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Analysis Terminology - 2003

The oldest analysis terminology is weight per weight; e.g. 55 weights of calcium per weight of soil, standardized on a million weight of soil, tissue or water or parts per million (ppm).  Since a liter of water weighs one kilogram, water analysis will often be expressed as weight of ion per liter of water either of water extracted from soil or irrigation water.  Milligrams per liter (mg/l) is interchangeable with ppm.

Another common expression is to put all the ions of interest on an equivalent charge basis.  For example sodium has an atomic weight of 23 and calcium has 40.  Calcium has twice the charge of sodium, though  and equal weight of the two  as ppm will have very different effects on plants, soil and water.  To show their equivalent charge they are expressed  as milliequivalents per volume or weight of soil or water (meq/100 grams or meq/l).  A milliequivalent is the ppm of that ion divided by its atomic weight per charge. 

          Example:  Ca2+ with atomic weight of 40 and a solution concentration of possibly 200 ppm.  Ca2+ has two charges per atom, so it has a weight of 20 per charge.  200 ppm divided by 20 = 10 meq of calcium for a liter of water.

Leave it to the Dutch to find a new expression, mmol/l.  This is taking the atomic weight of the ion and giving it a value of one mole (or in this case one millimole).  One mole of sodium has a different weight from calcium, but they both have the same number of atoms present, so it is another way of putting different things on an equivalent basis.

          Example: 100 ppm calcium = 100 mg/l = 5 meq/l = 2.5 mmol/l.

Total Dissolved Solids (TDS): measure of total salts in solution in ppm or mg/L

Electrical Conductivity (EC):  similar to TDS but analyzed differently.

          Units: deciSiemens/meter(dS/m)=millimhos/centimeter (mmhos/cm)=
1000 micromhos/cm (umhos/cm).

            Conversion TDS<->EC:  640 ppm=1 dS/m=1000 umhos/cm

Hardness:  measure of calcium and magnesium in water expressed as ppm CaCO3

pH:  measure of how acid or base the solution

Alkalinity:  measure of the amount of  carbonate and bicarbonate controlling the pH, expressed as ppm CaCO3.

Sodium Adsorption Ratio (SAR):  describes the relative sodium hazard of water

                    SAR= (Na)/((Ca+Mg)/2)1/2, all units in meq/l

                    There is also an Adjusted SAR which considers the carbonate and bicarbonate present, but does not do much better.