This web page was originally presented by us as a three part series on Turfgrass Rootzones. 

In this article Dr. Richard Duble (Texas A&M) provides an introduction to the importance of building and maintaining a quality rootzone.  He discusses soil textural properties and the soil textural triangle.  He also discusses key rootzone properties, such as, bulk density, pH, and organic matter content.  Finally, he delves into how the rootzone’s physical properties affect water management and nutrient management.

Introduction
Turfgrasses are grown on many types of substrates - gravel, sand, silt, clay, organic or combinations of these components. These components form a matrix (a rootzone) that is characterized by specific physical, chemical and biological properties. These properties of the rootzone determine the quality of turf that can be produced under different environment, management and use conditions.

For example, a fine bermudagrass turf can be maintained on a sports field or golf green if the rootzone is well-drained. A poorly drained rootzone leads to failure because of shallow rooting, compaction, wear and slow recovery. Likewise, buffalograss performs poorly on sandy rootzones; centipedegrass does poorly on alkaline rootzones and ryegrass performs poorly on saline sites - all properties of specific types of rootzones.

In addition to the quality of turf, the level of inputs - time and money - required to maintain turfgrasses on different rootzones may be equally important. A well-drained rootzone on a sports field or golf green needs fewer inputs than a poorly drained rootzone.

The durability and persistence of turfgrasses are also influenced by the characteristics of the rootzone. For example, wear injury is much greater on a compacted, wet rootzone, than on a loose, well-drained rootzone. And, some grasses are not persistent on sandy, alkaline or saline rootzones.

Fig. 1 Soil Textural Triangle

Rootzone Characteristics
The solid components of a rootzone largely determine its physical, chemical and biological characteristics. The solid components of a rootzone include minerals (gravel or coarse aggregates, sand, silt and clay) and organic matter. The relative proportion of these components, the uniformity of their distribution and their depth determine the physical, chemical and biological characteristics of a rootzone. Physical properties of a rootzone include texture, porosity, structure and bulk density.

Texture
The texture of a rootzone is determined by the relative particle size distribution of its mineral components (sand, silt and clay). Texture can be identified on the soil textural triangle by plotting its percentages of sand, silt and clay (see fig. 1). For a soil to be described as sandy it must have at least 50% of its mineral particles in the sand designation (above 0.05 mm in diameter). Likewise, a soil with 50% or more of its particles in the clay designation (less than 0.002 mm in diameter) is described as a clay soil. Soils with a wide distribution of particle sizes are usually described as loams.

Soil texture is important because it influences the surface area, the porosity and the density of a turfgrass rootzone. The surface areas of the various particles in a rootzone affect the chemical, biological and physical activity of the rootzone. Fine textured particles, such as silt and clay, contribute great surface area to a rootzone. For example, a volume of clay particles has at least 50 times the surface area of the same volume of sand particles.

Consequently, clay particles increase the chemical activity of a rootzone by providing more sites for holding and exchanging plant nutrients. Clay particles are, also, important cementing agents in a rootzone that contribute to the aggregation of soil particles into structural units or aggregates. Aggregation of soil particles into larger units increases the large pore spaces that allow for the movement of air and water into and out of the rootzone. Organic matter is another important cementing agent in the rootzone.

Texture also determines the porosity and the size distribution of pore space in a rootzone. Coarse textured rootzones (sands) have limited pore space with large pores making up most of the space. Fine textured soils (clay loam, silt loam, clay) have greater total pore space and small pores are dominant. Water occupies the smaller (capillary) pores. Consequently, fine textured soils hold more water than coarse textured soils.

Bulk Density
Bulk density, the weight of a given volume, is another physical characteristic of a turfgrass rootzone. Hard, compacted rootzones have relatively high bulk densities, above 1.6 grams per cubic centimeter; whereas, well aggregated soils and rootzones high in organic matter have bulk densities between 1.45 and 1.6. Rootzones with excessive organic matter are characterized by spongy, waterlogged conditions during wet weather. Such rootzones are not well suited for golf courses and athletic fields. A bulk density below 1.3 gms/cc may indicate such a condition.

pH
Soil reaction, or pH, is a measure of the degree of acidity or alkalinity of a soil, or rootzone. The relative amount of hydrogen ions (H+) and hydroxyl ions (OH-) in the soil solution determine the degree of acidity or alkalinity. A predominance of hydrogen ions makes a soil acid; a predominance of hydroxyl ions makes a soil alkaline.

