|by James C. Thomas C.P.Ag.
Presented December 4, 2000 at
the Texas Turfgrass Association Meeting in San Antonio, TX
To understand soil and its behavior, one must first understand its
composition and the interrelation between the soil components. If you
were to reach down and scoop up a handful of soil it could be divided
up or separated into 4 primary components: minerals, water, air, and organic
The mineral fraction consists of the actual soil mineral particles
sometimes referred to as soil grains. Soil scientists have traditionally
classified soil minerals on the basis of their size. The USDA has developed
a widely used and accepted Soil Textural Classification System which divides
minerals into sand, silt and clay particles based on their sizes. Sand
particles range from 0.05-2.0 mm in size, silt particles are in the range
of 0.002-0.05 mm, and clay particles are the smallest at less than 0.002
mm in size. To get an idea the importance of size, I have calculated the
approximate number of particle in 10 grams (about one teaspoonful) of
soil. If all the particles in a 10 gram sample were 1 mm sand particles,
there would be approximately 7,207 particles. The same weight of soil
would require approximately 7.2 million silt sized particles or 7.2 trillion
clay sized particles. Therefore, due to the large amount of fine particles
in a small volume or weight of soil, the fines exert a large influence
on the behavior of soil. It also follows that a small change in the percentage
of silt and clay makes a big difference in the number of particles and
in the performance of the soil. Thus, if the amount of silt and clay in
a root zone mixture increases from 1% to 4%, this represents the addition
of a very large number of fine particles.
The second component of soil is water. Water typically is found either
as free liquid in the pores or voids between soil particles or as thin
films surrounding soil minerals. Soil water is typically classified into
one of three groupings. The first class of soil water is "Gravitational"
or "Drainage Water". This is water which resides in the very large soil
pores and will drain away rapidly due to the pull of gravity. Thus, shortly
after rainfall or irrigation is over, the gravitational water is lost
from the profile and is not available for turf consumption. The second
class of soil water is called "Plant Available Water". This water is held
in the soil tightly enough to prevent its loss due to gravity but loosely
enough to allow plant roots to withdraw it. The final class of soil water
is "Hygroscopic Water". This is water that is held very tightly in thin
films around soil particles. Hygroscopic water is held too tightly to
be available to plants and cannot be removed except by drying the soil
at elevated temperatures.
The third component, soil air, is composed primarily of nitrogen, carbon
dioxide and oxygen. Air fills the large pores or voids between the soil
particles after the gravitational water has drained away. As more water
is used, more air is drawn into the soil. Then when rainfall or irrigation
events occur, the air is forced out of the soil and the process is repeated.
The oxygen in the soil air is needed to support the growth of turf roots.
Plant roots respire or breath much like animals taking oxygen from the
air and using it to oxidize sugars and carbohydrates to make energy needed
for growth and the uptake of nutrients. Ideally, a soil should contain
15-30% air-filled pore space, however, it can go as low as 10% with minimal
impact on the turf. When the air-filled porosity drops below 5% there
will generally be serious adverse effects on turf growth and quality.
The fourth and final component of soil is organic matter. Organic matter
is a rather diverse grouping that includes all materials that are primarily
composed of carbon, hydrogen and oxygen. The common or laymen's definition
of organic matter would be "living, dead, or partially decayed plant and
animal substances". This definition includes all the live and dead plant
roots, rhizomes, stolons and leaves. The entire soil microbiological community
is also a part of the soil organic matter pool and typically amounts to
approximately 30 lb/1,000 square feet. It also includes any peat moss,
rice hulls, or other organic matter sources that may have been added to
the soil at the time of construction. In the southern US, temperatures
and moisture are favorable for degradation of organic matter. Therefore,
organic matter content of southern soils tends to be much lower than that
of the more northern climatic regions.
