Gardening Articles: Landscaping :: Lawns, Ground Cover, & Wildflowers

Improving Soil With Gypsum (page 2 of 3)

by Garn Wallace with Shelly Stiles

Sulfur

The other component of gypsum, sulfate, supplies plants with sulfur, which is essential to protein synthesis. Sulfur is a constituent of the amino acids cystine, methionine and cysteine. It is also essential for nodule formation on legume roots, and for the characteristic odors of plants such as garlic and onions. Sulfur deficiencies are less common than calcium deficiencies, but can occur throughout North America, particularly in high-rainfall areas.

If sulfur is applied as elemental sulfur, it is not available until the soil bacteria oxidize it to sulfuric acid. Surface-applied sulfur is oxidized faster than incorporated sulfur, but because the required bacteria are frequently not present in alkaline soils, the oxidation may be very slow. Gypsum supplies sulfur as sulfate, the form plant roots can absorb.

Symptoms of sulfur deficiency include

  • Light green to yellowish young leaves.
  • Small and spindly plants.
  • Retarded growth rate and delayed maturity.

Gypsum as Soil Conditioner

Gypsum can reclaim high-sodium or "sodic" soils. Soils with a high proportion of exchangeable sodium (Na+) lose structure. Once soil is tightly packed, water and air penetration is reduced and root growth suffers. Water often puddles on the surface of these soils.

To revitalize sodic soils, incorporate gypsum into the soil and then apply six or more inches of water. The calcium ions will replace sodium ions that are attached to the soil particles. The sodium (as sodium sulfate, NaSO4) washes away through the soil into the groundwater, and your topsoil is left more porous, so is more supportive of root growth.

Sodic soils occur almost exclusively in the arid and semiarid West. If you suspect your soil is sodic and would benefit from this gypsum-leaching treatment, order a laboratory soil test to confirm the presence of excess sodium. Applying gypsum to the surface of a compacted soil won't work.

Compacted soils are potential problems anywhere, and organic matter--such as compost--is generally the best remedy. Gypsum helps you get the most benefit from organic matter added to the soil. Soil crumbs and aggregates that give structure to soil are cemented together with glues that include salts such as calcium. Leaching from salt-free rainwater increases acidity and decreases the salinity of soils, and this can cause soil to lose its physical structure despite the presence of adequate organic matter. Correcting acidity with limestone helps, but limestone does not contribute calcium ions to the soil. Gypsum also protects the soil from compaction and other types of degradation where irrigation water is reclaimed or of low quality.

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