This is further explained by the graph and comments shown below. Sodic soils are low in the kinds of salts found in saline soils, but they are high in sodium. The resulting high pH, which is more than 8.5 can limit the growth of many crops. For most crops, the main effect of sodium is the destruction of soil structure. When sodium ions saturate cation exchange sites, the colloids separate and disperse soil aggregates.
Tiny soil particles lodge in the soil pores, sealing the soil surface.
Tilth also suffers and crusts hard enough to stop seed germination may form. In many cases, sodic soils also show a poorly drained columnar subsoil structure. The effect of sodium is most extreme on fine-textured soils and least extreme on coarse soils. Crop plants may take up enough sodium to injure plant tissues. Crops vary in the tolerance to sodium.