Soil structure is impacted by exchangeable cations (positively charged ions). Multivalent cations (more than one positive charge) help hold soil particles together because they can have electrostatic (magnetic) attraction between two or more negative charge sites (soil clay and organic matter have a net negative charge). Multivalent cations include Ca2+, Mg2+, Zn2+, and Al3+. Monovalent cations (only one positive charge) cannot help with soil structure because of only one positive charge, and with sodium (Na+), for example, can degrade soil structure when large amounts occupy the soil exchange sites (also impacted by large ionic size of Na); thus, soils with low salt but high levels of exchangeable sodium (Na+) have poor soil structure. Except for a very small acreage of Napa soil in the Missouri River valley, excess Na is not a problem on Iowa soils, including those with high pH. In arid regions where salt and Na accumulates (saline-sodic soils), reclamation can include use of gypsum. Gypsum is used to add large amounts Ca2+ ions that displace the Na+ ions from the exchange sites, and when flushed with clean water both salts and Na are removed from the soil (gypsum is used instead of limestone because of higher solubility and no increase in soil pH). However, even in these sites this practice is not effective when subsoils have low permeability to water. If a soil only has high soluble salt, then gypsum is not used because it would add to the salt problem.

Soil structure also is greatly improved by soil organic material, which help “glue” soil particles together. Iowa soils have high organic matter content, which is just as important for good soil structure as exchangeable multivalent cations. The most detrimental effect on surface soil structure comes from the physical impact of raindrops. Surface residue is the best defense against this impact, and it comes at no cost from crop residue. Thus, improving water infiltration can be best achieved by limiting tillage to leave the most crop residue as possible rather than applying gypsum. Table 2 shows the lack of corn and soybean yield response to applied gypsum.

In summary, gypsum is an excellent fertilizer source of Ca and S. If application of these plant-essential nutrients is needed, then it works well. However, for Iowa soils both Ca or S are in good supply. Iowa soils inherently have a capacity for providing adequate levels of exchangeable Ca and S for crop production. Thus, more is not necessarily better.