 2%, additional N will be released to the soil through mineralization at a rate of 15-20 lbs N/a for each 1% of O.M. Therefore, N fertilizer rates can be decreased by 15-20 lbs N/a, if the soil has 3% O.M. or more. With small grain residues remaining on the soil surface and broadcast N applications, 10 lbs N/a can be added per 1000 lb residue/a, up to a total of 40 lbs N/a. Montana research indicates that additional N is not needed. Split N applications may be warranted on coarse-textured soils.

Phosphorous and Potassium

Phosphorus and K guidelines (Tables 18 and 19) are based on a sample taken from 6 inches in depth and assume band placement of fertilizer material. The P and /or K rate may need to be increased for broadcast applications particularly on low to medium testing soils or where past experience has indicated a response to applied fertilizer.

Potassium fertilizer response information is accurate approximately 30% of the time. Therefore, site- and year-specific response information is particularly important. Starter applications (10-20 lbs nutrient/a) are recommended for all crops (particularly spring crops), since the soils are typically cold and ideally wet, which limits the initial availability of residual N, P, and K.

Variability within Sites

The soil analysis values are based on soil samples that represent a field or areas within a field. Interpolation may be necessary to determine the suggested level of a specific nutrient to be applied. Special condition comments are provided to enhance nutrient management practices and, in particular, avoid problems and optimize inputs. P and K recommendations are independent of yield and are based on typical yields for Montana. Table 21 provides crop replacement/removal values for the harvested portion, if needed.

Applying Site-specific Information

When site-specific information is available or is known through actual field experiences, use it to develop unique guidelines for fertilizer applications. The research-based information presented in these tables is from multiple sites over multiple years, spanning unique environments and, ultimately, representing average response information for Montana and the Northern Great Plains. Therefore, this information will not take into account annual variability in climate prior to and during the growing season. Without question, local expertise should be used to ultimately determine fertilizer rates.

Adapting to your Conditions

The guidelines are for a single season of cropping and do not represent a build- or fertilize-the-soil philosophy. Based on economics, soil factors and level of management, a land manager may want to put more P and/or K in a field in a given year. Assuming no soil erosion occurs (for P and K losses) or leaching (for N losses), these agronomic rates will not impact water quality.

Specific differences in soil, climate, management intensity and other unique site factors should be integrated into final decisions on fertilizer rates. General crop removal rates for numerous essential elements are presented in Table 21. These are based on the dry matter unit presented, using the best available data. These should not be considered absolute, since factors such as cultivar, climate and agronomic practices can influence nutrient concentrations.

Sulfur and Micronutrients

Although sulfur (S) guidelines are not presented, deficiencies are increasingly common on Montana soils with low soil S levels, low gypsum levels, or when gypsum is present, but it is positioned in the soil profile out of the rooting zone. Compounding this diagnostic problem is the fact that the analytical procedures for S soil analysis sometimes do not accurately reflect or relate to crop response from S fertilizer applications. Canola and forages have been demonstrated to be responsive to S applications.

General micronutrient guidelines are presented based on a 6-inch sample depth. However, deficiencies are not common, so minimal research has been conducted on micronutrients in the Northern Great Plains.

Table 1. Alfalfa/grass N guidelines.‌

Special Conditions

• Inoculation with nodule forming N-fixing bacteria is advised for establishment of legumes on fields not previously cropped to legumes.
• Inoculation is essential to meet N demand from N fixation, if native inoculum is not sufficient.
• Plowed down alfalfa stubble adds some fixed N to the soil. In general, the first crop after alfalfa or sweet clover will add 35-50 lbs N/a.
• Broadcasting is the most efficient method of fertilizer application on established perennial crops. Recent data show deep band applications of P in old alfalfa stands is effective.
• Under dryland conditions and low P/K soil levels, it would be better to “build up” or increase the available level of P and/or K in the soil before planting alfalfa or other perennial hay crops.
• For established alfalfa when fertilizer is recommended, the above fertilizer is to be applied annually as long as the stand is maintained.
• For all new grass seedings, the above recommendation is for the seeding year and subsequent annual applications. For the seeding year, do not apply more than 20 lbs N/a.
• The above recommendation may be applied for cool season grasses in the late fall or early spring. For warm season grasses, apply about mid-May.
• N fertilization of grass-legume mixtures will usually increase the grass yield in relation to legumes. Legumes will be more competitive, if phosphorus (P) rates are adequate.
• Frequently, the legume percentage in the forage can be increased by applying high rates of P and little or no N fertilizer. If more than 50% of the plants are legumes, assuming good stands and available water, lack of P in soil is the major cause of poor production.