Organic matter has many beneficial functions in soil including: retaining
nutrients, retaining water, reducing the saturated hydraulic conductivity
of sandy soils, and acting as a binding agent between soil particles to
promote the formation of soil aggregates. Organic matter has a high cation
exchange capacity (CEC) which allows it to attract and retain cations
in an exchangeable form. Thus, as plants remove potassium, calcium, magnesium
and other positively charged nutrients from the soil water, they will
be replenished by some of the ones which were initially held on the organic
matter. This helps insure adequate plant nutrition between fertilizer
application events. The high CEC also tends to help buffer the soil pH
and prevent wild fluctuations in pH. Organic matter is also very effective
in absorbing and retaining pesticides and minimizing their potential to
wash off into surface water bodies. Finally, organic matter serves as
a food source for soil microbes and as such helps to maintain an adequate
and healthy population of soil microbes.
Another major property of soil is the unit weight or "Bulk Density".
Bulk density is defined as the dry weight of material per unit of volume.
It is most commonly expressed in units of grams per cubic centimeter (g/cc).
A low bulk density indicates an abundance of voids or pore spaces while
a high bulk density indicates an increased amount of minerals and a reduced
amount of pore spaces. Closely related to bulk density is the concept
of compaction. Compaction is defined as "the process by which soil grains
are rearranged to decrease the void space and bring them into closer contact
with one another, thereby increasing bulk density". Examples of processes
which result in soil compaction include: foot traffic, equipment traffic,
vibrating equipment, rollers, and packers. Essentially, increasing soil
compaction is much like a contest to see how many people you can fit into
the cab of a pickup. Initially with only a driver in the truck, there
is a lot of void space and the density is low. As you add more and more
people, the weight increases while the total volume remains constant.
Consequently, the volume of voids or air-filled pore space decreases.
Thus, the more people you put in the vehicle, the higher the density and
the lower the air-filled porosity.
Soil compaction results in numerous problems and may eventually prevent
the growth of turf. As previously discussed, compaction results in reduced
soil aeration. Since compaction reduces the amount of large pores in the
soil it also reduces the infiltration rate which is a measure of the rate
at which water enters the soil. Compaction also results in an increase
in the number of fine pores in the soil which causes the soil to remain
wet for prolonged periods of time after rainfall or irrigation events.
The combination of wet and poorly aerated soil results in shallow rooted
turf which has a reduced ability to withstand winter cold and summer heat
stresses. When the situation becomes severe, it may result in the formation
of black layer.
Compaction is primarily caused by traffic, be it foot traffic, maintenance
equipment or vehicular (golf carts, tractors, cars and trucks). In addition,
certain cultural practices may result in some soil compaction. Soil compaction
can be decreased by the addition of organic matter to the soil. Periodic
aerification can also help reduce compaction, particularly if the equipment
has the ability to move laterally while in the soil and impart a fracturing
or lifting effect on the soil. Tillage operations certainly decrease the
bulk density of the upper layers, however this procedure is very disruptive
and is only an option in the event of renovation of an area. Finally,
earthworms are nature's aerifiers and create tunnels and channels throughout
the soil profile.
The data in table 1 show typical values of bulk density and porosity
for a variety of materials. There is a clear inverse relationship between
density and porosity. For very porous materials such as potting mix, the
bulk density is very low and the porosity is very high. As you go down
the list to sands and then compacted sandy loam, the density increases
and the porosity decreases as more mineral matter is forced into a given
volume. Finally, for many rock materials the bulk density equals the particle
density and the porosity is zero.
Typical Values for Bulk Density and Porosity
Surface Clay (wet)
Golf Green Mix
Surface Loam Soil
Compacted Sandy Loam
It was mentioned earlier that traffic is the major cause of soil compaction.
While little research has been done on foot traffic, there has been some
published studies of compaction resulting from vehicular traffic. In general,
researchers have measured significant increases in bulk density to a depth
of 30 cm due to compaction from wheeled vehicles (usually farm tractors).
Another concept that we need to discuss is water movement through soils.
