Gardening Articles: Care :: Plant Care Techniques

Improving Soil With Gypsum

by Garn Wallace with Shelly Stiles

Gypsum is one of the best sources of calcium, the most important of the secondary plant nutrients. But calcium is more than a nutrient. It is the major balancing element in plants and soils. It protects, within limits, from nutrient excess and deficiency, problems caused by both high and low pH and heavy metal contamination. Along with organic matter, calcium improves soil structure.

Gypsum as Fertilizer

Pure gypsum is 23 percent calcium and 19 percent sulfate (CaSO4-2H2O). In the hierarchy of the 16 essential plant nutrients that begins with non-minerals hydrogen, oxygen and carbon, calcium is seventh and sulfur is ninth.

Calcium is the plant nutrient most likely to be unavailable to roots when needed. An essential nutrient itself, calcium also improves root uptake of other plant nutrients, especially ammonium nitrogen. Calcium does not move from old to new plant tissues, so a constant supply of soluble calcium is needed. The growing points of both roots and shoots are sensitive to insufficient calcium, but roots more so. The ratio of calcium to sodium is more critical than the actual concentration of calcium.

Symptoms of calcium deficiency include

  • Water-soaked, discolored areas on fruits, such as blossom end rot on tomatoes, peppers, melons and squash, or bitter pit or cork spot on apples and pears. Tip burn on young leaves of celery, lettuce and cabbage.
  • Death of terminal buds and root tips.
  • Abnormal dark green appearance of leaves.
  • Premature shedding of blossoms and buds.
  • Weakened stems.

Calcium is most often deficient in high-rainfall, acid-soil areas, such as the Pacific Northwest west of the Cascade Mountains, and east of the Mississippi River. These regions receive in excess of 40 inches of rainfall each year. Deficiency symptoms, such as tomato blossom end rot, often appear there after a period of heavy rainfall. Gardeners apply ground dolomite or limestone, primarily to raise soil pH, but also to replace lost calcium. But the calcium in limestone is not water soluble, however. It must be incorporated into the soil and brought into contact with soil acids before calcium ions (Ca++) are released.

Limestone does not migrate in the soil, and is effective only to the depth incorporated. In contrast, gypsum dissolves immediately in the moisture of the soil, allowing plant roots to absorb the calcium ions and the sulfate ions. Because it is dissolved in the soil solution, gypsum readily migrates into the lower depths in the soil.

Most western soils contain abundant calcium, but as very insoluble limestone in the form of calcium carbonate. Where pH is 7 or higher, the limestone is insoluble and the calcium it contains is not available to plant roots. Again, the calcium in applied gypsum is immediately available.

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