A Quick Glance

Soil & Water Notes

The Lab Report for Water and Soil-Water Samples: A Quick Reading

This is an overview of a lab report for water or soil-water samples. To prepare a soil-water sample for analysis, the lab personnel dry and ground the sample, then add distilled water to saturation. This water is then extracted by vacuum and used for analysis. Unless otherwise indicated, the measurements below are applied to both water and soil-water.

Saturation Percentage (SP)

This measurement characterizes soil texture. It is calculated by dividing the water weight required to saturate the soil divided by dry soil weight. A higher number indicates greater capacity for water in the soil pores.

pH measures Acidity or Alkalinity of water, based on hydrogen ion (H+) activity. pH figures in the selection of soil amendments.

• Pure water has a neutral pH of 7.0
• Acidic water has a pH < 7.0
• Alkaline water has a pH > 7.0

Electrical Conductivity (EC)

This is a reflection of total salinity, and is used in diagnosing salinity problems. This salinity is based on the presence of the charged ions discussed below.

• EC_e is for soil-water
• EC_w is for irrigation water
• Deciseimens/meter (dS/m) is the unit of measure for EC. This is the same as millimhos/centimeter (mmhos/cm), and is equal to: micromhos/cm (mmhos/cm) / 1000.

Salts: specifically, the charged ions: cations and anions

Ions bond together based on their opposing charges, but these bonds are broken in water. These ions are important in that they contribute to salinity in both water and soil-water samples are, and can be toxic in high concentrations. The ions of interest are:

• Positively charged cations: calcium (Ca⁺⁺), sodium (Na⁺), magnesium (Mg⁺⁺)
• Negatively charged anions: bicarbonate (HCO₃), carbonate (CO₃⁼), sulfate (SO₄⁼), chloride (Cl^-)

Millequivalents/liter (meq/L) is the unit of measure for  charged ions. Reporting in meq/L measures the concentration of the charge and is the most useful information on which to base decisions regarding soil amendments. When it is time to calculate the amounts of required amendments, meq/L can be converted to milligrams per liter (mg/L) by simple multiplication by a conversion factor specific to the ion.

• Boron (B) which can be toxic to plants, but does not contribute to salinity
• Nitrate-nitrogen (NO3^-N) which influences fertilizer decision-making, but does not contribute to salinity
• Potassium (K⁺) which can be a minor component of salinity   (verify?)

Milligrams per liter (mg/L) and ppm are the typical units of measure reported for the above ions. mg/L and ppm are approx. equal.

Sodium Absorption Ratio (SAR)

SAR is measured in irrigation water only, to evaluate sodium problems (sodicity).

• SAR is this ratio of soluble cations: Na⁺ to Ca⁺⁺ +  Mg⁺⁺.
• SAR is more useful than Na⁺ values alone, because more sodium can be tolerated when calcium increase proportionally.

SAR_adj: the adjusted SAR takes the levels of bicarbonate and carbonate into account, because these anions take calcium from the soil to form lime, thus increasing the ration of sodium to calcium. Water higher in these anions has an SAR_adj higher than the SAR.

Exchangeable Sodium Percentage (ESP)

ESP is a value for soil only, to evaluate sodium problems.

• ESP indicates the amount of Na⁺ bound to ion exchange sites in the soil.
• ESP is typically an estimated value, based on SAR, not directly measured.

Total Dissolved Solids (TDS)

TDS is the weight of all soluble salts in mg/L. EC, rather than TDS, is used in evaluating salinity problems.

Gypsum Requirement (GR)

GR is provided in lab analyses for soils with pH > 7.0, and SAR above 10 -15 (alkaline and high sodium soils).

Lime Requirement (LR)

LR is provided in lab analyses for soils with pH < 7.0 (acidic soils).

Is it a Good Report?

Check 1: Total Cation Charge should be approximately equal to the Total Anion Charge.
Na⁺ + Ca⁺⁺ + Mg⁺⁺ =  HCO₃^- + CO₃⁼ + SO₄⁼ + Cl^-

Check 2:  Total Salinity multiplied by 10 should be approx. equal to the Total Cation Charge and approx. equal to the Total Anion Charge: 
EC_w*10 = Na⁺ + Ca⁺⁺ + Mg⁺⁺
EC_w*10 = HCO3^- + CO3⁼ + SO₄ ⁼ + Cl^-

For either check: The totals should not be exactly equal, which would imply that some values were obtained by subtraction rather than measurement.