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Theoretical Gypsum Requirement (TGR) Models

July 6th, 2012

By John Ashworth,  ALS Environmental – Canada

ALS Environmental Lab TestingGypsum is often applied as an amendment to soils that exhibit a high sodium adsorption ratio (SAR).  The addition of gypsum can reduce a soil’s clay plasticity, thus improving drainage and ease of cultivation. Estimating the correct amount of gypsum required to remediate a particular site is an inexact science requiring experience and consideration of specific site history and conditions, but models described in the literature can produce theoretical estimates to provide guidance.  As a service to our clients, ALS now offers two theoretical calculations for gypsum requirement that are suited to two common categories of salt-impacted soil on the Canadian prairies.

  • Natural salt contamination – Sodic soils (e.g. solonetzic soils) contain naturally occurring sodium salts. After centuries of accumulation, sodium occupies a substantial percentage of the soil’s cation exchange capacity (CEC).
  • Man-made salt contamination – Spills of produced water (brine, sodium chloride) in the oil industry immediately increase the electrical conductivity (E.C.) of the soil solution, damaging seeds and hindering root uptake of water by growing plants. A secondary effect of brine spills is to increase soil SAR, thus affecting clay properties as already described.

Calcium amendments – Usually, calcium and magnesium ions predominate in soils, so that the SAR of a saturated paste extract is normally very low. Calcium-containing compounds can be used to bring high SAR values back below detrimental levels.  Soluble salts such as calcium nitrate (“Envirofloc”) and concentrated calcium nitrate solutions supplied in drums are fast-acting, but carry the risk of damage to growing plants due to high E.C. In contrast, insoluble calcium carbonate may have no immediate effect on SAR. Gypsum (calcium sulphate) has limited solubility causing only a modest E.C. increase, making it a widespread choice in SAR remediation.

Suitable rates of gypsum application – Rates of application range from a few tonnes per hectare up to as much as 50 t/ha. Due to its limited solubility, gypsum should be worked into the surface. Since soil cultivation depth is often approximately 15 cm, recommended rates are normally given in tonnes per hectare for a layer of soil 15 cm deep.

In estimating the theoretical gypsum requirement (TGR), it is important to distinguish man-made salinity – where the objective is to supply enough calcium to bring the SAR of the soil solution down to non-damaging levels – from natural salinity, where the need is to displace exchangeable sodium from the soil’s cation exchange complex.

Method 1: TGR for Brine-Contaminated soils (TGRbrine) – In this approach, an amount of gypsum is calculated that would supply enough soluble calcium to lower the soil’s SAR from its current, high value to a target saturated paste value considered non-damaging to soil structure.

TGRbrine (tonnes per hectare-15 cm) = 0.335 a2{(1/b2) – (1/c2)} x (% sat./100)   . . .   [1]

a = sodium ion concentration in saturated paste extract (meq/L),
b = the target SAR after soil remediation (ALS uses 7),
c = the current on-site SAR,
and % sat. = mL of water required to saturate 100 g of dry soil

Any exchangeable Na on the soil’s cation exchange complex is ignored. While some displacement of Ca and Mg cations by Na occurs after a salt spill, it is usually minor compared with Na in solution.

  • TGRbrine increases with electrical conductivity E.C. (due to increased sodium concentration) and also with increasing site SAR.
  • If the SAR is already below 7, the TGRbrine becomes negative and is set to zero.
  • Equation [1] can give very high TGR values; if TGRbrine exceeds 50 t/ha, a default rate of 50 t/ha is recommended. In such cases, the client will need to re-visit the site later to assess progress.
  • On brine-contaminated sites with high E.C., gypsum treatment is usually not a cure in itself; salts often need to be physically removed, by diverting saline pore water into trenches, bell-holes or tile drains, followed by safe (e.g. deep well) disposal.

Method 2: TGR for Naturally Sodic Soils (TGRsodic) – In this approach, an amount of gypsum is calculated that would supply enough exchangeable calcium to lower the soil’s exchangeable sodium percentage (ESP) from its current, high value to a target value considered non-detrimental to soil structure. Empirical allowance is made for incomplete displacement of Na by Ca.

TGRsodic (tonnes per hectare-15 cm) = 0.021 x CEC x (I – F)                        . . .  [2]

CEC = the soil’s cation exchange capacity in cmol(+)/kg  [= meq/100 g],
I = the initial, existing ESP value,
and F = the final, target ESP (ALS uses 8%, corresponding to an SAR of ~ 7)

Any sodium in the soil solution is neglected, since it is minor compared with the amount of exchangeable sodium naturally present on the cation exchange complex of sodic soil.

  • CEC can be estimated from % sat. because both parameters are governed by the amounts of clay and organic matter in the soil (USDA 1954).
  • For sodic soils, the initial ESP can be estimated from the SAR (USDA 1954).
  • TGRsodic increases with increasing SAR, but is unaffected by soil E.C.
  • If the SAR is already below 7 the calculated ESP is below 8, so that TGRsodic becomes negative and is set to zero.
  • TGR calculated by eqn. [2] is usually well below its theoretical maximum of approximately 50 t/ha.

Conclusions: – TGR values obtained by the above methods can differ considerably (see examples below). It is important, therefore, to use the approach that is appropriate to the chemistry and history of a particular site, and also to recognize that gypsum amendments alone are not always sufficient or practical for the remediation of all salt-impacted sites.

Soil condition EC (dS/m) SAR TGRbrine




Brine-contaminated 14 17 31  t/ha 2  t/ha
Naturally sodic 3 20 2 11

TGR values should always be interpreted as estimates. Calculated TGR values are best regarded as an index of salt contamination and of progress towards site remediation, rather than as a definitive application rate. TGR values cannot be as accurately predicted as, for example, fertilizer recommendations for a crop planted on a field tested for available nutrients. However, TGR values are useful in keeping application rates consistent, either from one site to another or from one application date to the next.

TGR is normally reported as tonnes per hectare (t/ha) to a treatment depth of 15 cm.  To convert to kg of gypsum per cubic metre of soil, multiply by 0.67. To convert to units of tons per acre at the same treatment depth, multiply  by 0.45.


Ashworth et al. 1999. Canadian Journal of Soil Science 79: 449-455.

Oster and Frenkel 1980. Soil Sci. Soc. Amer. J. 44: 41-45.

U.S. Department of Agriculture. Handbook No. 60, 1954 (available on-line).

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