MN - Vegetable IPM Newsletter

In this issue:

Crop Water Use Update

Get Planting! 1999 Sweet Corn Variety Trial Results

Calcium and Magnesium Management

Vegetable Resources for Growers, Processors - 2000

MDA Plant Pest Survey - News

Vol. 2 No. 4   May 5, 2000

Calcium and Magnesium Management

By Carl J. Rosen, Professor and Extension Soil Scientist, Dept. of Soil, Water, and Climate, Univeristy of Minnesota

Summary: The need for Ca and Mg can be determined using a soil testing program that defines a sufficient level of each nutrient for crop production. In acid soils, liming according to soil test recommendations with dolomitic will generally satisfy Ca and Mg needs. In some situations localized Ca deficiency may occur even on high testing Ca soils. This problem is more the result of inadequate transport of Ca within the plant rather than an inadequate supply of soil calcium. Adjusting Ca/Mg ratios in the soil has generally not been shown to be a useful or economic means of determining Ca and Mg needs for crop production.

Calcium (Ca) and magnesium (Mg) are secondary macro nutrients essential for the growth and development of plants. Deficiencies of these nutrients are generally not as common or widespread as those with the primary macronutrients: nitrogen, phosphorus and potassium. Most soils contain sufficient amounts of Ca and Mg to meet the requirements for vegetable crop production, especially if soil pH is in an optimum range or when a proper liming program is followed. However, under some soil situations and for certain crops, Ca or Mg amendments may be needed or beneficial. The topics related to Ca and Mg addressed in this discussion include: 1) characteristics of these nutrients in the soil 2) deficiencies and their correction, and 3) soil testing and validity of using Ca/Mg (cation) ratios to determine amendment rates.

Calcium: Calcium is available to plants as the Ca²=+ ion. Because of its positive charge, Ca is attracted to negatively charged clay particles and organic matter and held by the soil. Calcium in plants provides structure to cell walls and is also involved in membrane transport. Calcium is immobile in plants, so deficiency symptoms are first apparent in young expanding tissue. Typical symptoms include death or die back of growing points. Localized Ca deficiency can also occur and is discussed in more detail below. Because of localized deficiency, tissue analysis for Ca is generally not useful in diagnosing the problem.

Calcium deficiency due to low soil Ca is rare, but may occur in acid sandy soils. Soil tests are useful in identifying low Ca soils. Levels less than 400 ppm of ammonium acetate extractable Ca are considered low. For most crops, Ca needs can be met by liming according to soil test recommendations for pH adjustment. The lime source to use can be either dolomitic or calcitic. Both sources are effective and the decision on which one to use should be based on cost. Potato scab is potentially more severe in limed soils. Therefore, fields with potatoes in the rotation for many years may be low in Ca because lime is not usually applied. For potatoes on low Ca soils where maintenance of acidity is often desired, Ca needs can be met by applying low rates of lime (approximately 1000 lb/A) during the rotation year. An alternative is to broadcast and incorporate calcium as gypsum (calcium sulfate - 20% Ca) at rates of 100 to 200 lb Ca/A (500 to 1000 lb gypsum/A).

Some vegetable crops are susceptible to Ca deficiency even when adequate levels of Ca are present in the soil. These problems are usually due to poor transport of calcium to developing tissues rather than a deficiency of Ca in the soil. Examples of these physiological disorders related to a localized Ca deficiency include: blossom end rot in tomatoes, black heart in celery, and leaf tip-burn in lettuce, cabbage, and cauliflower. In soils where pH has been adjusted to 6.0 or above, additional soil applied Ca fertilizer does not usually correct these physiological disorders. These disorders can often be related to cultivar, excessive ammonium fertilization, and/or excess or lack of water. To prevent or minimize Ca disorders, the following control measures can be taken:

  • if soil pH is less than 6.0, then lime to a pH of 6 to 7
  • avoid ammonium forms of nitrogen
  • ensure adequate supply of water, too much or too little water can induce the disorder
  • change to a cultivar less susceptible to the disorder
  • apply foliar Ca sprays: calcium chloride at the rate of 5-10 lbs per 100 gal per acre or calcium nitrate at the rate of 10-15 lbs per 100 gal per acre should be applied directly to the sensitive tissue. Multiple applications are usually required. Refer to manufacturer's recommendations for use of other Ca foliar products.

Magnesium: Magnesium is available to plants as the Mg²+ ion. Similar to Ca, Mg is attracted to clay particles and organic matter and is held by the soil. Magnesium is a constituent of the chlorophyll molecule and is also involved with other metabolic functions such as protein synthesis, enzyme activation, and energy transfer. Being mobile within the plant, Mg deficiency symptoms occur first on older leaves. Typical symptoms include a yellowing between the veins of older leaves. Tissue analysis is useful for confirming a suspected Mg deficiency.

Magnesium deficiency may occur in acid sandy soils. Soil tests less than 100 ppm Mg are considered low. If Mg deficiency is known or suspected on acid soils, the use of dolomitic limestone is the best long range approach. Apply low rates (approximately 1000 lb/A) if maintenance of soil acidity is desired. Other more immediately available sources of magnesium include potassium-magnesium sulfate (11% magnesium) or Epsom salts (10% magnesium). General recommendations on low Mg testing soils are to broadcast and incorporate 50 to 100 lb Mg/A. Deficiencies of Mg can be induced by high or excessive applications of potassium fertilizer.

Cation Ratios: Recommendations for Ca and Mg fertilizer applications can vary with the soil testing philosophy. The recommendations discussed above are based on the sufficiency level approach where, based on previous research, an established cut off level is established above which no fertilizer is recommended. Another approach that has been proposed for many years is the cation saturation ratio approach where ideal ratios of potassium, calcium, and magnesium on the soil exchange complex are established. Cations are simply positively charged ions such as potassium, magnesium and calcium. Many studies have shown that plants can tolerate wide ranges of cation ratios provided that sufficient levels of each nutrient are present. Trying to adjust to an ideal ratio can be costly with little probability of realizing a return on investment. Therefore, emphasis on calcium or magnesium needs should be based on maintaining sufficient levels in the soil rather than adjusting cation ratios.

Co-Editors:Bill Hutchison, Department of Entomology, University of Minnesota,
Jeanne Ciborowski, IPM Program, Minnesota Department of Agriculture,
Cindy Tong, Department of Horticulture, University of Minnesota,
Production Editor: Suzanne Wold, Research Specialist, University of Minnesota,

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Last Revised May 4, 2000.
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