A system of expressing soil reaction in terms of pH was developed by a Danish biochemist. To avoid more complicated terms he defined pH as the hydrogen ion concentration. For example, a soil with a concentration of 1x10-7 hydrogen ion moles per liter has a pH of 7. At a pH of 7 the concentrations of hydrogen and hydroxyl ions in the soil solution are equal and the soil is considered neutral. As the hydrogen ion concentration increases the pH is lowered and the soil becomes acid. Since the scale is logarithmic, a change of one pH unit represents a ten-fold increase or decrease in hydrogen ion concentration. Likewise, a change of two pH units represents a hundred-fold increase or decrease.
 

An extremely low or high soil pH is toxic to grass roots and leads to the loss of turfgrasses. However, nutrient availability is affected by only moderate deviations from a neutral soil pH. At pH levels below 6, nitrogen and phosphorus availability is reduced. And at pH levels above 7.5 the availability of most minor nutrients, iron in particular, is reduced. The chlorotic condition of grasses in alkaline soils is frequently due to an iron deficiency. Soil organisms are also sensitive to only moderate changes in pH. Thus, it is important for the turf manager to monitor soil pH and to add the amendments (limestone, gypsum or elemental S) needed to neutralize soil acidity or alkalinity.

Organic Matter
Organic matter is another component of the turfgrass rootzone. The organic fraction of the rootzone consists of plant residues in various stages of decay, grass roots, microorganisms and their amendments (such as peat, rice hulls, etc.) that may have been added to the rootzone during preparation. On a weight basis the organic fraction of a turfgrass rootzone may range from 1 percent, or less, to 8 to 10 percent. In some areas of the U.S., turfgrass is produced on muck soils that contain 30 to 40 percent organic matter on a weight basis.

On a volume basis the organic fraction constitutes a much higher percentage. For example, a one to one mixture of peat and soil on a volume basis may be only 5 percent organic matter on a weight basis. Thus, organic matter adds "bulk" to the soil and reduces the density of mineral soils. A mineral soil may have a "bulk density" of 1.6 gms/cc, whereas, a muck soil may have a "bulk density" of only 1.2 gms/cc.

Organic matter contributes significantly to the physical and chemical properties of a turfgrass rootzone. Organic matter reduces bulk density, increases porosity, increases nutrient and water holding capacity, increases soil aggregation, increases aeration and water movement and provides a source of plant nutrients.

Turfgrass rootzones that are low in organic matter are typically hard, droughty, compacted and deficient in plant nutrients. At the other extreme, turfgrass rootzones high in organic matter are typically soft (spongy) and waterlogged after rain or irrigation. A heavily thatched turf would be an example of a rootzone with excessive organic matter.

Organic amendments commonly added to turfgrass rootzones include peat, rice hulls, sawdustand bark residues. Fresh organic residues such as rice hulls or sawdust must undergo decomposition before they benefit the soil. Fresh organic residues may tie-up plant nutrients and heat-up the soil to the degree they cause problems. Supplemental nitrogen and limestone may be needed to breakdown fresh organic materials. Also, uniform mixing with the soil is essential to prevent "hot spots" in the rootzone. Heating, which produces the "hot spots", is associated with the decomposition of fresh organic materials and can reach temperatures that kill or injure grass roots.

Rootzone Management
Turfgrass managers must consider the soil as the growing media for their crop, sports fields, lawns, golf courses, etc. For any other crop, particularly horticultural crops, growers go to great efforts, and expenses, to provide an optimum growing media. Unfortunately, people tend to assume that grass can grow on any site without regard to soil conditions. People plant grass on hard, compacted soils, poorly drained or waterlogged soils, clay pans or layered soil profiles, and other conditions where one would not consider trying to grow a tree or shrub.

