Fertilizer Rate Calculations

Fertilizer need = Crop need – Available soil nutrient content +/- Adjustments

This guide explains the adjustments and summarizes how to calculate optimal fertilizer rates for crop yield and to reduce loss of nutrients to the environment. The guidelines presented are considered the best averages for the entire state. Adjust fertilizer rates up or down if the published guidelines consistently under- or over-fertilize. See Developing Fertilizer Recommendations for Agriculture for more information on fertilizer calculations.

Soil Analysis Conversion

Soil test laboratories report nutrients in parts per million (ppm) or pounds per acre (lb/acre). Determining N application rates requires the conversion of ppm to lb/acre if the lab does not report nitrate-N in lb/acre (Calculation Box 1).

Fertilizer Recommendations

Nitrogen

The total available N needed (soil nitrate-N + fertilizer N) is based on the crop and yield goal. Guidelines for most Montana crops are available in Fertilizer Guidelines for Montana Crops and crop specific Extension bulletins and MontGuides. Fertilizer N is calculated by subtracting soil nitrate-N (from lab report) from the total available N needed and including any adjustments, which include:

1. Soil organic matter (SOM): This relates to how much N will be released during the growing season. Decrease N rate by 15-20 lb N/acre for each percent SOM above two percent (up to 5%). Increase N rate by 20 lb N/acre if SOM < 1%.
2. N ‘credit’: Residue remaining on the field from legumes and brassica vegetables (e.g., cauliflower), contribute to total available soil N. The N credit for legumes is subtracted from the fertilizer need as follows:
   • Perennial legume: 40 lb N/acre
   • Annual legume for 1 rotation: 10 lb N/acre
   • Annual legume for 3 or more rotations: 20 lb N/acre
   • Legume cover for 1 to 2 rotations: 20-30 lb N/acre
   • Legume cover for 3 or more rotations: 30-50 lb N/acre

Soil sample in the spring after brassica vegetables.

3. Immobilization: Non-legume surface stubble tends to slow decomposition and tie up (immobilize) soil- and surface-applied N. Therefore, apply more N the first few years after conversion from till to no-till (Calculation Box 2). For more details see Nutrient Management in No-till and Minimum Till Systems.

   CALCULATION BOX 2. Determining N adjustment for remaining stubble

   CALCULATE STUBBLE REMAINING Stubble remaining = total stubble (lb/acre) - stubble removed (lb/acre) Total stubble can be estimated as 100, 80, 54, 85, and 60 lb stubble per bushel winter wheat, spring wheat, barley, canola, and corn respectively. CALCULATE N ADJUSTMENT FOR STUBBLE REMAINING N adjustment = 10 lb N/1000 lb x stubble remaining (lb/acre)                           =  X  lb N/acre (add X to N rate, up to 40 lb N/acre)

4. Soil sampling time: MSU guidelines assume soil sampling is done in the spring. Fall samples may not give an accurate representation of what will be available at spring seeding. Decomposition of SOM and crop residue from fall to spring can increase soil nitrate-N on average by about 25 lb N/acre following broadleaf crops and 15 lb N/acre following fallow and small grains. Subtract these amounts from fertilizer N need IF you sample in fall (Calculation Box 3). Nitrogen release from some crops such as broccoli and cauliflower can be much higher, but also variable. Conversely, soil N can be lost over winter to leaching or denitrification (N-gas loss). Unknown amounts of overwinter N gains and losses are why spring sampling is preferred.
5. Sampling depth: Most nitrate-N uptake occurs in the top 2 feet of soil. To account for any available nitrate-N below 2 feet, assume 50% of nitrate-N at depths from 2-4 feet will be taken up. 

Phosphorus & Potassium

There are 3 approaches used to determine P and K fertilizer rates:

1. Sufficiency: Apply the minimum amount of fertilizer necessary to maximize yield. Tables 18 and 19 in Fertilizer Guidelines for Montana Crops  provide rate guidelines based on crop and soil analysis.
2. Maintenance: Replace only the nutrients removed at harvest, generally used when P soil test levels are above 16 ppm, and K above 250 ppm. Nutrient removal estimates are available at https://landresources.montana.edu/soilfertility/nutuptake.html and Table 21 in Fertilizer Guidelines for Montana Crops.
3. Build: Because P and K are relatively immobile in the soil, their levels can be ‘banked’ in the soil over time to save on fertilizer in future seasons. Do not bank P above 30 ppm, or K above 300 ppm.

Sulfur

MSU fertilizer guidelines for S are being updated. For now:

1. Fertilize brassicas with 7-10 lb S/acre regardless of soil S test. ​
2. Fertilize lower S need crops (e.g., pulses, small grains, non-brassica vegetables) with 5 lb S/acre if sulfate-S soil test in top 2-feet is < 40 lb S/acre (< 5 ppm) ​
3. For a maintenance rate, match fertilizer S amount with aboveground S removal (e.g., 4 ton alfalfa removes ~20 lb S/acre). See Fertilizer Guidelines for Montana Crops(Table 21) or https://landresources.montana.edu/soilfertility/nutuptake.html

It is economically riskier to under-fertilize with S than over-fertilize in soils that have the potential to have a yield increase from S. If unsure, use field test strips with soluble S fertilizer (not elemental S) to determine whether S fertilizer will increase yields or protein.

Micronutrients

Montana’s micronutrient fertilizer guidelines (for B, Cu, Fe, Mn, and Zn) are independent of crop. Boron and Mn deficiencies are uncommon in Montana. General guidelines for micronutrient fertilizer rates based on soil test results are given in Developing Fertilizer Recommendation for Agriculture  . Soil analysis is not a good predictor of Cl fertilizer need.  Wheat is the most sensitive to Cl deficiency. Apply about 20 lb Cl/acre if plants appear Cl deficient or soil levels are below 12 lb Cl/acre. Applying 50 lb KCl (0-0-60-50Cl)/acre should meet wheat’s Cl needs.

Fertilizer Rate and Costs

Because fertilizers are not 100 percent nutrient, the fertilizer rate is not the same as the nutrient need. To calculate fertilizer rate and cost of nutrient supplied per ton or per acre, the percentage of a nutrient in the fertilizer (grade) needs to be considered (Calculation Box 4).

For more information:

Resources mentioned can be found at MSU Soil Fertility Extension Bulletins and MontGuides

Posted 2019, revised September 2025