I Have My Soil Test Results - Now How Much Fertilizer Should I Apply?

Why is it that you sometimes receive different fertilizer recommendations from the same soil test results? Agronomists and farmers may use different assumptions and objectives when determining how much fertilizer and manure to apply. The main approaches for determining fertilizer application rates from a soil test result include:

  1. sufficiency; and
  2. build-up (or draw-down) and maintenance.

Sufficiency Approach

The sufficiency approach rate is the most economic during the year of application. The value of the yield response is expected to pay for the cost of the fertilizer within that year. The sufficiency approach is designed to minimize the cost of fertilizer relative to yield within a given year. It requires frequent soil testing and annual fertilization, unless the soil test is high. It is most suitable for short-term decisions, including annual land rental agreements or limited cash flow. It is associated with a low risk of over-fertilization. Recommendations in OMAFRA Publication 75, Agronomy Guide, use the sufficiency approach.

Build-up & Maintenance

Sometimes also known as "target and maintenance" it is a longer-term approach that targets an ideal soil test value to meet the crop needs. Then over time, "builds-up" or "draws-down" the soil test to this level and then maintains it.

  • For build-up of a low testing soil to target level, the rate is calculated by determining and adding:
    • the crop removal amount estimated from the expected yield, and
    • the additional fertilizer needed to increase the soil test to the target level over time.

  • Once in the target range, the soil test level is maintained by applying the amount removed by the crop.
  • Above the target level, no fertilizer is applied, allowing soil level to draw down to the target range.

Suggested target soil tests levels are 15 ppm P and 125 ppm K. Within ranges (12 - 18 ppm P, 100 - 150 ppm K) the recommended fertilizer rate for maintenance would be the same as crop removal. For suggested crop removal rates, refer to Table 9-14, Publication 811, Agronomy Guide.

Above the target soil test ranges, no fertilizer would be applied until the levels are drawn-down into the ranges. Below the target soil test ranges, fertilizer amounts would be made by adding the removal rate amounts and an amount that will build-up the soil test level over time to the target level. Although the required build-up and draw-down amounts will vary from soil type to soil type, it is generally assumed for these calculations that to move a soil test (up or down) by 1 ppm, it is equivalent to:

  • 35 lb/acre P2O5
  • 20 lbs/acre K2O.

The target and maintain approach is more suitable for owned land and long term leases. It provides long term returns from the investment in building soil test nutrient values into the optimum range. It is associated with some risk of over-fertilization. Regular soil testing is required to verify that crop removal and soil test response estimates are reasonably accurate. The length of the build-up time (eg. 1 - 8 years) will determine how the cost will be amortized. Also, this approach provides some financial flexibility by:

  • deferring fertilizer purchases to when cost is lower relative to crop revenue
  • more application during years with crops more responsive to freshly applied fertilizers or when efficient fertilizer application is more feasible (such as use of equipment capable of sub-surface placement).

Example Calculations Using the Build-up Approach

Assume a soil test of 10 ppm P and 90 ppm K.

Target soil test:

  • 15 ppm P
  • 125 ppm K

Build-up amounts:

  • Assuming a 5 year build-up period
  Desired change in soil test (ppm)
[target -actual]
Total nutrients to build-up to target (lb/ac)
[soil test ppm change X nutrient addition per change in ppm]
Annual nutrient rate for build over 5 yr (lb/ac/yr)
[total nutrient # years]
P 15 -10 = 5 ppm 5 ppm X 35 lb P2O5/ppm=175 P2O5 175lb P2O5/ac÷ 5 yr = 35 P2O5
K 125 -90 ppm = 35 ppm 35 ppm X 20 lb K2O/ppm=700 K2O 700lb K2O/ac÷ 5yr = 140 K2O

Crop removal rates (maintenance) of 40 bu/ac soybeans

  • 40 bu/ac X 0.84 lb P2O5/bu = 34 lb/ac P2O5
  • 40 bu/ac X 1.4 lb K2O/bu = 56 lb/ac K2O

Fertilizer Application Rates (during the soybean year)

  • total of maintenance + build-up amounts
  • P2O5 34 lb/ac + 35 lb/ac = 69 lb/ac
  • K2O 56 lb/ac + 140 lb/ac = 196 lb/ac

For more information:
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E-mail: ag.info.omafra@ontario.ca