Table 2. Feed and malt barley N guidelines based on soil analysis.‌

Special Conditions

• Drill-row applications of N+K₂O should not exceed 30 lbs/a. When using urea as the N source, drill-row applications of N+K₂O should not exceed 15 lbs/a with a 6-7 inch row spacing. When using a wider row spacing, do not apply any urea with the seed. With newer drills and openers, the mixture of seed, fertilizer and soil is much greater, so more fertilizer can be placed in the “row” due to the dilution of potential detrimental impacts from salts and ammonia on germination and growth.
• Applying N fertilizer on well-drained sandy soils in the fall is not recommended because of possible loss by leaching.
• If protein levels of malt barley produced on your fields have been over acceptable levels, reduce the recommended rates by 20 lbs N/a.
• Fertilization with K at 20 (dryland) to 30 (irrigated) lbs K₂O/a is generally recommended for malt barley regardless of soil analysis.
• Barley grown for hay should be fertilized with the above N guidelines. If any plant stress (e.g. drought) is present, the potential hay should be checked for nitrates.

Table 3. Dry Bean N guidelines based on soil analysis.

Special Conditions

• Dry edible beans are legumes which respond to N fertilizer and are very salt sensitive. If N is applied as a starter, it should not be in contact with the seed. Inoculation is essential to help meet N demand from N fixation. Some bush-type varieties will use more N for pod production compared to vine-type varieties.
• It usually takes several weeks after emergence for legumes to start producing their own N and dry beans are very ineffective in fixing N. Use of higher rates of N, however, may slow maturing and increase harvest problems. Responses to P and K are not always noticeable in the year of application. Approximately 60 percent of the P and 50 percent of the K used by edible bean plants is removed from the field when the seed is harvested.
• Dry bean can be N deficient even though they are legumes. Such deficiency can occur with cool wet growing conditions, especially in the first weeks of growth. Also, poor nodulation and/or inefficient strains of Rhizobia will fix less N than the plants require, resulting in N deficiency.
• Dry bean is sensitive to Zn and Fe deficiencies. High P can induce Zn deficiencies, even at adequate soil test levels.
• With an Fe soil test below 3.0 ppm, Fe availability is low. With an Fe soil test of 3.0-5.0 ppm, Fe availability is marginal. Direct Fe fertilization usually does not produce an economic return, and as the season warms up and the soil dries out, more Fe may become available to the crop. Incorporation of O.M. and improved drainage can help Fe availability. Also, avoid planting dry bean after sugarbeet, particularly if residual soil NO₃-N levels are high.
• Zn recommendation is based on use of inorganic product such as zinc sulfate which is broadcast and plowed down. One application should be effective for 2 to 4 years. Banding near the seed (2x2) has been found to be more effective per lb Zn than broadcast or incorporated applications.

Table 4. Buckwheat N guidelines based on soil analysis.‌

Special Conditions

1. N+K₂O applications in contact with the seed should not exceed 10 lbs/a. N from urea, and DAP (18-46-0) should NOT be placed with the seed.

Table 5. Canola/mustard/rapeseed N guidelines based on soil analysis.‌

Special Conditions

• N+K₂O applications in contact with the seed should not exceed 10 lbs/a. Nitrogen from urea, and DAP (18-46-0) should NOT be placed with the seed.
• Apply 20 lbs S/a as sulfate for each canola crop in a preplant application.

