Interpreting Soil Sample Results

  • Soil analysis is a great tool to assess what soil amendments are needed for optimum plant growth and yield potential.
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  • Soil test results list the soil test concentration for specific parameters along with an interpretation value (low, optimum, and high) and a recommendation.
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Soil Sampling and Lab Testing

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To obtain quality soil test results, soil samples must be taken properly. Each sample should be representative of the entire field or specified sampling unit. Samples must be taken at the proper depth during the same time frame every year. Sample depth can vary by test, but are usually 6 to 12 inches. For more information on best management practices refer to the agKnowledge Spotlight - Soil Sampling.

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Lab Results

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Table 1. Phosphorus recommendations for corn and soybean production when utilizing various extraction methods.
PPM
Very Low Low Optimum* High Very High
Bray P and Mehlich-3 P 0-8 9-15 16-20 21-3031+
Olsen P 0-5 6-9 10-13 14-18 19+
Mehlich-3 ICP P 0-15 16-25 26-35 36-45 46+
PPM
Corn 10075 58 0 0
Soybean 80 60 40 0 0
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Soil lab results list the test, the result, and may include interpretation or recommendation. When reviewing lab results, it is important to know what extraction method was used. Labs may report results in parts per million (ppm) or lbs/acre. To convert ppm to lbs/acre multiple ppm by 2 (lbs/acre = ppm x 2). To convert lbs/acre to ppm divide lbs/acre by 2 (ppm = lbs/acre ÷ 2).

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Soil Parameters

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Plants require macronutrients in large amounts. Nitrogen (N), phosphorus (P), potassium (K), sulfur (S), calcium (Ca), and magnesium (Mg) are all macronutrients. Micronutrients are needed for plant function in trace amounts. Plant micronutrients are: copper (Cu), iron (Fe), manganese (Mn), zinc (Zn), boron (B), chloride (Cl), nickel (Ni), and molybdenum (Mo). Other soil characteristics that may be included in soil lab results are organic matter (OM), soil pH, soluble salts (salinity), and cation exchange capacity (CEC).

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Nitrogen (N) - An adequate supply of N is associated with high photosynthetic activity, vigorous growth, and dark green plant vegetation. There are two forms of plant available N: nitrate (NO3-) and ammonium (NH4+). Nitrate is measured most often in soil tests. Soil test results report NO3- N in lb N/acre. When soil is saturated, nitrate can be lost by leaching and denitrification. It is important to remember that nitrate levels on a soil test reflect what is immediately available and not what will be available in the future from mineralization of organic matter. The Late Spring Nitrate Test, also known as the Pre-sidedress Nitrate Test (PSNT), may be used in-season when corn plants are 6 to 12 inches tall, to determine how much N should be side-dressed. Since nitrate soil test thresholds are regional, consult the nitrate soil test thresholds specific to your geography.

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Table 2. Potassium recommendations for corn and soybean production when using the ammonium acetate and Mehlich-3 Extractable K method.
PPM
Very Low Low Optimum* High Very High
Dry 0-120 121-160 161-200 201-240 240+
Field-moist and Slurry0-5051-85 86-120 121-155165+
PPM
Corn
Fine Textured 130 90 40 0 0
Sandy Textured 1107040 0 0
Soybean
Fine Textured 12090 660 0
Sandy Textured 10085 660 0
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Phosphorus (P) - Phosphorus is needed by the plant to store energy created from photosynthesis and carbohydrate metabolism to be used for plant growth and reproductive processes. Phosphorus is not as naturally abundant in the soil as other macronutrients and is relatively immobile because of its negative charge that binds to positively charged particles, such as calcium. The amount of plant available P in the soil solution is related to soil pH. Different P extraction methods are used for lab tests depending on the soil pH: Bray P (acidic soils), Mehlich-3 P (acidic soils), and Olsen P (neutral to alkaline soils). Test results may vary based on the extraction method or test used for measuring P. When interpreting results it is important to know which extraction method and test were used, and how the results were reported. Table 1 provides phosphorus recommendations for corn and soybean production based on soil test results from each of the 3 soil extraction methods.2

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Potassium (K) - Plants require potassium to activate enzymes, help draw water into the roots, produce phosphate molecules and CO2, help translocate sugars, and uptake and assimilating N. Potassium is present in large quantities in most soils, although it is not always available. The K cycle is always changing and concentrations will fluctuate seasonally due to differing environmental conditions. Comparing soil tests over time is the best method of evaluating nutrient management decisions. Soil testing in the fall or spring are acceptable for determining K soil concentrations, as long as there is consistency as to when samples are taken. Soil test K recommendations for corn and soybean production can be found in Table 3.

