Soil Test Interpretation Sheet
Test Interpretation Saturated Paste Extract Saturated Paste Extract (SPE) was developed to determine the electrical conductivity (EC w ) and sodium hazard of soil. Compared to exchangeable nutrient tests such as Mehlich III, Olsen, and Bray P-1, a SPE test is not a good indicator of soil nutrient reserves. Estimated Nitrogen Mineralization – Estimated amount of organic nitrogen that will mineralize to an inorganic form, becoming plant available, through the next 12 months. Electrical Conductivity ( EC w ) – measurement of soluble salts in soil solution. Measured by saturated paste extract test. Sodium Adsorption Ratio (SAR) – Measure of the ratio of sodium to calcium and magnesium in soil solution. Measured by a saturated paste extract test. Higher values indicate greater sodium relative to calcium and magnesium. Soils with high SAR values may experience sodium toxicity, poor water infiltration, and soil deflocculation. Effects are generally most detrimental in soils with high EC w (McCarty, 2005).
Soil Test Types and Applications Saturated Paste Extract (SPE) – Standard procedure for measuring soil electrical conductivity (EC w ), sodium adsorption ratio (SAR) of soil, and boron concentration. Do not used SPE results to assess soil fertility beyond soil boron (Carrow et al. 2004). Bray-P1 – Full name: Bray-Kurtz P1 Test, recommended as a test for plant-available soil phosphorus in neutral and acid soils (pH ≤ 7), not for alkaline soils (pH > 7) (Bray and Kurtz, 1945). Olsen – Recommended as a test for plant-available soil phosphorus test in alkaline soils (pH >7) (Olsen et al. 1954). Mehlich III – Recommended for evaluation of soil phosphorus, potassium, calcium, magnesium, sodium, manganese, and zinc. Mehlich III extracted soil P may differ from Bray-P1 and Olsen extracted soil P under some conditions. Common Terms and Abbreviations pH - Indication of soil alkalinity or acidity. Scale of 1-14; 1 being most acidic and 14 being most alkaline. Ideal soil pH for turfgrass growth is between 5.5 and 6.5. Above or below this range, nutrient availability may become limited. Figure 1 illustrates the effect of nutrient availability on soil pH.
Table 1. Saturated paste extract test interpretation
Measurement
Minor Problems Increasing Problems
Severe Problems
Electrical Conductivity (EC w ) 1 Boron (ppm) 2 Sodium Adsorption Ratio (SAR) 2 1 McCarty 2005 2 Carrow et al. 2003a
<4
4 to 12
>12
<2 <3
2-6
>6
3 to 12
>12
The sodium adsorption ratio (SAR) describes the relative concentration of soil solution sodium to calcium and magnesium. Higher SAR values indicate an increased likelihood of soil deflocculation, and inhibition of water movement and air exchange. The SAR formula describes ion concentrations in meq/L and soil test values must be converted prior to utilizing the equation. To convert ppm (or mg/L) to milliequivalent per liter (meq/L) use the following formula:
Figure 1. Relative nutrient availability as influenced by soil pH.
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Cation Exchange Capacity (CEC) – The sum total of exchangeable cations that a soil can adsorb. Expressed in milliequivalents per 100 grams of soil. Plant nutrient cations include: potassium (K + ), calcium (Ca 2+ ), magnesium (Mg 2+ ), iron (Fe 2+ ), zinc (Zn 2+ ), copper (Cu 2+ ), manganese (Mn 2+ ), ammonium (NH 4+ ), and sodium (Na + ). Higher values indicate a greater cation-nutrient holding capacity. Organic Matter Percent (OM%) – Percent organic matter by weight. Organic matter can influence nutrient holding capacity, soil structure, water infiltration and retention, and nitrogen mineralization.
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Table 2. Milliequivalent weight of elements used to calculate sodium adsorption ratio.
Element Sodium Calcium
Milliequivalent Weight
23 20
Magnesium
12.2
+
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