Table 6. Grain and silage corn N guidelines based on soil analysis.‌

Special Conditions

• If starter fertilizer is used, with seed and fertilizer applied together, rates should not exceed 5 to 10 lbs N/a, 10 lbs of P₂O₅/a and 5 lbs of K₂O/a. On sandy soils, such rates may damage germination because these soils are more likely to dry out before the crop is up. If the fertilizer implement places the fertilizer in a band that is to the side and below the seed, you can generally apply the entire recommended rate with the planter.
• Corn is sensitive to inadequate levels of Zn, and deficiency symptoms are occasionally observed in irregular patterns on soils of the Yellowstone Valley, especially where topsoil has been removed by land leveling. Zinc deficiencies may be aggravated by high rates of P.
• Reduce seed placed fertilizer rate by 50%, when soil conditions are dry or sandy, particularly with N as urea, DAP (18-46-0) and ammonium thiosulfate (12-0-0-26).
• Sandy soils may require split N applications.
• Corn is sensitive to Zn and Fe deficiencies. High P can induce Zn deficiencies, even at adequate soil test levels.
• With an Fe soil test below 3.0 ppm, Fe availability is low. With an Fe soil test of 3.0-5.0 ppm, Fe availability is marginal. Direct Fe fertilization usually does not produce an economic return, and as the season warms up and the soil dries out, more Fe may become available to the crop. Incorporation of O.M. and improved drainage can help Fe availability.
• Zn recommendation is based on use of an inorganic product such as zinc sulfate which is broadcast and plowed down. One application should be effective for 2 to 4 years.

Table 7. Flax N guidelines based on soil analysis.‌

Special Conditions

• N+K₂O applications in contact with the seed should not exceed 10 lbs/a. Nitrogen from urea, and DAP (18-46-0) should NOT be placed with the seed.

Table 8. Grass N guidelines based on soil analysis.‌

Special Conditions

• Fall N application on sandy soils is not recommended. On all other soils, apply fertilizer in late fall or early spring. Continued application of N in late fall or early spring will favor the growth of cool season grasses at the expense of warm season grasses in native pasture, to enhance or promote the growth of warm season grasses, apply N in early summer.
• With a fluctuating water table (subirrigation), fertilize as though it is irrigated with a higher yield potential; however, be sure to apply P fertilizers in the fall or late winter.
• Hay meadows with reasonably good drainage can be fertilized any time from early winter to early spring. Wet meadows should be fertilized as close to spring as practical. Experiments show less yield increase from fall applications on very wet soils, but they are often profitable, so late winter fertilization may be the best alternative.
• Split N will generally not increase total production. If seeding on summerfallow, O. M. levels of 3% or more may produce enough plant available N to reduce rate of N fertilizer for 2 or 3 years.
• Do not exceed 60 lbs N/a during the seeding year, or within 9 months after a fall seeding date. If ammonium phosphate fertilizer is banded with the seed, for better efficiency of low rates, do not exceed 15 lbs N/a for 14 inch or less row spacings, and do not exceed 10 lbs N/a for rows spaced 18 inches or more. If N is placed with the seed, do not exceed 15 lbs N/a.
• If P is supplied by ammonium phosphate banded below the seed, do not exceed 15 lbs N/a for 14 inch or less row spacings and do not exceed 10 lbs N/a for rows spaced 18 inch or more. Fall application of P may give better first season response than spring application, particularly for low-medium P soils.

Table 9. Lentil/chickpea/pea N guidelines based on soil analysis.‌

Special Conditions

• Legumes without nodules or with ineffective nodules will respond to N applications. Since legumes have the ability to fix N, it is important to inoculate the seed just before planting. This is especially true on fields that have not been recently planted to either crop.
• It is important to use the proper inoculant for pea and lentil, since specific legumes require specific strains of Rhizobia bacteria.
• Starter applications of 10 lbs N/a have been shown to minimize N deficiency during early nodulation particularly on soils low in clay content or with high levels of small grain residues.
• Pea and lentil crops fix from 20% to 80% of their N requirement and obtain the remainder from the soil or fertilizer.
• K application rates when applied with seeding should not exceed 15 lbs K₂O/a due to the potential for seedling damage. If N is applied, the K₂O rate should be decreased by one pound for each pound of nitrogen added with the seed.
• Montana research has shown that the N benefits following pulse crops (chickpea, lentil, pea) averages about 10 lbs/a, but can vary from 0 to 20 lbs N/a depending upon climate and soil conditions. N contributions from green fallowed pulse crops can be substantially higher, but research has not determined actual numbers.

Table 10. Millet/canary seed/sorghum N guidelines based on soil analysis.‌

Special Conditions

• Drill-row applications of N+K₂O should not exceed 10 lbs/a to avoid the possibility of germination damage.