As water enters the soil, it fills up the large pores first and then spreads
into the smaller pores. The water then moves downward from the surface
to deeper depths in the soil in response to the pull of gravity. When
the water encounters a layer of coarser gravel, further downward water
movement will stop temporarily until the bottom of the upper layer becomes
nearly saturated. Once a sufficient "head" or pressure is developed, then
water will move into the underlying coarse material. The underlying principle
that is responsible for this behavior is that the soil has finer pores
in it than does the coarser gravel. Thus, the soil has a greater attraction
for the water. Until this attraction is satisfied and some excess water
builds up to overcome the surface tension, water will not flow into the
underlying coarser gravel layer. This is known as a perched water table
and is a primary principal upon which the USGA recommendations for putting
green construction were based. The use of a gravel bed under the entire
green or athletic field results in more uniform soil moisture conditions
as compared to the same root zone mixture directly overlying the drain
lines without a gravel blanket.
The principles of soil composition, particle size, density, compaction,
and water movement can be directly applied to field situations and everyday
problems. Issues about layering, root zone depth, topdressing sand selection,
aerification, and fairway soil selection all involve the above principles.
Making decisions when building a new facility or when managing an existing
facility should at a minimum include a careful consideration of the physical
aspects of soils.
One issue that frequently arises is that of layering. Since very coarse
sand particles are difficult to work down into the turf and damage mowers,
greens and athletic fields are often top dressed with sands that are finer
than those used in the original root zone mixture. After some years of
topdressing with these finer sands plus the accumulation of some organic
materials, a layer of fine topdressing sand mixed with organic matter
develops at the soil surface. Because of the fineness of the sand, this
layer retains a higher amount of water than the original root zone mixture.
This is further aggravated by the high amount of organic matter in the
layer. Thus, the upper layer stays too wet, and is poorly aerated. Thus,
the layer is not well suited for good root development and the roots tend
to stay close to the surface where they can get adequate oxygen. If the
condition develops to the point that it becomes devoid of oxygen, black
layer conditions can develop.
Occasionally we encounter the reverse problem with a layer of coarser
sand over a finer material. While this problem is less serious, it is
still a management problem. First, the coarse sand will likely have a
low water holding capacity and will likely be droughty. This may cause
difficulties in managing water during the grow-in stage until the turf
develops an extensive root system. The rapid saturated hydraulic conductivity
and low cation exchange capacities typical of coarse sands will promote
leaching of plant nutrients - particularly nitrogen and potassium. A third
problem associated with coarse sands placed over finer materials is the
need for drainage. Since the coarse sand will have a relatively high infiltration
rate, water will rapidly enter this layer until it becomes saturated.
When the rainfall or irrigation event is over, provision must be made
to allow the gravitational water to escape. This may require the installation
of a subsurface drainage system in some instances. In some cases, the
permeability of the underlying soil may be sufficient to handle the drainage
water in a reasonable amount of time. In either case, suitable provision
must be made to prevent the water from remaining in the coarse layer for
an extended period of time or it will drown the plant roots and result
in anaerobic soil conditions.
Another question which is frequently asked is "How important is it
to have a uniform depth of root zone mixture?" Since the water films in
the soil are all interconnected, water at the surface of the soil is connected
to a hanging water column which is as deep as the root zone mixture. Furthermore,
the laws of physics tell us that the longer the water column is, the more
water it will remove and the drier the soil will be. Figure 1 shows the
moisture content of a 90/10 sand/peat root zone mixture at tensions ranging
from 0-40 cm. These tensions represent soil depths ranging from 0-40 cm.
The data indicate that the root zone mixture loses a large amount of water
between 20 cm and 40 cm tensions. Thus, changing the depth of placement
by only 1-2 inches can have a significant effect on the amount of water
retained in the soil. Therefore, areas with a greater thickness of root
zone mixture will be drier, while shallower areas will be wetter. Because
of this, it is recommended that a root zone mixture be placed at a uniform
thickness of ?0.5 inch. In this way the spatial variability in moisture
contents will be minimized.
