Turfgrass Rootzones

Richard L. Duble, Turfgrass Specialist
Texas Cooperative Extension
Text and images copyright © Richard Duble.

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.

Thus, rootzones not only affect the quality of turf produced, but, also affect the durability, the persistence and the level of maintenance required.

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 orgnaic 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. 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 (Figure 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 (Figure 2).

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 densitites 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.

Chemical properties of rootzones, also, impact turf management pracrices and the quality of turf produced. Fine-textured soils and soils high in organic matter have much higher nutrient holding capacities than coarse textured soils. The nutrient holding capacity of a soil varies inversely with the size of its particles. The nutrient (cation) exchange capacity (CEC) of soils range from 2 to 4 milliequivalents per 100 grams of soil (meg/100 gms) for sands to 40 to 60 meg/100 gms for clay soils. Soils high in organic matter may have CEC's over 100 meg/100 gms.

As a result, nutrients may readily leach through a coarse textured rootzone, such as sand or sandy loam; while nutrients are held in reserve in finer textured soils. The nutrient retention capacity of a rootzone is one measure of its "fertility". Sandy textured soils low in organic matter are usually light colored and considered "infertile"; while loam and clay loam soils high in organic matter are dark colored and considered "fertile".

The relative amount of each cation held by the clay particles is closely associated with specific properties of the rootzone. Highly acidic soils (pH below 6.0) have a high percentage of hydrogen ions (H+) adsorbed on the soil particles and in the soil solution. Alkaline soils (pH 7.5 to 8.5) have a high percentage of calcium ions (Ca++) adsorbed on their soil particles. And highly dispersed soils with low infiltration rates hve a high percentge of sodium ions (Na+) associated with their soil particles.

Thus, the CEC of a rootzone and the relative abundance of the various plant nutrients determine the "fertility" of the soil. Those with high CEC's, a high percentage of calcium associated with the soil particles and an abundance of plant nutrients are considered "fertile" rootzones. Such rootzones,a re typically well aggregated, well-drained and resistant to rapid leaching or loss of plant nutrients.

Soil reaction, or pH, is a measure of the degreeor 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 degreeof 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 from 1x10-7 to 1x10-6, the pH is lowered from 7 to 6 and the soil becomes acid (Table 2). 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.

In soils, hydrogen ions are found in solution and adsorbed on soil particles. As they are removed from the soil solution by plants or microorganisms they are replaced by those on the soil particles. Thus, the soil is resistant to a change in acidity or alkalinity. Only by adding massive amounts of calcium ions to replace hydrogen, or vice versa can we change soil reaction.

For example, the addition of several tons of limestone per acre may raise the pH of an acid soil by the following reaction:

(H+) + CaCO3 ® H2O + CO2 +Ca++
(acid soil) + (limestone)

In other words, the limestone reacts with the acid soil and the hydrogen ion is replaced by the calcium ion.

Likewise, the addition of elemental sulfur to an alkaline soil reduces the pH by the following reaction:

elemental S + 4H2O ® H2SO4 + 6H+
(water) (acid)

In this case, the hydrogen ions (H+) replace calcium and other cations in the soil solution to reduce soil pH.

In alkali soils (pH above 8.5), the soil particles are saturated with sodium ions (Na+) which disperse soil particles (destroys soil aggregates) and seal the soil surface. Alkali conditions can be treated with large applications of gypsum (CaSO4). Gypsum reacts with the alkali soil by the following mechanism:

CaSO4 + Na+ - clay ® NaSO4 + Ca++ - clay

If the site has adequate drainage, the soluble NaSO4 is carried away with the drainage water. Sometimes tile drains must be installed for the procedure to be effective. Without adequate drainage the problem is only aggravated by the addition of gypsum. In calcareous soils, elemental sulfur can be added instead of gypsum. Sulfur reacts with calcareous soils to produce gypsum as follows:

S + 4H2O ® H2SO4 + 6H+

H2SO4 + CaCO3 ® CaSO4 + CO2 + H2O

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 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 (less than 1 percent by weight) 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 water-logged after rain or irrigation. A heavily thatched turf would be an example of a rootzone with excessive organic matter.

Perhaps 2 to 5 percent (by weight) organic matter would be ideal for a turfgrass rootzone. That amount of organic matter would add resilience to the turf, increase soil aggregation, provide adequate water and nutrient holding capacities and contribute to the nutrition of the turf.

Organic amendments commonly added to turfgrass rootzones include peat, rice hulls, saw dust and bark residues. Fresh organic residues such as rice hulls or saw dust 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 break-down 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.

Biological (microbial) activities of turfgrass rootzones are affected by soil texture, soil aggregation and organic matter. Fine textured soils, well aggregated soils with good drainage and soils high in organic matter have high biological activities.

