Types Of Secondary Nutrient Amendments (Ca, Mg, S) For Michigan Soils

Michigan soils are diverse — from sandy dunes and outwash plains to fine-textured glacial tills and organic peats — and each soil type presents different needs for secondary nutrients: calcium (Ca), magnesium (Mg) and sulfur (S). These three elements influence plant structure, chlorophyll production, protein synthesis, soil structure and the behavior of other nutrients. This article explains the common amendment materials, how and when to use them in Michigan, testing and interpretation, application methods and rates, and practical takeaways for growers, turf managers and landscapers.

Why Ca, Mg and S matter in Michigan soils

Calcium strengthens cell walls and drives cation exchange reactions that govern soil structure and nutrient availability. Magnesium is the central atom of the chlorophyll molecule and is critical for photosynthesis and enzyme activity. Sulfur is a component of amino acids (cysteine, methionine), vitamins and plant secondary metabolites.
Historically, atmospheric deposition supplied substantial sulfur to agricultural soils. Reduced atmospheric S emissions over recent decades mean S deficiencies are now more common, especially on coarser-textured soils or where manure and organic S sources are limited. Michigan’s sandy soils and organic matter-poor fields are most vulnerable to S and Mg deficiency and to imbalanced Ca:Mg ratios that affect structure and nutrient uptake.

Soil testing and interpretation for secondary nutrients

Soil testing is the starting point. Michigan State University Extension and other labs report exchangeable Ca and Mg, percent base saturation or meq/100 g of soil, and sulfate-S in ppm for topsoil. Key practical interpretation points:

• Exchangeable Ca: evaluates available calcium; low values or low Ca base saturation can indicate the need for lime (calcitic or dolomitic) or gypsum depending on pH goals and subsoil needs.
• Exchangeable Mg: low Mg or low %Mg of the cation exchange complex can reduce chlorophyll and cause interveinal chlorosis on susceptible crops.
• Sulfate-S (soil test S): values below about 10-12 ppm (top 0-6 inches) often indicate a likelihood of response to S fertilization for many crops; values >20 ppm are generally adequate. (Target values vary by lab and crop; use the specific lab interpretation.)
• Soil pH: determines whether lime is needed. If pH is low and Mg is low, dolomitic lime (provides both Ca and Mg) is often appropriate. If pH is low and Mg is adequate, calcitic lime supplies Ca without adding Mg.

Always follow the issuing lab’s interpretations and recommendations. Soil tests also indicate CEC (cation exchange capacity), which affects how much Ca or Mg the soil can hold and how rapidly S can leach.

Common amendment types and what they supply

Liming materials: calcitic and dolomitic lime

• Purpose: raise soil pH (neutralize acidity) and supply Ca; dolomitic lime also supplies Mg.
• When to use: use calcitic lime if pH correction is needed and Mg is adequate. Choose dolomitic lime when both pH and Mg correction are required (low exchangeable Mg or low Mg base saturation).
• Notes: lime effectiveness is expressed as neutralizing value (CCE). Product chemistry varies; use lab recommendations to calculate required tons/acre.

Gypsum (calcium sulfate dihydrate)

• What it supplies: soluble Ca and sulfate-S without raising soil pH.
• When to use: to supply S and Ca where pH adjustment is not wanted or where subsoil Ca enrichment or sodic conditions benefit from added Ca. In Michigan, gypsum is commonly used to correct S deficiency, improve crusting and reduce dispersive behavior on fine-textured or compacted layers.
• Typical analysis: gypsum contains roughly 18% S and about 23% Ca by weight (product labels should be checked). It is soluble and moves into the soil profile with water.

Elemental sulfur (S0)

• What it supplies: acidifying S that must be oxidized by soil microbes to sulfate; effective for lowering pH and providing long-term S.
• When to use: to gradually acidify high-pH soils or as a slow-release S source. Oxidation is temperature- and moisture-dependent; it works slowly (weeks to months).
• Notes: useful in small, targeted applications; not recommended for immediate S correction when plants are already deficient.

Magnesium sulfate (Epsom salt) and kieserite

• Epsom salt (MgSO4*7H2O): water-soluble, common for foliar correction or small soil applications; contains roughly 10% Mg (approximate) and is fast-acting but not economical for large-acreage soil Mg correction.
• Kieserite (MgSO4*H2O): more concentrated source of Mg than Epsom salt, supplies both Mg and sulfate-S, is water-soluble and suitable for fertigated systems or broadcast applications where a soluble Mg source is desired.
• When to use: when Mg is deficient but pH does not require liming; for quick correction use Epsom salt foliar sprays or soil-applied kieserite. For long-term correction of low Mg with low pH, dolomitic lime remains preferred.

