Overview
Excessive sodium in irrigation water  promotes soil dispersion and structural breakdown but only if sodium exceeds calcium by more than a ratio of about 3:1. Such a relatively high sodium content (>3:1) often results in a severe water infiltration problem due to soil dispersion and plugging and sealing of the surface pores, in much the same way as does the very low salinity water (ECw<0.2 dS/m). This is due to lack of sufficient calcium to counter the dispersing effects of the sodium. Excessive sodium may also make it extremely difficult to supply enough water to meet the crop water demand. Other related problems such as soil crusting, poor seedling emergence, lack of aeration, plant and root diseases, weed and mosquito control problems caused by the low rate of infiltration may further complicate crop management.

The sodium adsorption ratio (SARw) is an indicator of the relative proportion of sodium ions in a water sample to those of calcium and magnesium. SARw is used to predict the sodium hazard. The sodium adsorption ratio is used to predict the potential for sodium to accumulate in the soil, if sodic water was in constant use.

Adding a soluble form of calcium to the water or soil  to balance the overabundance of sodium applied in irrigation water  helps  maintain a soil's physical integrity.

This calculator, based on the measured chemical properties of irrigation water, helps calculate a rate of gypsum sufficient to balance the calcium input to the soil based on the waters Sodium Adsorption Ratio (SAR). The equations to calculate SAR (adj) are based on Suarez (1981).

1. Enter soil water chemical properties in appropriate cells  in tab "SAR".
2. Determine the target SAR from the Interpretation data in "SAR tab" according to soil type.
3. Adjust the Ca (red) value in the "Water Target SAR" column upward until the SAR adj is about the same  value as the desired level of hazard for the soil type.
4. Enter the gypsum purity. [HINT: gypsum purity % = (S% of gypsum purchased/18.5)*100].
5. The gypsum rate is calculated per ML of water.
6. Enter  the quantity of water used per hectare for the period between gypsum applications, result is  the rate of gypsum require for the quantity of water to be applied, in tonnes/ha.

Below is a list of other products (and there equivalent rates)  that can be substituted for gypsum where cost effective.

Other a
mendment tonnes equivalent to 1 ton of 100 percent gypsum
Gypsum (CaSO4 . 2H2O) = 1.00
Sulphur (S) = 0.19
Sulphuric acid (H2SO4) =  0.61
Ferric sulphate (Fe2(SO4)3 . 9H2O) = 1.09
Lime sulphur (9 percent Ca + 24 percent S) = 0.78
Calcium chloride (CaCl2 . 2H2O) = 0.86
Calcium nitrate (Ca(NO3)2 . 2H2O) = 1.06
Calcium carbonate3 (CaCO3) = 0.58  (acid soil only)

Source (FAO Bulletin, Water quality for agriculture, Ayers and Wetscot, 1994, Infiltration problems, [ http://www.fao.org/docrep/003/t0234e/t0234e04.htm])
 
Water -  Current       Water  - Target SAR        
  meq/L mg/L     meq/L mg/L    
EC (dS/m)     EC (dS/m)      
Calcium (Ca2+)   Calcium (Ca2+)
Increase the Ca until the SARadj below is at the top end of the acceptable range
   
Magnesium (Mg2+)   Magnesiun (Mg2+)      
Sodium (Na1+)   Sodium (Na1+)      
Bicarbonate (HCO31-)   Bicarbonate (HCO31-)      
Carbonate (CO32-)   Carbonate (CO32-)      
                 
Ca difference meq/L            
SAR adj     SAR adj      
                 
Gypsum purity               
Gypsum requirement kg/ML   Interpretation SAR Hazard  
Water Rate ML/ha   Soil type No hazard Slight-Moderate Hazard Severe Hazard  
Gypsum Rate t/ha   2:1 clays <6 6 - 9 >9  
        1:1 clays <16 16 - 24 >24  
        Sand Ecw>1.5 dS/m <16 16 - 24 >24  
        Sand Ecw<1.5 dS/m <6 6 - 9 >9