Here’s another “high tech” blog post. Like my last blog post, this topic may be out of reach for the standard turfgrass warrior. But hey, if you’ve read my blog posts from the beginning, you will see we are past the 100 or 200 level courses. We are now in advanced studies! As I’ve said before, I think it is good to know about some of the many issues out there when it comes to turf care and maintenance. You can never know too much!
In this post, I’m going to discuss soils that are resistant to water. Soils that are difficult to “wet” (wet – used as a verb in this case) are branded as hydrophobic soils. The soil repels water. The soil is difficult to wet because it resists penetration by water. The infiltration of water into this type of soil can often be enhanced by applying a non-ionic surfactant, more commonly called a wetting agent. Wetting agents are detergent-like substances that reduce the surface tension of water, allowing it to infiltrate and wet the soil more easily. Soap is one of the most common surfactants. And a “SURFace ACTive AgeNT” causes change to the soil at the surface.
Understanding how wetting agents work will give you a better idea of the situations for which they are best suited. There are three “forces” that act upon the movement of water into the soil. The first is gravity. Gravity is a constant force that pulls the water downward. The second force is cohesion, the attraction of water molecules to each other. Cohesion is the force that holds a droplet of water together. It creates the surface tension, which causes the droplet to behave as if a thin, flexible film is covering it, keeping the water molecules apart from other substances. The third force is adhesion, the attraction of water molecules to other materials. This force causes water molecules to adhere to other objects, such as soil particles.
The chemist on my staff, Jake, says I should mention that the forces discussed above are actually electromagnetic forces. And, Jake says I should be more precise by explaining the properties of cohesion and adhesion, specifically detailing hydrogen bonding and capillary action. Okay, Jake looks at everything down to its molecular structure. If he gets real excited, he’ll start seeing atomic valences. That’s what chemists do. That’s why I engage the best minds in the business to ensure you have flawless analysis.
Hydrogen bonds form between the hydrogen of one water molecule and the oxygen of the neighboring water molecule. In ice, all the molecules of water are held in their places by a network of hydrogen bonds. In water, only some molecules at any given time will be attached, in a constantly changing process. The hydrogen bonds have to be broken in order for ice to melt, and more for water to become steam, and because this takes a lot of energy we see a higher than expected melting and boiling point.
Capillary action is caused by the water molecules pulling one another along via the hydrogen bonds. Surface tension is caused by the water molecules on the surface holding onto one another in a network, like giving enough support for a small bug to walk on the surface. If the bonds within a liquid are strong enough, then the liquid will have a large surface tension. Since hydrogen bonds are one of the strongest intermolecular forces known (apart from covalent and ionic bonds) liquids with large surface tensions tend to have hydrogen bonds.
With regard to capillary action, there are two forces (Watch Jake tell me it is an electromagnetic force.) acting on a molecule of the liquid: an attractive force exerted by other liquid molecules, and another attractive force exerted by the walls of the capillary. (A capillary is like a small tube or straw inserted in the water.) Hydrogen bonds are an example of the attractive forces that can be exerted by the liquid molecules (e.g. between water molecules, or between ethanol molecules), or the attractive forces that can be exerted by the capillary walls.
By considering the strength of the hydrogen bonds between the molecules in a liquid and the strength of the attractive forces between the capillary wall and the liquid molecules, one can determine whether capillary action can take place with the liquid being observed.
The effects of these forces can also be simply illustrated by placing a drop of water on a paper towel and another drop on a piece of wax paper. On the paper towel, the force of adhesion between the water molecules and the paper molecules is greater than the force of cohesion that holds the water molecules together. Therefore, the water droplet spreads out and soaks into the paper towel. On the wax paper, the water "beads up" - that is, the droplet remains intact. The water molecules are not attracted to the wax that coats the paper's surface. Instead, the water molecules stick to each other. When the adhesive forces between water molecules and an object are weaker than the cohesive forces between water molecules, the surface repels water and is said to be hydrophobic.
Most wetting agents have polar and non-polar distinctiveness. These characteristics allow the water to cling to, or even soak into certain organic matter. In hydrophobic soils, the soil particles are coated with substances that repel water, much like the wax coating on leaves. In studies of localized dry spots in turfgrass, the soil particles were found to be coated with a complex organic, acidic material that appeared to be similar to the mycelium of a fungus. My blog post “Turfgrass Diseases” discusses funguses a little.
Nearly all of the soil wetting agents used in turfgrass management are non-ionic surfactants. Non-ionic surfactants, or surface active wetting agents, decrease the surface tension of water, allowing the water molecules to spread out. When applied to water-repellent soils in high concentrations, surfactants can improve the ability of the water to penetrate the soil surface and thus increase the permeation rate. Remember, I’m talking about water penetrating soil, not water penetrating the surface of leaves on plants.
Hydrophobic soils can cause problems on golf courses, playing fields and other turf areas. Water repellant soils can also be found in gardens, shrub beds, nurseries, greenhouses, and in open fields.