How to choose between amendments: practical rules of thumb

• If the primary problem is low pH: choose a liming material. If Mg is low as well, choose dolomitic lime. If Mg is adequate, choose calcitic lime.
• If pH is adequate but S is low: use gypsum (fast sulfate supply) or soluble sulfate fertilizers. Elemental S is slower and mainly used when acidification is also desired.
• If pH is adequate but Mg is low: use kieserite for soil correction or foliar Epsom salt for rapid short-term green-up. For long-term structural correction on acidic soils, dolomitic lime is better.
• For subsoil Ca enrichment or improvement of dispersive subsoil layers: gypsum can be effective because it supplies soluble Ca without changing surface pH much.

Application rates, timing and methods (practical calculations)

Always calculate rates from the product label nutrient concentration and the desired nutrient addition. Example methods and approximate guidelines:

• Gypsum for sulfur: to add 20 lb S/acre and using gypsum at ~18% S: required gypsum = 20 lb S / 0.18 110 lb gypsum/acre. For many S corrections, growers apply 200-1000 lb gypsum/acre depending on crop demand and soil tests. For soil structural improvement or subsoil Ca enrichment, 1-4 tons/acre may be used.
• Dolomitic lime: rates are based on neutralizing value (CCE) and buffer or target pH; follow soil test lime recommendations (commonly 1-4 tons/acre depending on initial pH and soil texture).
• Kieserite/Epsom salt for Mg: kieserite contains substantially more Mg than Epsom salt per pound. A typical soil correction might be several hundred pounds per acre of kieserite; for foliar or quick fixes, Epsom salt at 25-50 lb/acre split or as foliar applications can produce transient greening. Always compute using the percent Mg on the product label.

Timing and placement:

• Broadcast and incorporate lime in fall or spring prior to planting so pH adjusts before peak crop demand.
• Gypsum can be applied anytime but spring or early fall incorporation helps move sulfate into the root zone.
• Elemental S should be applied well before the period of need (fall or early spring) to allow microbial oxidation.
• Soluble Mg and S sources for in-season correction may be applied via sidedress, fertigated systems or foliar spray for rapid response.

Crop-specific considerations for Michigan

• Corn and soybeans: responsive to S on sandy fields and where S fertilizer history is low. Small S additions (20-30 lb S/acre) often prevent yield loss; soil testing and tissue testing (young leaves) guide in-season needs.
• Forages and alfalfa: sensitive to pH and Mg; dolomitic lime is commonly used where pH is low and Mg deficiency is present. Alfalfa yield and protein content respond to adequate S.
• Potatoes and vegetables: shallow-rooted and responsive to immediate sulfate; gypsum or soluble sulfate sources are useful. Beware of chloride-containing fertilizers near chloride-sensitive crops.
• Turf and ornamentals: show Mg and S deficiency on sandy rootzones; soluble Mg (foliar or light soil applications) gives quick visual recovery. Gypsum helps alleviate surface crusting and improves rootzone calcium without pH change.

Common mistakes and interactions to avoid

• Applying dolomitic lime when pH does not need raising: unnecessary Mg addition can alter Ca:Mg balance and soil structure.
• Assuming “sulfur is everywhere”: reduced S deposition means more soils are S-responsive; do not ignore soil or tissue testing.
• Over-relying on Epsom salt for broad-acre Mg correction: it is often not economical compared to dolomitic lime or kieserite for field-scale correction.
• Failing to account for CEC and soil texture: sandy soils have low CEC and require smaller, more frequent S applications because sulfate can leach; chisel or deep-placement of gypsum may be needed to affect the subsoil.

Practical takeaways for Michigan land managers

• Test before you apply. Use soil and, when appropriate, tissue tests to target Ca, Mg and S corrections. Test depth and timing matter (top 6 inches commonly).
• Match the amendment to the objective: raise pH (lime), add S without pH change (gypsum or sulfate fertilizer), add Mg quickly (kieserite or foliar Epsom), or acidify slowly (elemental S).
• Calculate rates from product labels: convert desired nutrient pounds/acre to product pounds/acre using the product’s percent nutrient.
• Consider timing: apply lime well before planting, elemental S ahead of need, and soluble sulfates/Mg for in-season corrections.
• Watch soil texture and CEC: sandy Michigan soils lose sulfate quickly and may need split or timely applications; fine-textured soils may benefit from gypsum’s structural effects.
• Monitor crop response with tissue testing mid-season, and follow up with soil tests every 2-4 years to track base saturation and S status.

Final thoughts

Ca, Mg and S are essential, interacting nutrients whose management depends on soil texture, pH, crop and history of inputs. In Michigan, the decline of atmospheric S deposition, variability of glacial soil types, and crop rotations mean targeted, test-driven amendment strategies pay off. Match materials to goals (pH correction vs nutrient supply), compute rates from product nutrient content, use appropriate timing and placement, and re-test to confirm success. Practical, measured adjustments based on sound soil testing will maintain productivity, improve soil structure and avoid unintended nutrient imbalances.