If we consider the soil as the growing media for turfgrass then we are more likely to be concerned with the environment that a particular soil provides. For example, a hard compacted soil on a poorly drained site provides a harsh environment for plant growth. We would not consider planting a tree of shrub on such a site, but we frequently plant grass on similar sites. Ideally, those soils would be modified to provide conditions favorable for plant growth. But, usually we are left to manage such soils and are expected to produce fine quality turfgrass.

Water Management
The first priority with respect to rootzones for growing turfgrasses is water management. The turf manager who can control water in the rootzone has a great advantage over those who cannot. The ideal rootzone would hold adequate available moisture for turfgrass growth for 5 to 7 days, yet permeable enough so that water would not stand on the surface for more than a few minutes following heavy rainfall or irrigation. Deep sandy loam soils with organic amendments incorporated in the rootzone usually have those characteristics. As the rootzone deviates from this ideal, water management becomes more difficult.
Slowly permeable soils need adequate surface drainage to aid water management, since standing water creates a totally unfavorable rootzone environment for turfgrasses. Where standing water consistently occurs after rainfall or irrigation, drains must be installed to remove excess water. Narrow trenches, 10 to 14 inches deep, backfilled with sand or gravel or geotextile fabric covered drains provide excellent pathways to remove excess water. Properly installed, these subsurface drainage systems can remove surface water within 30 minutes following a rainstorm.

Core aeration also helps get water into a slowly permeable soil by increasing the surface area of the rootzone and by breaking up surface crusts or impenetrable layers near the surface. Aeration provides only temporary improvement in water management and must be repeated when surface crusts and layers develop. Topdressing with a permeable medium such as sand or sand and organic amendments helps keep the vertical cores open after aeration. Repeating these practices three or four times for several years can significantly improve surface conditions. However, modifying the surface few inches of the rootzone does not solve the drainage problems. The combination of providing surface drainage, installing subsurface drains and modifying the surface of the rootzone, improves the ability of the turfgrass manager to manage water in the rootzone.

Nutrient Management
The nutrient status of the rootzone may be the second priority of the turfgrass manager. Growth rate, density, root development and color are some of the responses to the turfgrass nutrient status of the rootzone. If nutrients are not present in required amounts or are not available to the grass for some reason, then weak turf, poor color and slow recovery will be apparent.
Growth rate, color, leaf tissue analyses and soil tests provide the means for the turf manager to determine the nutrient status of the rootzone. Each of these indicators provides useful insight into the nutrient status of the rootzone, and each should be evaluated on a regular basis.

Moisture, pH, texture and biological activity of the turfgrass rootzone all influence the availability of nutrients. Even though nutrients may be present in adequate amounts, they may not be available to the grass in saturated soils, highly acid or alkaline soils, compacted soils or in soils with very low biological activities. To maintain conditions favorable for nutrient uptake, the turf manager must control soil moisture through irrigation and drainage. Excessive irrigation causes leaching of some nutrients, especially nitrogen and potassium. Also, saturated rootzones result in anaerobic conditions where nutrients are not available and gases toxic to grass roots are produced. Denitrification occurs in saturated soils. Matching irrigation rates to water use rates will reduce problems associated with saturated rootzones.

Soil texture and structure also influences the nutritional status of a rootzone. Coarse textured soils have very low cation exchange capacities and, consequently, low nutrient retention capacities. Ammonium nitrogen, nitrate nitrogen, potassium and other nutrients readily leach through a sandy soil. The addition of organic materials greatly improves nutrient retention in coarse textured soils. In contrast to sandy soils, clay soils and soils high in organic matter have very high cation exchange capacities.
Compacted soils, or soils with poor structural characteristics, do not provide adequate nutrients for good growth of turfgrasses. Aeration and the incorporation of organic matter usually improves the nutritional status of compacted soils.

Soil microbes also play a significant role in the availability of plant nutrients. Rootzone conditions such as compaction, saturation, salinity, acidity, and low organic matter that reduces microbial activity also reduces nutrient availability.

Send mail to turfdiag@turfdiag.com with questions or comments about this web site.
Copyright © 2005