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Sulfur (S) - Sulfur has many important functions in plant growth and metabolism. Deficiency symptoms resemble those of N; however, S deficiency is found in young tissue where as N deficiency can be found in both young or old plant parts. Only a small fraction of the total soil S is readily available to plants and that form is sulfate (SO42-). Sulfur can be mobile or immobile in soil depending on microbial activity and the quantity of carbon (C), N, and P. S-deficient soils have soluble SO42- concentrations less than 5 to 10 ppm.5

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Table 3. Zinc recommendations for corn production utilizing the DTPA Extractable Zn extraction method.
Zn Soil Test Zn application
PPMBroadcastBand
Low 0-0.04 10 2
Marginal0.5-0.8 51
Adequate0.9+ 00
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Calcium (Ca) and Magnesium (Mg) - Calcium enhances NO3-N uptake and also regulates the uptake of cations, such as K+ and sodium (Na+). High Ca concentrations typically result in low concentrations of undesirable cations, but a low Ca content can contribute to soil acidity. Magnesium (Mg) is needed for photosynthesis and in many other physiological and biochemical functions within the plant. Both Mg and Ca ions can easily be exchanged or taken off of negative soil colloids. Mg deficiencies are not widespread, but can occur. Concentrations of Mg2+ in the soil are commonly 5-50 ppm in temperate soils but can be much higher.5

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Micronutrients - Micronutrients are needed in trace amounts for plant function: copper (Cu), iron (Fe), manganese (Mn), zinc (Zn), boron (B), chloride (Cl), nickel (Ni), and molybdenum (Mo). Although many of the micronutrients are reported on soil test reports their levels do not currently affect fertilizer recommendations, with the exception of Zn. Soil test Zn recommendations for corn are shown in Table 3.

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Organic Matter (OM) - Organic matter affects many soil biological, chemical, and physical properties that influence nutrient availability. A general guideline is to reduce N recommendations by 20 lb/acre for soils with >3% OM and increase N recommendations for soils with <1% OM.4 Consult your regional guidelines for a more precise influence of OM on nutrient availability.

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Soil pH - Soil pH is an indicator of the level of acidity or alkalinity of the soil, ranging from 0 to 14. A reading of 7 is neutral, crops typically grow best when pH is between 6 (slightly acidic) and 7.5 (slightly alkaline). Results of soil pH are reported on a logarithmic scale; therefore, a soil with a pH of 6 is 10 times more acidic than a soil with a pH of 7, and a soil with a pH of 5 is 100 times more acidic than a pH of 7. Nutrient availability may be hindered if soil pH is not within the optimum range and can result in available nutrient deficiencies.

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A buffer pH (BpH) test is used to determine lime rate requirements.5 The amount of lime needed to increase soil pH to a desirable level can be estimated by mixing a buffer solution (with a known pH) to soil and then measuring the change in pH. If the change in pH is large after the buffer is added, the soil pH is easily changed and a low lime recommendation rate will be made and if the change is small it means the soil pH is difficult to change requiring a larger lime recommendation.

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Soluble Salts - High soluble salt content (or salinity) can cause water stress, nutrient imbalances in plants, and affect nutrient uptake. Seedlings are more sensitive to higher than normal soluble salts compared to older plants. High soluble salt levels above 4 mmhos/cm (or ‘dS/m’) can potentially damage plants.4 Salinity levels in soil can change rapidly due to leaching; therefore, sampling should take place periodically within the growing season.

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Cation Exchange Capacity (CEC) - CEC is not always part of soil analysis. If it is included on a lab result, a CEC above 10 milliequivalents per 100 grams (10 meq/100g) is considered adequate.4 A high CEC is sought because it indicates a high capacity for the soil to hold cations (positively charged particles), such as, K+, NH4+, Cu2+, Fe2+, and Mn2+.

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Summary

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The soil parameter descriptions and optimum values provided within this spotlight can help assess your soil fertility program and reach optimum yield potential. Due to variability in soil, lab analysis, and reporting, guidelines specific to your region may exist. A local agronomist or extension specialist can provide information specific to your area.