Interpreting Soil Test Results
Whether using precision or standard sampling methods, soil test results ultimately serve as the basis for making P and K rate recommendations. Soil test interpretations and fertilizer recommendations vary among regions and states, but most approaches can be described in terms of two dominant fertility paradigms, nutrient sufficiency and build and maintain:
Nutrient Sufficiency is a philosophy for P and K fertility that focuses on applying the minimum amount of fertilizer needed to maximize profitability in the year of application, with no concern for future soil test values or fertilizer requirements. Generally, recommendations based on nutrient sufficiency will provide 90 to 95% of maximum yield and a high rate of return per unit of fertilizer applied. The nutrient sufficiency approach is most logical when:
- fertilizer prices are high relative to grain prices
- resources are limited in a particular year
- growers are operating under a short-term land tenure situation
- soils have a high capacity to convert readily available P and K to forms that are unavailable to crops in the short-term
Disadvantages of nutrient sufficiency-based recommendations include:
- the need for regular and accurate soil testing
- precise knowledge of optimum application rates
- risk of limiting long-term crop productivity
Build and Maintain fertility programs contrast with the nutrient sufficiency approach in that they are not intended to maximize economic returns in any given year. Rather, they are designed to provide flexibility and consistent economic returns over the long-term by removing P and K as yield-limiting factors. At low soil test levels, build and maintain recommendations focus on increasing P and K to the critical test level and maintaining soil nutrient supply at or above this point through application of additional fertilizer to account for crop removal (see Table 1 for critical levels and crop removal rates). Build and maintain programs also advise that fertilizer be applied to account for crop removal in the optimum soil test range. Generally, recommendations based on a build and maintain philosophy will provide 100% of maximum yield with low risk of yield loss due to insufficient fertility.
Table 1. Critical P and K soil test levels and crop removal rates for corn and soybeans (Warncke, et al., 2004).
1Critical soil test level for K = 75 + (2.5 x CEC) for corn and soybean.
The build and maintain approach is attractive when:
- grain prices are high relative to fertilizer
- recent or complete soil test data are unavailable
- crop yields are expected to increase in the future
- resources are currently available and fertilizer
prices are expected to increase in the future
Disadvantages of build and maintain-based recommendations include:
- higher fertilizer cost during the build phase when soil test levels are below the critical value
- risk of sub-optimal economic return in a given year
It is important to note that application of P and K at higher than economically optimal rates in a particular year can offset fertilizer requirements in future years. Both P and K are relatively stable in soils and can be, "banked" for later use if economically advantageous.
Equipment advances allow for accurate fixed- or variable-rate application of dry fertilizer. Photo courtesy of Case-IH.
Rate recommendations for P and K fertilization based on the nutrient sufficiency and build and maintain paradigms are presented in Table 2 and Table 3.
Table 2. Phosphorus rate recommendations for corn and soybean based on nutrient sufficiency and build and maintain approaches. Adapted from Warncke, et al., 2004.
1Calculated as: [(Critical level - soil test) x5]
2Nutrient Sufficiency + crop removal at 175 bu/acre yield
3Nutrient Sufficiency + crop removal at 60 bu/acre yield
4Fertilize high-testing soils only under favorable crop and fertilizer prices or as a band at planting
Table 3. Potassium rate recommendations for corn and soybean based on nutrient sufficiency and build and maintain approaches. Adapted from Warncke, et al., 2004.
1Recommendations based on a K critical value of 125 ppm at CEC of 20 meq/100 g
2Calculated as: [(Critical level - soil test) x5]
3Nutrient Sufficiency + crop removal at 175 bu/acre yield
4Nutrient Sufficiency + crop removal at 60 bu/acre yield
5Fertilize high testing soils only under favorable crop and fertilizer prices or as a band at planting
It is often said that nutrient sufficiency recommendations focus on feeding the crop, while build and maintain recommendations focus on feeding the soil. Both approaches are valid. The decision to adopt one strategy over another ultimately depends on market conditions, management style and risk position (Leikam et al., 2010).
In reality, P and K rate recommendations provided by most university extension services incorporate elements of both nutrient sufficiency and build and maintain strategies. For example, Iowa State University's recommendations fall between strict interpretation of either paradigm at low soil test levels, but conform to the build and maintain philosophy at and above the optimum soil test range. In contrast, Michigan State University's recommendations embrace a build and maintain philosophy but include underlying equations allowing users to determine rate recommendations based on either approach. Kansas State University offers separate nutrient sufficiency and build and maintain rate recommendations.
Regardless of which paradigm or set of guidelines is used to develop rate recommendations, the following general rules of thumb apply (Figure 3):
- Always fertilize when soil test levels fall below the optimal range. Risk of yield loss is high and return to fertilizer investment is greatest for very low- and low-testing soils.
- Avoid application on high-testing soils and never apply on soils that test in the very high range. Return on fertilizer investment decreases as soil test level increases.
- When in doubt, fertilize based on expected rates of crop removal.
Figure 3. Fertilizer response, risk factors and general fertility guidelines with respect to soil test category.