Selection of suitable topdressing sands is another issue that is frequently
raised. As noted above, a sand with the same particle size distribution
as that used to construct the area being top dressed is ideal. If an exact
match can not be found, then you should select one which is slightly coarser
than the original. This will avoid the formation of an excessively wet
and poorly aerated layer near the surface. Given the very tight turf mat
formed by some of the new ultra-dwarf grasses, serious consideration to
having suitable topdressing materials should be given during the construction
Questions often arise about the need to fill aerification holes. It
has been my observation that aerification holes left open will collapse
and fill in by themselves in a few days. Essentially, what happens is
that the compactive forces of traffic cause the soil to shift together
and fill in the hole. Once this happens, you have lost the majority of
the benefits of aerification. If, however, the aerification holes are
filled with a clean topdressing sand, the channels will remain open and
effective in transmitting water and air much longer. Examination of samples
from turf areas often show a shallow overall turf root depth with isolated
clumps of much longer roots running down sand filled aerification holes.
A major question which inevitably arises during construction of a new
facility is the issue of fairway soils. Since fairways occupy a large
area, it adds a large amount (approximately one million dollars) to the
initial capital cost for a golf course if the fairways must be plated
with a 6 inch layer of imported soil or sand. This must be balanced against
some of the potential disadvantages of using the existing soil. Consideration
needs to be given to the depth and uniformity of the existing soils, their
texture, the amount of play anticipated, the acceptability of restricted
cart usage during wet weather, the potential for loss of revenue due to
closing the course due to wetness, and the potential need for additional
equipment and supplies to manage the existing soil. Slowly permeable native
soils may require the installation of an extensive network of surface
drains to prevent wet spots. On the other hand, the use of a highly permeable
top soil may require the installation of an extensive subsurface drain
Whether we realize it or not, irrigation scheduling is directly related
to the physical properties of the soil. A sandy soil with a high infiltration
rate can accept water rapidly, however it will only hold a relatively
small amount prior to drainage. Clay textured soils are just the opposite
and only allow water to enter them very slowly, however they hold a large
amount of water. Thus, the physical properties of a soil control both
the rate at which water may be applied and the total amount of water to
be applied. Consider a 12 inch depth of sand based root zone mixture which
has a capillary porosity of 20%. This root zone would retain a maximum
of 2.4 inches of water (12 inches x 0.2). Thus to take this root zone
mixture from "oven dry" to the maximum water content without any drainage
water loss, will require the addition of 2.4 inches of water. In a field
situation one would normally irrigate long before the soil moisture content
approached oven dry. Therefore, it is doubtful that you would ever need
to apply more then 1.0-1.5 inches of irrigation. Research has shown that
turf has a maximum water use of 0.5 inch per day in the hottest part of
summer. Therefore, even a sandy root zone mixture may be able to support
turf for 2 days in the summer. Dr. White at Texas A&M has shown that turf
responded well to an irrigation frequency of as great as once every 4
days. Thus, irrigation scheduling should be done carefully to water deeply
but less frequently, yet at a rate that can infiltrate into the soil.
Superimposed on all the above issues are special considerations of
items which will vary from location to location. Attention needs to be
given to the annual rainfall rate and to the monthly distribution of rainfall.
This will provide some information on how to prepare for adequate drainage
and irrigation of the facility in question. In addition, irrigation water
quality is becoming a major factor in many of the golf courses being built
recently. The use of effluent water is becoming more common and consideration
needs to be given to the proper management of the associated salts and
suspended solids. Special provision may be needed to allow periodic leaching
of salts from the soils. Obviously, it is also necessary to research the
availability of local soils and building materials.
As with all businesses, money plays a major role in the decision making
process. As in many other situations, the cheap way out may not always
be the best and may cost more in the long run. For instance, problems
with excessive salts in the soils and irrigation water may require replanting
of certain areas and delay the opening date. If the problems persist,
they may reduce the number of rounds that can be played annually and in
severe cases may reduce the sell ability of associated development property,
not to mention the increased cost of maintenance. Therefore, do your homework
initially before selecting soils for use on your course giving careful
thought to the future implications of your decisions. Problems with limited
drainage, poor aeration, layered soils and varying soil depths are not
easily corrected and will make management difficult for the lifetime of