"Sometimes advertisements for wetting agents and the labels on these products claim or imply that they are universally effective under all soil conditions. These claims are misleading. Tests in which wetting agents have been applied to normal, wettable soils have failed to substantiate claims that these products will increase water infiltration, plant population, nutrient uptake, and crop yield. They are effective only on soils that are at least somewhat water repellent.
Several methods can be used to determine the extent to which a soil is water repellent. The most precise methods require laboratory facilities, but several tests can be conducted in the field. The one most useful for preliminary tests is simply to place a drop of water on the soil surface and observe how long it takes to penetrate the soil. On a wettable soil, the water drop will flatten and move into the soil within a few seconds. On more water-repellent soils, the drop of water will stand more upright and will move more slowly into the soil.
As mentioned earlier, water infiltrates more slowly into fine-textured soils than into most coarse-textured soils. Poor tillage practices can also reduce infiltration rates. Before spending money on a wetting agent, be sure that slow infiltration is being caused by water repellency, not some other factor. Wetting agents will improve infiltration rates only in soils that have water-repellent properties, regardless of their texture, tilth, and aggregation." SOURCE