Six major microbial groups are associated with biological activity in turfgrass rootzones: bacteria, actinomycetes, fungi, algae, protozoa and viruses. Biological activities associated with these microbes include turfgrass nutrition, thatch accumulation (or decomposition), chemical (pesticide) decomposition and soil aggregation (associated with humus...a product of microbial activity).

Soil microbes have a major impact on plant nutrition. Nearly all of the nitrogen and much of the phosphorus and sulfur, as well as other plant nutrients, is bound in the soil and unavailable for use by turfgrasses. Through the activity of soil microbes, these nutrient are made available for uptake by grasses and other plants. Nitrogen, for example, is converted from an organic form to a nitrate (NO3-) form by bacteria. Sulfur and phosphorus are also converted from unavailable forms to sulfate (SO4-) and orthophosphate (HPO4-) which can be utilized by turfgrasses.

The role of fungi in the decomposition of plant residues can be demonstrated by thatch accumulation in turf frequently treated with fungicides. Other biological organisms such as earthworms also have a significant role in the decomposition of plant residues (thatch).

Soil microbes demonstrate remarkable capacitites to breakdown chemical pesticides to compounds that are not considered toxic. Without this microbial degradation we could not use many of the pesticides we depend on today to keep turfgrasses healthy and weed-free.

Soil aggregation -- the binding together of soil particles -- is due in part to the physical binding of soil particles by the structures of fungi and actinomycetes. The formation of humus -- the final product of microbial degradations -- also aids in the aggregation of soil particles.

All of these biological activies which depend on soil microbes, are sensitive to changes in the environment of the turfgrass rootzone. Changes in pH, moisture, temperature and aeration can significantly alter the relative composition of soil microbes and, consequently, alter biological activity. For example, reducing soil pH through the use of nitrogen fertilizers over a period of years causes a shift in the population of soil microbes responsible for nutrient conversions. As a result, nitrogen, phosphorus and other nutrients are not as readily available to the turfgrass. Nitrification, the conversion of organic nitrogen to nitrate (NO3), decreases with increasing acidity and is not observed at pH levels below 4.5. Likewise, increasing pH through prolonged use
of high sodium water inhibits microbes that degrade certain groups of pesticides.

Aeration has a dramatic effect on the population of soil microbes. In a well drained soil aerobic microbes are dominant. Aerobic microbes utilize oxygen to breakdown organic residues with the release of carbon dioxide (CO2). In poorly drained soils anaerobic microbes are dominant. Anaerobic microbes reduce sulfur to hydrogen sulfide (H2S) which produces black layers in soils with the putrid odor of rotten eggs. The H2S gas is toxic to grass roots and can lead to the loss of grass. These conditions develop in turfgrass rootzones that are overwatered or where impermeable layers develop that restrict water movement.

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 minues 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 impereable 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 ona regular basis. Certainly, growth rate and color should be evaluated weekly, or even daily. Leaf tissue analyses must also be evaluted on a regular basis to develop benchmarks for making comparisons. Monthly tracking of nutrient tissue levels during the growing season may be adequate for all but the most intensively maintained facilities such as golf greens. Deficiencies of phosphorus, potassium, iron and other nutrients can be determined from tissue analyses (Figure 4). Soil tests may be conducted annually, or semi-annually on golf greens to determine the availability of nutrients.

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.

Like soil moisture, soil pH influences the availability of nutrients. At very low and very high pH levels phosphorus and some minor nutrients are not readily available. The efficiency of nutrient uptake is also reduced at low pH levels. Nitrogen utilization, for example, may be reduced 50 percent at pH 5.5 compared to pH 7.0. At high pH levels, above 8.0, iron availability reaches critical levels. Annual soil tests allow the turf manager to monitor pH levels and identify problem areas. Where pH levels require amendments more frequent testing should be done.

Soil pH levels can be adjusted with limestone on acid soils and sulfur on alkaline soils. Again, soil test information should be used to determine the amount of limestone or sulfur needed to adjust pH to the desired level.

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 nutritonal status of compacted soils.

Soil microbes also play a significant role in the availability of plant nutrients. Nearly all of the nitrogen and most of the phosphorus and sulfur, as well as other nutrients, are bound in soil organic matter. In this form these nutrients are largely, or entirely, unavailable for utilization by grasses. It is only through microbial activity that the vast store of nitrogen and the reserve phosphorus and other nutrients are made available to the grass. Thus, rootzone conditions such as compaction, saturation, salinity, acidity, and low organic matter that reduces microbial activity also reduces nutrient availability.

For example, in compacted soils, or in poorly drained soils, populations of microbes shift from those that function in aerobic conditions to those that function under aerobic conditions. As a result, hydrogen sulfide (H2S) rather than CO2 becomes the primary product of decomposition and grass roots deteriorate rapidly.

Also, under compacted soil conditions bacterial reduction of nitrate to nitrogen (denitrification) results in significant losses of nitrogen from the rootzone. Denitrification is favored by anaerobic conditions; thus aeration will reduce those losses from compacted soils.