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  Understanding Soil Physical Properties
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 matter.

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

Material

Bulk Density
(g/cm3)

Porosity
(%)

Potting Mix

0.25-0.75

92-72

Surface Clay (wet)

1.12

58

Golf Green Mix

1.25-1.60

53-40

Surface Loam Soil

1.28

52

Surface Sand

1.50

44

Subsoil Sand

1.61

39

Compacted Sandy Loam

1.90

28

Sandstone

2.12

20

Rock

2.65

0

 

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.

Figure: 1

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 phase.

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 system.

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 the facility.

 

 
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