Secondary Nutrients

What are secondary nutrients?

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Secondary nutrients are nutrients that slightly limit crop growth and are moderately required by plants. These nutrients are calcium (Ca), magnesium (Mg), and sulfur (S). Secondary nutrients are as significant as primary nutrients in plants, but they are needed in smaller quantities.

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Regional Educator ResourcesOther Educator ResourcesTechnical Resources
Title Source Resource type and Date Short Summary / Preview
Sulfur, Calcium and Magnesium Kansas State University Extension Training Manual
Not Dated
This is a section from an extension handbook. This section is on sulfur, calcium, and magnesium requirements for common row crops.
Secondary Nutrients Kansas State University Presentation

Not Dated

This is a Powerpoint slide set developed and used for extension training purposes. It contains information on sulfur, calcium, and magnesium with respect to the amounts needed for producing row crops.
Other Nutrients SARE Book Excerpt


Although farmers understandably focus on nitrogen and phosphorus—because of the large quantities used and the potential for environmental problems—additional nutrient and soil chemical issues remain important.
Essential Elements for Plant Growth – Primary and Secondary Nutrients University of Wisconsin University Webpage

Oct 1995

The ability of soils to supply secondary nutrients to plants indefinitely is is subject to the law of conservation of matter and is therefore dependent upon nutrient cycling. Continued crop removal of Ca, Mg, and S requires replentishment just as surely as primary nutrients, but most likely less frequently.


Title Source Resource type and Date Short Summary / Preview
Sulfur for Minnesota Soils University of Minnesota Extension Webpage


This is an in-depth publication of sulfur for Minnesota soils, which goes into detail about its availability to growing crops from different soil types and from non-harvested crop residues. This resource describes several sources of sulfur, describes removal of sulfur from the soil system, such as crop uptake, removal and leaching, and describes deficiency symptoms in crops, such as alfalfa and corn. Sufficiency levels of sulfur in the tissue of several crops are listed, soil test interpretation of sandy soils and response to sulfur from other soil types with different organic matter levels are given.  Recommendations, methods of application and sources of sulfur fertilizer are provided.
Sulfur Deficiency Purdue University Extension Publication

Jul 2017

This publication from Indiana first explains some of the reasons why we are seeing sulfur deficiency symptoms in crops in recent years. It describes how more sulfur is being removed in crops and how soil properties and crop management affect sulfur availability. It describes the relationship of soil testing and sulfate-sulfur trends over time, how to diagnose sulfur deficiency, and using tissue sampling with soil testing to determine sulfur deficiencies. Methods and fertilizer materials are listed and best management practices are discussed.  Impact of sulfur containing fertilizers on soil pH is described.

Calcium and Magnesium Ratios

Title Source Resource type and Date Short Summary / Preview
Calcium and magnesium: the secondary cousins University of Minnesota Lesson

Not Dated

This lesson focuses on the importance of Ca and Mg in Minnesota soils and their role in plant nutrition. Frequently, Ca and Mg are grouped with potassium (K) and referred to as the basic cations.
Soil calcium to magnesium ratios— Should you be concerned? University of Wisconsin Extension Fact Sheet

Nov 1993

While some sales people and scientists have claimed that there is an “ideal” soil calcium to magnesium ratio (Ca:Mg), and that Wisconsin soils contain too much magnesium for the amount of calcium present, research in Wisconsin shows this is not true.
Soil calcium:magnesium ratios Iowa State University Extension Webpage

Apr 2003

All soils contain calcium ions (Ca2+) and magnesium (Mg2+) cations (positively charged ions) attracted to the negative exchange sites on clays and organic matter (cation exchange complex of the soil). The amount and relative proportion usually reflect the soil’s parent materials.
Soil calcium-to-magnesium ratios should not concern most farmers Michigan State University Extension Blog Post

Nov 2016

If soil calcium and magnesium levels are adequate and soil pH is acceptable, variations in the calcium-to-magnesium ratio between 2 and 8 have been shown to have no influence on crop yield.


