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My Blog
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Wetting Agents
| Posted on February 3, 2015 at 2:38 PM |
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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. |