Getting Water In
Water repellency is an asset to a duck, but a problem in sand-based root zones.
Utility is a function long associated with turfgrass. For this reason, rapid soil drainage is a highly favorable trait of turfgrass systems. A well-drained soil enables activities, such as golf or other athletic events, to rapidly commence following an untimely rainstorm. Furthermore, unconditional air-filled porosity facilitates gas exchange and promotes root growth and health. Most turf managers know sand is the low-cost mineral fraction/amendment to achieve well-drained soil when used as the primary component of constructed root zones.
However, water repellency in the surface of sand-based athletic fields and golf course greens is a common management problem. Because of the inherent uniform coverage of turfgrass, irregular wetting patterns and pockets of water repellency result in unsightly vegetative desiccation (see photo above), often referred to as localized dry spots. This leaves many managers wondering: Where does water repellency come from and how quickly can it develop?
The cause of water repellency is hydrophobic coatings on soil surfaces, and their origin has long been recognized as decomposing plant litter. Fresh soil organic matter additions, acting as reservoirs of hydrophobic waxes, are prime substrates for soil microbial populations. Over the course of a day, an average soil microorganism can exude and liberate hundreds of organic compounds (fatty acids, extracellular enzymes and polysaccharides). These waxy, organic compounds diffuse through solution and readily coat sand particles (see Fig. 1 below). This phenomenon of soil water repellency has long been associated with basidiomycete fungi activity; yet, fungi are not a prerequisite for water repellency development.
Certain factors that contribute to repellency are reasonably understood but difficult to prevent. Soil water repellency increases with age of resident vegetation, and is most severe in soils with a per-manent vegetative cover. Wetting and drying cycles have also been implicated in water repellency development. When researchers mix sand with intrinsic particulate organic matter and induce wetting/drying cycles, water repellency develops in the previously wettable sand. Systematic wetting/drying cycles (without any additional intrinsic particulate organic matter) continue to increase water repellency of sand, identifying soil wetting/drying cycles as a contributing factor.
Sands are more prone to develop water repellency than finer soil textural classes. This is attributed to the low surface area of sand. If limited surface area facilitates water repellency development, then marginal additions of clay should reduce hydropho- bicity of a water-repellent soil, right? Yes. Researchers have successfully reversed water repellency by the addition of clay minerals, but only after a wetting/ drying cycle. Microscopy subsequently revealed the clay colloids physically coated the hydrophobic surfaces, allowing the soils to absorb and retain water. However, inclusion of fine particles in athletic root zones defeats the purpose of the sand-based design through gradual reduction of drainage, air-filled porosity and compaction resistance.
For all of these reasons, soil water repellency continues to be problematic to the health and performance of high-maintenance turfgrass systems (photo, on page 16). This fact is exacerbated by irrigation water quality and quantity limitations being imposed on turfgrass managers. Non-ionic soil surfactants (wetting agents/soil penetrants) are commonly applied to water repellent sand root zones to coat hydrophobic coatings and render them wettable. This is likely the most rapid and effective cultural practice used by managers.
Given microbial action on soil, organic matter is the primordium of hydrophobic coatings on soil particles, requisite time and degree of use imposed may govern water repellency development rate. Our experimental objectives were to determine: 1) the temporal dynamics of water repellency formation in 80:20 (v:v) sand/sphagnum peat moss root zones established with creeping bentgrass [Agrostis stolonifera L. ‘Crenshaw’]; and 2) how water repellency formation rate is influenced by repeated compressive impact forces applied to the surface of these simulated root zones.
We thoroughly mixed quartz sand, sphagnum peat moss and organic fertilizer (6-2-0; Milorganite, Milwaukee) to contain 80 percent quartz sand and 20 percent peat moss by volume, and composed of coarse, medium and fine sands in proportions commonly used for athletic field/putting green root zones. Once homogenized, we transferred root zone mixes to 1.3-foot-long sections of polyvinyl–chloride (PVC) pipe (2-inch ID).
Once constructed, we sodded columns with ‘Crenshaw’ creeping bentgrass and irrigated as necessary to prevent wilt. Columns were mowed regularly, yet we applied no topdressing, pesticides or supplemental fertilizer following establishment. We applied compactive force applications to half the experimental units, twice every 6 months. We initiated the compaction treatments about 130 days after sodding, by dropping a 2.7-pound cylindrical stainless-steel weight from a 2.6-foot height onto each turfgrass surface three consecutive times. We made a second application of three impact events about 20 days later. We reapplied these six impact events as the compaction treatments every 6 months.
Following 6-, 12- or 18-month experimental periods, we sectioned the surface 2.7 inches of the root zone mixtures and measured the percolation rate by laboratory methods. We then carefully extruded the root zone from each column intact, and divided it at the 1.3-inch depth. We discarded turfgrass leaves and shoots, and separated the root zone by gentle elution. We air-dried all remaining sand and particulate organic matter (not identifiable as roots) to a constant mass and analyzed them for soil water repellency by three standard laboratory methods.
