Does your soil need a salt shaker?  Probably not, although it can happen through application of manure and/or byproducts that may contain sodium (Na) or excess salt (salinity), and through irrigation water.

Sodium has a dispersive effect on soil clays.  It can cause the clay particles to separate from each other, the particles will clog the soil pores, and cause a thin layer of slowly permeable material near the soil surface.  This effect is more serious in fine-textured soils than in coarse textured.  Soils with a high water table or poor drainage are more susceptible to salt or sodium accumulation.

Salinity can make plants work harder.  They must expend more energy to extract water from the soil.  Excess salinity can retard cell enlargement and division, as well as the production of proteins.  

Visible crop injury symptoms, such as leaf burn, are likely to occur only at high salinity levels.  However, yield losses can occur at lower levels, depending on the crop's sensitivity.  Corn & alfalfa are classified as "moderately sensitive", while soybeans are classified as "moderately tolerant."

Have more questions?  Start with a soil test.  If you or your agronomist suspect this may be involved, then talk with the lab where the samples are going.  I recommend Midwest Labs (www.midwestlabs.com), but most labs will be able to help you with this.  They may have specific guidelines that they recommend following to help you identify what is going on.