Golf course greens keepers commonly report troubles with localized dry spots on their greens. Nursery operators sometimes encounter hard-to-wet soil in pots and greenhouse beds. And, some farmers who work soils high in organic matter complain that the soil wets too slowly, reducing crop output. Problems with hydrophobic soils are also commonly associated with citrus production areas down south. In some locations where mine wastes have been deposited, and with burned-over forestland and grassland, there tends to be hydrophobic soils.
If water cannot readily enter and wet the soil, the availability of moisture to plants is reduced, decreasing the germination rate of seeds, the emergence of seedlings, and the survival and productivity of plants. Lack of water penetration in the soil also reduces the availability of essential nutrients to plants, further limiting growth, health and productivity. In addition, water that cannot penetrate the soil can run off the surface and increase erosion. Water repellency often occurs in localized areas. As a consequence, the soil wets inconsistently and dry spots occur.
In the majority of situations, low water penetration rates are caused by factors other than water repellency. Water naturally moves more slowly into fine-textured (clay) soils because the spaces between the soil particles are just too small to allow rapid water movement. Cultural practices that promote good soil tilth and particle aggregation can improve the infiltration rate on these soils. On the other hand, activities that lessen soil tilth and aggregation make problems worse. Tillage pans (an induced layer of soil which has high bulk density – see my blog post on “High Traffic Areas”) and compaction by personnel and/or machinery also reduce infiltration. In these circumstances, wetting agents will have little or no effect. Remember what I’ve said about soil compaction. There’s not much you can do about it except physically manipulate the soil. There’s nothing you can dump out of a bag or a jug that can reverse soil compaction.
How effective are these wetting agents? Research has been conducted on hydrophobic soils and on the effectiveness of wetting agents. Some of these studies have focused on localized dry spots in turf grown on naturally sandy soils and on formulated materials high in sand content. As one would expect, these dry spots become a grave turf management dilemma during the summer months, especially during periods of drought. In spite of frequent irrigation, the soils in these spots refuse to give in to wetting, resulting in patches of dead or severely wilted turf. The water applied wets the turf but does not adequately penetrate the soil surface to reach the root zone. This is another key – not penetrating leaves (like I said before), but penetrating the soil AND penetrating the soil deep enough to reach the root zone.
In one study of dry spots in turfgrass, it was found that the hydrophobic condition was restricted to the top 1 inch of soil. The infiltration rate in the dry spots was only 20 percent of that measured in normal areas. In other analysis, the hydrophobic layer was from 5 to 18 inches thick. Keep in mind, many different species of turf, crops, shrubs – all kind of plants have varying root zone depths. Applying wetting agents reduced the severity of the condition, but the most effective solution was to use wetting agents in combination with core aeration. Also, keeping the soil consistently moist seemed to be the best protection against the increase of dry spots. Allowing the soil to dry out intensified the predicament.
Many agronomists and other specialists in the management of turfgrass, range land, and forestland have tested the effects of wetting agents on the rate of water infiltration into disturbed and undisturbed soils. Undisturbed soils are like soils in their natural state, whereas disturbed soils are soils where the soil structure is not in its natural state. Somebody has messed with it – either through farming, construction, etc. In general, the results have shown that the extent of improvement in infiltration rate is affected by the type of wetting agent used, its dilution, earlier use of wetting agents on the soil, and the water content of the soil at the time water is applied. Numerous studies have shown that the permeation rate of a hydrophobic soil, once it has been wetted, remains higher than it was before it was wetted, even if it is allowed to dry out again.
Studies have also been conducted to determine whether wetting agents have any toxic effects on plants. In tests on barley shoots grown hydroponically (that is, in a nutrient solution rather than in soil), a wetting agent concentration of 300 parts per million (ppm) in the solution caused a reduction of about 70 percent in the dry weight of the shoots. However, the same concentration in water applied to barley shoots growing in soil or in a sand-peat mixture increased shoot growth only slightly. When wetting agents are applied to soil, the concentration would have to be much higher than 300 ppm before plant growth would be impaired.
Just like many of the products I have discussed in earlier blog posts, you have to take a hard look at what producers or manufacturers are saying. Occasionally 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 not always truthful. Tests in which wetting agents have been applied to normal, “wet-able” soils have failed to substantiate these claims. Some of these exaggerated claims are 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.
A number of techniques 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 real simple and most useful test is simply to place a drop of water on the soil surface and observe how long it takes to penetrate the soil. On a “wet-able” 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 stated before, water infiltrates more slowly into fine-textured soils than into most coarse-textured soils. Poor tillage practices can also reduce penetration rates. Before spending money on a wetting agent, be sure that slow infiltration is being caused by water repellency, not some other factor (Like soil compaction!). Wetting agents will improve infiltration rates only in soils that have water-repellent properties, regardless of their texture, tilth, and aggregation.