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Soil Amendments

Amendments to Improve Infiltration

Amendments target soil structure. The goal is to decrease soil resistance to water movement, thus improving water infiltration. Excessive sodium (Na+) causes swelling and dispersion of clay particles, blocking soil pores. In contrast, calcium (Ca+) will stabilize these particles. A water infiltration problem may result if Na+ exceeds Ca+ in the soil by more than a ratio of about 3:1. Under these conditions, there is not enough Ca+ to counter the negative effects of Na+.  Similar problems can be caused by Mg+ and K+, and can also be countered by the addition of Ca+.

Soil & Water


Sanden, Blake, Allan E. Fulton amd Louise Ferguson. Managing Salinity, Soil and Water Amendments. (UC Cooperative Extension Kern County) Retrieved Jan. 26, 2012.

 When soil and water lab reports can show the high SAR and low EC values that indicate an infiltration problem, amendments can be added to offset excessive Na+ with Ca+. Two common approaches are: adding free Ca+ (a calcium salt) or breaking down native soil lime to release Ca+ in the soil (acid-forming amendments). Applying amendments through irrigation water is typically more expensive than soil applications, because it requires high-grade product and special injection equipment.

Free Calcium Added to Irrigation Water

Calcium salts add free Ca+ to alkaline soils (pH >7.0). The most frequently applied calcium salt is gypsum. Calcium chloride (CaCl) and calcium nitrate (CaNO3) are also used, and all three are soluble and can be applied through irrigation water. This is a good approach when thin soil surface crusting is a problem, as the amendment is delivered to the soil surface. Lime and dolomite are insoluble Ca salts, not appropriate for injection into irrigation systems, and typically applied to the soil increase pH.

Example: Gypsum Added to Microirrigation Systems: This presents a lab irrigation water report, then calculates the amount of soluble gypsum to apply to lower the SAR to an acceptable level. In this example the SAR of 17.5 = approx. ratio of Na to Ca + Mg: 9.6 / SqRt [(0.5 + 0.1) /2)] SAR=Na/ SqRt[(Ca+Mg/)2]. This is higher than 5 x EC, so a potential infiltration problem exists. ECw = approx. (Ca + Mg + Na+) /10.
Note the following:

  • A target is to maintain SAR < 5 x EC
  • 1 to 3 meq/l of Ca is considered a low - moderate injection
  • 3 - 6 meq/l of Ca is considered a moderate - high injectiion

Table 1: Lab irrigation water analysis report
a/ ions measured in meq/l unless indicated in ()

Iona ECw (dS/m) Ca+ Mg++ Na+ HCO3- Cl- B (mg/l) SAR
Lab Report Value 1.0 0.5 0.1 9.6 4.2 4.6 0.7 17.5
  • From Table 3 (below) see that for gypsum, to add 1 meq/l free Ca+, apply: 234 lbs/acre-ft of water
  • From Table 4 (below), see that for bulk solution gypsum: purity is 92%
  • Using this infomation, calculate an application rate:  234 / 0.92 = 254
  • Conclusion: to add 1 meq/l free Ca+, apply :254 lbs of bulk soluble gypsum/acre-ft of water
  • Extrapolate: to add 2 meq/L free Ca+, apply: ca. 500 lbs of bulk soluble gypsum/acre-ft of water
  • Calculate new Ca+ : 0.5 + 2.0 = 2.5
  • Calculate new ECw: (2.5 + 0.1 + 9.6) / 10 = approx. 1.2
  • Calculate new SAR : 9.6 / SqRt [(2.5 + 0.1) /2] = 8.4
  • The new SAR is below the target: 8.4 < 5 x 1.2

Table 2: Analysis including calculation for new ECw and SAR with addition of amendment: which adds 2 meq/L free Ca+
a/ ions measured in meq/l unless indicated in ()

Iona ECw (dS/m) Ca+ Mg++ Na+ HCO3- Cl- B (mg/l) SAR
Lab Report Value 1.0 0.5 0.1 9.6 4.2 4.6 0.7 17.5
Calculated Values with Amendment 1.2 2.5 0.1 9.6 4.2 4.6 0.7 8.4

Gypsum Added to Flood irrigation Systems

For orchards without microirrigation systems. injecting gypsum into surface-irrigated furrows can be effective in improving infiltration. However, this requires a silo and solutionizer machine, and the use of high grade gypsum. A 4-year trial using coarse Lima gypsum applied in May and again in July improved infiltration in almonds (Sanden 2007).