Table 11. Oat N guidelines based on soil analysis.‌

Special Conditions

• For oats, N+K₂O fertilizers should be limited to 25 lbs/a when placed in contact with the seed in 6 or 7 inch rows. Reduce these values correspondingly for wider row width. Reduce these amounts by half for dry or coarse textured soils.
• Oats grown for hay should be fertilized with the above N guidelines. If any plant stress (e.g. drought) is present, the hay should be checked for nitrates.

Table 12. Potato N guidelines based on soil analysis.‌

Special Conditions

• With an Fe soil test below 3.0 ppm, Fe availability is low. With an Fe soil test of 3.0-5.0 ppm, Fe availability is marginal. Direct Fe fertilization usually does not produce an economic return, and as the season warms up and the soil dries out, more Fe may become available to the crop. Incorporation of O.M. and improved drainage can help Fe availability.
• Zn recommendation is based on use of an inorganic product such as zinc sulfate which is broadcast and plowed down. One application should be effective for 2 to 4 years.

Table 13. Safflower N guidelines based on soil analysis.‌

Special Conditions

• N+K₂O applications in contact with the seed should not exceed 10 lbs/a. Nitrogen from urea, and DAP (18-46-0) should NOT be placed with the seed.
• Drill-row applications of N+K₂O should not exceed 15 lbs/a to avoid the possibility of germination damage.
• Safflower is an excellent N scavenger to depths of 6 feet. Where long-term crop-fallow has been practiced, there is usually sufficient N below 4 feet to supply the majority of N required.

Table 14. Soybean N guidelines based on soil analysis.‌

Special Conditions

• Inoculation with nodule forming N-fixing bacteria is advised for establishment of new legumes or fields not previously used for legumes or combined fields with different cropping histories.
• Soybeans that have been well inoculated are not likely to respond to additional N fertilizer. Inoculation is essential to help meet N demand from N fixation.
• When planted in 30 inch rows, do not apply fertilizer in contact with the seed. When planted in 7 inch rows, limit seed placed N+K₂O to 5 lbs/a, but do not use urea, UAN or DAP (18-46-0) and limit 0-46-0 to 100 lbs/a.

Table 15. Sugarbeet N guidelines based on soil analysis.‌

Special Conditions

• A general guide for spring applied N is to reduce the recommended N rate by 10 pounds for each week that planting is delayed after May 20.
• If the amount of NO₃-N in the 2- to 4-foot depth is more than 30 lbs N/a, the N recommendation should be reduced by 4 pounds for each 5-pound increment above 30 pounds found in the 2- to 4-foot depth.
• 10 to 15 lbs of N/a in the top 6 inches of soil should be adequate to establish the crop.
• Fall applied N fertilizer is not recommended on sandy soils or soils with a high water table.
• Late season release of N from manure can reduce sucrose percentage by stimulating top growth. For this reason, do not apply more than 15 tons of manure/a for a sugarbeets.
• In rotations with sugarbeets, if the tops remain in the field, reduce N requirements by 40 to 50 lbs N/a due to the release of N from this sugarbeet material.
• Sandy soils may require split N applications.
• Ridged beets should have no more than 80 lbs N/a applied preplant broadcast. This is more critical with urea compared to ammonium nitrate.

Table 16. Sunflower N guidelines based on soil analysis.‌

Special Conditions

• Some N may be applied in combination with starter fertilizers, but the rate should be less than 10 lbs N/a. Most efficient use can be obtained by applying N just ahead of planting. However, apply all of the fertilizer before heading (bud stage) to maximize yields and N use efficiency.
• Sunflower roots grow quickly into the soil between the rows. Sidedress N fertilizers early in the growing season to avoid root pruning.
• When planted in 30 inch rows, do not apply fertilizer in contact with the seed. When planted in 7 inch rows, limit seed placed N+K₂O to 5 lbs/a, but do not use urea, UAN or DAP(18-46-0) and limit 0-46-0 to 100 lbs/a.
• Sunflower is an excellent N scavenger to depths of 6 feet. Where long-term crop fallow has been practiced, there is usually suf?cient N below 4 feet to supply the majority of N required.

Table 17. Spring and winter wheat N guidelines based on soil analysis.‌