Surfactants or soil wetters can help soils that repel water, but Washington State University’s Troy Peters found no benefits for normal, nonrepellent soils in a test he conducted.
Various soil-wetting agents are available that claim to improve water penetration and distribution uniformity in the soil as well as soil moisture retention, water use efficiency, and drainage. These are commonly referred to as surfactants, and they claim to make water "wetter." The question is, are these worth the money? The answer, as usual, is "it depends."
The polar nature of water makes it want to stick together (cohesion). Since a water molecule would rather be in with other water molecules, there is an apparent repulsion of anything that is nonpolar, including air, at the interface. This is referred to as surface tension. Most soil particles have negatively charged surfaces that attract the positively charged water molecules (adhesion) and cause water to stick to them. This makes most soils hydrophilic or "having an affinity for water." The combination of this attraction and surface tension causes water to be drawn up into the cracks between soil particles (capillary forces) and therefore enables soil to hold water in reserve and makes life possible on this planet. However, sometimes soil particles get nonpolar coatings on them and become hydrophobic (water repellent). Most of these coatings are formed from organic processes and compounds within the soil. Sandy soils (because of a low surface area to volume ratio), soils with high organic matter content, and soils that cycle to very dry conditions are more susceptible to water repellency. Dry soils and higher organic matter content tend to be more common near the soil surface, and, therefore, water repellency issues are more prevalent nearer the soil surface.
Because water doesn’t stick to these soil particles as readily, water will tend to bead up on the soil surface instead of going into the soil (poor infiltration). This results in higher runoff and poor uniformity of water distribution. The water that does go into the soil will tend to move in concentrated fingers of flow instead of being distributed throughout the soil profile. This will result in localized dry spots in some places and deep percolation in others, leading to poor irrigation efficiency and crop performance.
How soil wetters/penetrants help
Surfactants or soil wetters/penetrants all generally overcome these problems by the addition of molecules in the water that have both a polar (hydrophilic, water-loving) end and a nonpolar (hydrophobic, water repelling) end. The non-polar end will adhere to the water-repellent coatings on the soil and draw the water in behind it, allowing the problem soil to be wetted. There are a large number of different soil surfactants available. They vary greatly in their molecular weight, size, shape, structure, and the methods that they use to bind to the soil. They also vary in their recommended application rates and the length of time that they are effective in the soil. Be sure to read the label carefully to compare these different products.
Good for everybody?
In order to test the manufacturer’s claims, two common southeastern Washington soils were obtained that had no known soil issues (Warden silt loam and Quincy sand). Four different surfactants were selected; one nonionic, two block polymer, and one anionic. Laboratory tests were performed with the four surfactants applied at the labeled rate, and compared with a water only control, for infiltration rate (how fast water goes into the soil), water-holding capacity (how much water the soil can hold), unsaturated hydraulic conductivity (how good is the soil at moving water from wet areas to dry areas), and capillary rise (soil’s ability to move water upwards and/or sideways). These were done with basic soil physics methods, mostly on six-inch-diameter clear soil columns, and each test was replicated four times.
No significant differences were found in infiltration rate, water content, and unsaturated hydraulic conductivity between the surfactant treatments and the water-only control for these soils. There were very significant differences in the silt loam columns for capillary rise rates, with two of the surfactants performing slightly worse (slower capillary rise) than the control. Although there are demonstrated benefits for water-repellent (hydrophobic) soils, these tests fail to substantiate claims that surfactants or soil penetrants/wetters provide any benefit for normal, non-water-repellent soils.
The simplest way to determine the degree of a soil’s water repellency is to place several droplets of water on some air-dried, disturbed (powdered) soil. If the water droplets flatten out and go into the soil within a few seconds (zero to five seconds), then the soil can be considered to not have water repellency issues. Research does not support the cost-effective use of surfactants on non-water-repellent (hydrophilic) soils. If it takes longer than about 5 seconds for the water droplets to penetrate, then the soil may have some degree of water repellency and a surfactant or soil penetrant/wetter may provide some benefit in your situation.
Low infiltration rates may be due to heavy-textured soils (high clay content), compaction, poor tillage practices, and/or very dry soils. When water repellency occurs, it can be mitigated by not allowing soils to get very dry (irrigate more frequently in smaller amounts) and good tillage practices (avoid compaction, maintaining the soil structure, increase surface storage by roughing up the surface).