Acid-forming Amendments Added to Irrigation Water

These supply Ca indirectly through the breakdown of soil lime that is usually present in high pH soil. Common acid-forming amendments include sulfur (S), sulfuric acid (H2SO4), and lime sufur. Sulfur compounds undergo microbiological reaciotns to oxidize S to H2SO4. Acid materials react immediately with soil-lime. All of these materials can reduce soil pH. The example below uses the lab report in Table 1. To increase Ca, Instead of adding gypsum, sulfur is added in the form of lime sulfur. Acid should be added to microirrigation systems with caution, as it can damage components.

Example: Sulfuric Acid Added to Irrigation Water

  • From Table 3 (below) see that for 100% sulfuric acid, to add 1 meq/l free Ca+, apply: 133 lbs/acre-ft of water
  • From Table 4 (below), see that for sulfuric acid: purity is 98%
  • Using this infomation, calculate an application rate: 133 / 0.98 = 136
  • Conclusion: to add 1 meq/l free Ca+, apply: 136 lbs of sulfuric acid/acre-ft of water
  • Extrapolate: to add 2 meq/L free Ca+, apply: ca. 272 lbs of sulfuric acid/acre-ft of water
  • The rest of the calculations and the values in Table 2 are the same as in the gypsum example.

Note: irrigation rates are based on plant needs (link here).
Irrigation system settings: To set flow rates and scheduling of amendment injections for various field sizes and microirrigation systems: suggsted publications & resources (link here)

Table 3. Water-soluble Ca+ -supplying Amendments: Amount (in lbs/acre-ft of water) required to increase the Ca+ content of irrigation water by 1 meq/l of free Ca+
a/ assumes 1 meq K beneficially replaces 1 meq Na in addition to the acid generated by the S. b/ combined acidification potential from S andoxidation of N to NO3 to release Ca from soil lime. c/ acidification potential from oxidation of N to NO3 only.

Amendment Trade Name & Composition Am't Amendment Trade Name & Composition Am't
Gypsum CaSO4.2h2O
234 Ammonium thiosulfate Nitro-sul
12% N, 26% S
Ca chloride Hi-Cal
13% Ca
418 Ammonium polysulfide Nitro-sul
20% N, 40% S
Sulfur S
43.6 Sulfuric acid H2SO4
Ca polysulfide Lime-sulfur
23.3% S
191 Monocarbamide
dihydrogen sulfate / sulfuric acid
N-phuric, US-10
10% N, 18% S
Potassium thiosulfate
KTS - 25%
K20, 26% S



Table 4. Approx. bulk purity for Ca+ amendments
a/ free lime must be present in soil. b/ supply some Ca

Ca-salt Amendments Average
Purity (%)
Acid-forming Amendments Average
Purity (%)
Bulk Solution Gypsum (delivered) 92 Soil Sulfura (popcorn) 99
Lima Gypsum (Ventacopa) 75 Soil Sulfura(granular) 99
Westside Pit Gypsum 50 Sulfuric Acida (applied) 98
Ground Wallboard 92 Thio-Sula,b (delivered)
N-12, S-26
Beet Lime (acid soil only) 60 Lime Sulfur a,b (delivered) Ca-6, S-23

  N-pHuric 10/55 a,b (delivered) N-10, S-18

Amendments Added to Soil

Broadcast amendment over soil, followed by irrigation.
This is a more economical alternative to water-run application, because the material used, commonly gypsum, is less expensive. To be most effective, the application should be properly timed. If infiltration is a problem in the summer, the application should be made at the onset of summer.

In-season application for broadcast gypsum: 250 - 1000 lbs/acre, applied monthly in June, July and August. (fine, evenly- graded 100% gypsum preferred)

Dormant season application for broadcast gypsum: apply once. Slow-dissolve with irrigations throughout season (coarse gypsum, such as ground wallboard)

Application of sulfur: The acid-forming process to generate Ca occurs under extrended wetting of the soil/sulfur interface. Coarse sulfur (gravel-sized) can take up to 10 years for the process to fully occur. Fine sulfur, incorporated below the soil surface, will speed the process.

Banding of Amendments: this is typically done to lower the pH to increase the availability of micronutrients. This process will also release free Ca from the soil lime, thus improving infiltration.In established orchards, applications should not exceed 1500 lbs/acre (3 - 4 ft spray band), to avoid crop damage. Even smaller amounts of acid or fine sulfur, shanked into the soil under emitters can be effective. A cost-effective method for banding gypsum along drop lines and microsprinklers.

Broadcst amendment over soil, followed by tilling: This is most appropriate when the objective is site reclamation of the root zone, not just he surface of the soil, and may also be part of orchard site preparation (link), if warranted.

Schoonover Gypsum Requirement

If the SAR of the saturation extract >10, confirmation of the indicated sodium problem is required by the laboratory. Confirmation is by the Schoonover Gypsum Requirement test