Percolation rate is an important attribute of turfgrass root zones. Compaction treatments significantly lowered percolation rate, regardless of experimental period. Despite these measured reductions, saturated conductivity rates of compacted root zones fell within the accepted range for putting green root zones (greater than 6 inches per hour). These data show that compaction did not have increasingly deleterious effects on percolation over time, allowing early-time water infiltration rate (how we measure water repellency) to be measured on root zones of varying age without bias from percolation rate change.
Upon completing the analysis, we concluded that the greatest cause of water repellency in the upper sand profile of the root zones was time (see Fig. 2, on page 17). Root zone maturity, or length of time sands were exposed to the turfgrass system, was directly related to increasing soil water repellency. This significant trend was indicated by every soil water repellency measure employed in the study. This significant time effect likely culminated from the following naturally occurring soil processes: particulate organic matter deposition, wetting/drying cycles and saprophytic assimilation of soil organic matter by indigenous soil microbes.
The effect of compaction treatment on water repellency development was not quite as stark as the effect of root zone maturity. Of the three water-repellency measures conducted, one indicated a significant effect-of-compaction treatment, and another affirmed a compaction- by-age interaction. All methods indicate soil water repellency to be directly related to root zone age, and this effect generally supercedes that of compaction. However, at the 6-month root zone age, water repellency in the compacted columns was significantly greater (6 percent) than levels observed in the non-compacted columns (see Fig. 2 at right). We observed no significant differences following the 1- or 1.5-year experimental periods. These data indicate compaction and compressive forces contribute to water repellency in the early formation stages.
How do compaction forces accelerate water repellency development? Frequency and force of compressions on root zones containing organic matter may promote release and diffusion of hydrophobic/waxy com- pounds to nearby solid surfaces. Mechanisms responsible for dispersal of non-polar waxes under repeated compressions (traffic) include shearing force (on particulate organic matter) and enhanced organic matter/mineral fraction intimacy. Again, these compaction effects are generally masked by over- riding contributions of common root zone maturation processes.
WHAT CAN BE DONE?
These and other results show sand-based turfgrass systems are prime candidates for water repellency development. The perennial nature of turfgrass systems, their excessive organic matter production rate, and limited surface area of their predominantly sand sur- face horizon all result in virtually dependable development of resistant, non-polar, organic coatings on sand grains of the upper root zone. Our results indicate that turfgrass managers should monitor the severity of water repellency within four months of turfgrass establishment. In the time period immediately following establishment, there appears to be an acceleration of repellency in the sand-based root zones of areas receiving repeated compaction. Thus, high traffic areas of sand-based root zones (interior of hash marks, putting green plateaus or entry/exit regions) may deserve extra soil sampling attention when assessing water repellent conditions. One year following establishment, water repellency is more likely to be severe, and this trend appears to continue indefinitely, regardless of degree of compaction.
Although development of soil water repellency in turfgrass sand root zones seems rather certain, cultural practices can help you delay its manifestation, if you implement them early and often. The most obvious is topdressing with wettable sand. Water repellency is always greater in the upper regions of sand root zones, and lessens with increasing depth. Thus, reducing water repellency at the root zone surface will improve infiltration rate and enhance irrigation efficiency. Topdressing with wettable sand dilutes or offsets the repellent nature of this uppermost hydrophobic layer. Similarly, core aerification displaces these uppermost bands of water-repellent sand. Harvest and removal of evacuated cores effectively disrupts continuity of the surface-repellent sand layer, combating localized dry spot development. Likewise, dragging cores back into aerification holes may not remove these coated-sands outright, but will disrupt the coated-sand continuity and lessen their concentration at the root zone surface. A novel approach to preventing repellency development may be to minimize drying of the root zone. However, irrigating daily to minimize repellency development sounds more like problem-trading than problem solving!
Incorporation of internally-porous inorganic amendments (zeolite, calcined clay or diatomaceous earth) into sand-based root zones has been shown to reduce severity of water repellency. Current research at Penn State University is evaluating the roles wetting/drying cycles and surface area play in development of water repellency in sand-based root zones. Various soil amendments are being tested, and results will be made available in 2006.
In the meantime, systematic application of wetting agents is likely the most effective and least disruptive short-term treatment of water-repellent sand root zones. Wetting agent efficacy varies by soil chemistry, thatch content, environmental conditions, and repellency severity. GCSAA and USGA sponsored research on the comparative effects of several popular wetting agents. You can access the research results at www.gcsaa.org/gcm/2005/april05/04Re2.asp. The author recommends in-house testing of different samples of wetting agents before deciding which is best for your particular root zone.
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