Soil Testing Methods and Fertilizer Recommendation Strategies

by | Jan 4, 2019 | Ag

Reviewing and interpreting soil tests is a significant aspect of many agronomists’ responsibilities. This information is relied on to provide guidelines and recommendations to growers for improvements in crop production, often in both quantitative and qualitative ways.

Why you should know the difference between soil tests

Soils can contain higher amounts of nutrients than represented on a soil test. The lab results describe the nutrients available for plant growth and are estimates based on different testing methods. Different labs use different testing methods and it is important to know if the methodology used is appropriate for the specific conditions of your growers’ fields.

For example, phosphorus is often measured using a Bray-1, Mehlich-3 or Olsen test.

With the wide range of soils, parent materials and climate differences, field testing of crop response to added nutrients are necessary. This correlation allows for crop response to be predicted with a given soil test value. In general, labs that exist in your geography are already calibrated for the right test, however, you should be proactive in ensuring this is the case with the investment your customers are putting into the soil testing process.

Some various approaches to fertilizer recommendations include:

  • Buildup and maintenance
  • Sufficiency
  • The basic cation saturation approach

Buildup and maintenance

The build and maintain approach uses the strategy of maintaining soil fertility values for future years. The goal is to apply more nutrients than the crop removes so that the soil nutrient level does not limit yield. Nutrient availability in the soil is increased over time for future years. This approach requires more fertilizer, which decreases the risk of nutrient deficiencies.


The goal of the sufficiency approach method is to maximize profitability in a certain year by minimizing fertilizer application and costs. When soil test values are low, recommended fertilizer rates are higher than the nutrient level that the crop will remove.  When soil test values are above the critical soil test level, (established via field testing), recommendations will decrease to zero. This is the approach used by most laboratories and universities for their fertilizer recommendations but is focused on phosphorus and potassium.

The basic cation saturation approach

The basic cation saturation approach assumes that a specific ratio of cations must exist in the soil to achieve maximum yield. This typically results in a goal for 65-85% calcium, 12-15% magnesium and 3-5% potassium. Drawbacks to this approach include its unsuitability for use on sandy soils due to their low holding capacity and very little correlation between the suggested ratios and crop yields.

Having a basic understanding of soil test methods used for the nutrients you are recommending to your growers and the recommendation strategy utilized are important factors in being a successful agronomist. They are even more critical during times of economic challenges, such as what growers are faced with today.

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