SportsTurf

June 2012

SportsTurf provides current, practical and technical content on issues relevant to sports turf managers, including facilities managers. Most readers are athletic field managers from the professional level through parks and recreation, universities.

Issue link: https://read.dmtmag.com/i/69161

Contents of this Issue

Navigation

Page 33 of 47

Irrigation&Drainage | ByBob Oppold Water movement in turf: the root of soil physics N ITS MOST BASIC TERMS, soil physics is concerned with the delicate balance of water and oxygen in soil. Be- cause this equilibrium is so vital to the health and appearance of turf, all sports turf man- agers are trained in managing soil water. Water is released from soil three ways: I through internal drainage, evaporation, and transpiration. If a field has poor drainage, it becomes more dependent on evapotranspira- tion to remove excess moisture. If a field re- mains saturated, the turf experiences stress typically referred to as "wet wilt." While ex- tended saturated conditions can cause roots to rot, most wet wilt stress is actually caused by an oxygen deficient environment. In essence, the roots are drowning. When we consider the life processes of the grass plant, we typically focus on photosyn- thesis. The plant takes in carbon dioxide, water, nitrogen, and other nutrients while the chlorophyll captures photons and through the "miracle" of photosynthesis converts light into simple sugars such as glucose. The byproducts of photosynthesis are oxygen and water vapor. However, the life process does not end at microbes will pull it off of other compounds in order to live. This is a simplistic way of de- scribing the anaerobic processes. Carbon diox- ide, methane, and hydrogen sulfide are common byproducts of this anaerobic decom- position. These three gases are toxic to the turf in low oxygen environments. A release of toxic or potentially toxic soil measures the gravitational flow of water. It is related to saturated hydraulic conductivity, which measures the capillary rise of water to the surface. As such, the infiltration rate is an indirect measurement of capillary rise and can also give clues to the level of aeration/oxy- genation. Water does not move freely through the small spaces classified as water pores. Rather, water molecules remain inside the pores due to the water's adhesive and cohesive proper- ties. Some pores are so small that the water is not plant available. The root hairs cannot overcome the water's adhesion and cohesion properties in many of these smallest spaces. Water does move through the larger air pores, both gravitationally and to the surface through capillary rise. The air pores also provide space for oxy- gen and for root growth. This illustration shows glucose production. The sugar must be con- verted into energy at the cellular level in order for the organism to live. Oxygen is required to convert the glucose into energy and the byproducts of this ADP-ATP cycle at the cel- lular level are carbon dioxide and more water vapor. The roots are a primary gatherer of the needed oxygen but they also deposit water and carbon dioxide back into the soil. THE IMPORTANCE OF OXYGEN IN THE SOIL will be oxygen deficient. In both cases there is insufficient space for oxygen diffusion, which compounds the challenges roots already face to survive. The roots are in constant competi- tion with the microbial populations for soil oxygen. At this level, Oxygen is a depleting re- source that must be constantly replenished from the atmosphere. Bacteria and fungi need oxygen to break If soil is saturated or simply too dense, it down—or decompose—their food, primarily organic matter. If there is insufficient free- floating O2 in the soil, the bacteria and other 34 SportsTurf | June 2012 gases coupled with the diffusion of oxygen from the atmosphere into the soil is the process we call aeration. The evaporation of water at the surface draws water and soil gases from the soil column up toward the surface. As water and soil gases are removed from the soil through aeration, space is created for oxy- gen. The efficiency of the aeration process is determined primarily by a soil's physics. It is also supplemented by the turf 's access to di- rect sunlight, which is needed for transpira- tion, and surface airflow, which is needed for evaporation. [NOTE: We recognize that there are other factors that affect evapotranspira- tion. Ambient humidity is an example.] SOIL COMPOSITION AFFECTS WATER MOVEMENT are infiltration rate, total porosity (which is broken down into water porosity and air porosity), bulk density, and water holding. The composition of the soil (distribution of the sand, the gravel component, whether silt, clay, or organic matter is present) dictates the soil's physical properties. The infiltration rate The properties associated with soil physics pores. The soil loses total porosity and perme- ability, as measured by the infiltration rate. Bulk density measures compaction, but it how larger particles create air pores while smaller particles compress tightly to create water pores. As compaction tightens the soil, many of the air pores are converted into water is a "relative" value in that one must know the relative weight of the material in the soil be- fore evaluating soil density. Some products claim they reduce compaction because they lower bulk density. However, a decline in bulk density may not be a true indicator of reduced compaction. If the product weighs less than the material comprising the rootzone, bulk density will decline due to the fact that a lighter material has been introduced. It may or may not have truly reduced compaction. It is helpful to think of a sports field as a dynamic organism. It is constantly changing through (a) compaction, (b) the deposit of water borne contaminants that are present even in potable water, (c) the byproducts of the organisms that live in the soil, and (d) the organic matter deposited by the turf. This sec- ond illustration shows how the air pores are gradually converted to water pores with the resultant loss of permeability. The roots will prune to the surface where they will be vul- nerable to multiple stressors. www.sportsturfonline.com

Articles in this issue

Links on this page

Archives of this issue

view archives of SportsTurf - June 2012