Interpretation of Soil Chemical Tests

Parameter and

Test Result Interpretation pH of saturated soil paste <4.2 Too acid for most crops

5.2-5.5 Suitable for acid tolerant crops

5.5-8.4 Suitable for most crops

>8.4 Too alkaline for most crops

Cation exchange capacity (CEC) (mEq/100 g)

1-10 Limited adsorption (sandy soils) 12-20 Moderate adsorption (silt loam)

>20 High adsorption (clay and organic soils)

Exchangeable cations

Calcium 60-70

Potassium 5-10

Exchangeable sodium percentage (ESP) (as % of CEC) <5 Satisfactory

>10 Reduced permeability in fine-textured soils

>20 Reduced permeability in coarse-textured soils

Electrical conductivity (EC) (mmhos/cm) <2 No salinity problems

2-4 Restricts growth of very sensitive crops 4-8 Restricts growth of many crops

8-16 Only salt-tolerant crops will grow

>16 Only a very few salt-tolerant crops will grow

Source: Adapted from USEPA, Process Design Manual: Land Treatment of Municipal Wastewater, EPA 6125/1-81-018, CERI, Cincinnati, OH, 1981.

The nutrient status of the soil is important if vegetation is to become a component in the treatment system or if the soil system is otherwise to remove nitrogen or phosphorus. Potassium is also important for proper balance with the other nutrients. The N, P, and K ratios for wastewaters and biosolids are not always suitable for optimum crop growth, and in some cases it has been necessary to add supplemental potassium (see Chapter 3).

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