Title Source Resource type and Date Short Summary / Preview
Magnesium for crop production University of Minnesota Extension Webpage


Although magnesium (Mg) is an essential element for plant growth, its use in a fertilizer program receives only minor emphasis in Minnesota.


Title Source Resource type and Date Short Summary / Preview
Calculating Cation Exchange Capacity, Base Saturation, and Calcium Saturation The Ohio State University Extension Webpage

Aug 2019

The purpose of this fact sheet is to define soil cation exchange capacity, base saturation and calcium saturation, and demonstrate how these values are calculated in soil test reports.
Properties of Gypsum That Provide Benefits for Agricultural Uses The Ohio State University Chapter 2 of Extension Publication


This publication discusses how sulfur deficiencies are more common today and gypsum can be a good source of sulfur, especially for alfalfa, increasing yields. It explains how gypsum is a good source of calcium and this can improve quality of horticultural crops and enhance peanut production. It explains that gypsum as a soil amendment can improve soil physical properties, like overcoming the dispersion effects of sodium and magnesium and improving water infiltration.
Gypsum for Agricultural Use in Ohio—Sources and Quality of Available Products The Ohio State University Extension Publication

Aug 2005

This article summarizes gypsum’s role as a soil amendment and lists different sources and mineral compositions for gypsum. The physical properties, price, plant nutrient content and trace mineral content of different gypsum materials are also listed.
Title Source Resource type and date Short Summary
Primary and secondary minerals University of California, Davis University Webpage

Not Dated

For sedimentary and metamorphic rocks, the elemental and mineralogical composition is determined by the chemical and mineralogical composition of the sediments and rocks from which they form.
Macro and secondary nutrients – soil diagnostics Ontario Ministry of Agriculture Governmental Webpage

Mar 2009

From the Ontario Ministry of Agriculture, Food and Rural Affairs this article summarizes the three types of macronutrients and three secondary nutrients (including sulfur) which crops need including how to identify sulfur deficiency and potential sources of sulfur.
Secondary Macronutrients: Cycling, Testing and Fertilizer Recommendations Montana State University Lesson

Jul 2002

This module is the sixth in a series of Extension materials designed to provide Extension agents, Certified Crop Advisers (CCAs), consultants, and producers with pertinent information on nutrient management issues.
Secondary Plant Nutrients: Calcium (Ca), Magnesium (Mg), and Sulfur (S) University of California Extension Fact Sheet

not dated

This article (Part 3) discusses the use of soil tests to evaluate levels of the secondary nutrients calcium (Ca), Magnesium (Mg), and sulfur (S) in orchard soils.
Secondary Plant Nutrients: Calcium, Magnesium, and Sulfur Mississippi State University Extension Publication

Jul 2019

A summary of the three principal secondary plant nutrients, how and where to look for secondary nutrient deficiencies and where to find reliable sources of sulfur should soil require it.
Soil Nutrient Management For Forages: Phosphorus, Potassium, Sulphur, and Micronutrients Montana State University Extension Publication


Improvements in forage production have the potential to increase income and significantly reduce livestock production costs. Soil fertility is important for forage production, stand health/longevity, and forage quality.
Calcium and Magnesium For Mississippi Crops University of Mississippi Extension Webpage

Feb 2011

Calcium (Ca) and (Mg) magnesium are positively charged secondary nutrients. They are generally adequate in most Mississippi soils with favorable pH and organic matter levels.
Title Source Resource type and Date Short Summary / Preview
Patterns of Soil Calcium and Aluminum Across the Conterminous United States (Chapter 9 of Forest Health Monitoring: 2008 National Technical Report) U.S. Department of Agriculture Forest Service Book Chapter

Jun 2012

Calcium (Ca) is a cation—positive ion—used by plants to build cell walls. It is also involved in root and leaf development and the activation of plant enzymes

Technical review: March 18, 2020