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<br />110 THE TURFGRASS ENVIRONMENT
<br />the sponge is not uniform, but moisture is predominantly within
<br />the lower levels.
<br />The sponge analogy facilitates understanding of some soil
<br />the
<br />water phenomena. In a USGA green, the sponge represents p th the
<br />surface soil medium, and the air, or "free space,
<br />sponge represents the coarse sand or gravel layer underlying
<br />the surface medium. A workable greens design depends upon
<br />rather precise application of principles of soil physics.
<br />settled depth (depth after soil settling) of the surface medium
<br />must be correct, and the physical composition of the medium
<br />must be such that air and moisture within the turfgrass root
<br />zone are sufficient to sustain growth. The USGA green and
<br />other designs are discussed further in Chapter 9.
<br />SOIL AERATION
<br />The process by which soil air is replaced by atmospheric
<br />air is called soil aeration. Soil air differs from atmospheric air
<br />in that it has higher concentrations of carbon dioxide and water
<br />vapor but less oxygen. These differences are due to the con -
<br />sumption of oxygen and production of carbon dioxide by soil
<br />organisms. The magnitude of these differences depends upon
<br />the rate of gaseous exchange between the atmosphere and soil.
<br />Aeration is brought about by processes of diffusion and mass
<br />flow. Diffusion is the movement of gases through air - filled pores
<br />from regions of higher to lower concentration of the gas, and is
<br />proportional to air- filled porosity. Diffusion is low in com-
<br />pacted soils because of reduced pore size and number and dis-
<br />continuity of soil pores. Similarly, diffusion is low in wet
<br />soils because of the absence or reduction of air - filled pores and
<br />the extremely low diffusion rate of air in water.
<br />Mass flow occurs as a result of
<br />1. Expansion and contraction of soil gases due to tempera-
<br />ture and barometric pressure changes
<br />2. Soil air removal, through precipitation and irrigation,
<br />,
<br />and replacement, as water is removed by drainage, plant
<br />and evaporation
<br />3. Wind action causing air to be forced into the soil at
<br />some locations and pulled out at others. Relative to diffusion,
<br />mass flow is considered to have a minor influence on soil
<br />aeration.
<br />Poorly aerated soils are often deficient in oxygen. Oxygen
<br />is utilized by plant roots and soil organisms for respiration, and
<br />EDAPHIC ENVIRONMENT 111
<br />of
<br />carbon dioxide is evolved. ohe Without
<br />soil, adequate
<br />oxygen levels de line
<br />gases between the atmosphere
<br />and carbon dioxide o
<br />i levels
<br />reduced absorption nutrients and water by plant roots, since
<br />they must have suffici�ne oxygen respiration
<br />actgvity for de-
<br />energy necessary for processes.
<br />composing organic matter is also inhibited in oxygen- deficient
<br />itro-
<br />soils, as is the bacterial oxidation of ammonia to nitrate nN20
<br />gen. Denitrification, the conversion of nitrate to N2
<br />gases, occurs in persistently wet soils, resulting in a loss of soil
<br />nitrogen to the atmosphere.
<br />Turfgrass communities growing in compacted or p ersis-
<br />tently wet soils are often invaded adaptation of plants to poorly
<br />reflects, in part, the differential P grow under
<br />aerated soils. Some weed possess typically
<br />transmit foliar-
<br />absorbed conditions may p
<br />absorbed oxygen to their roots to satisfy respiratory require -
<br />man. Thus ass cles through their ability to 1persist under ethese
<br />many y turfgr
<br />conditions.
<br />SOIL TEMPERATURE
<br />Many of the physical, chemical, and biological events that
<br />take place in soil are strongly temperature adependent. Soil conditions tem-
<br />perature affected by: (a)
<br />(i.e., re is, in and solar radiation),
<br />(i.e., air temperature, moisture, wind,
<br />(b) thermal absorption and nfluenceston vity soil f temmperature he (v)
<br />plant cover. Atmospheric
<br />been discussed in an earlier section of this chapter.
<br />Thermal absorption is a function of the color, moisture
<br />level, and organic matter content of the soil. Generally, darker
<br />soils, high in organic matter, are a absorption occurs absorbing as
<br />heat from the atmosphere.
<br />drier soils, since the heat (
<br />neces-
<br />sary to raise ep ratureof1gram of a substance by 1 °C)
<br />humus is 1.0, 0.2, and 0.4,
<br />for water, dry mineral soil, and dry
<br />respectively. Therefore, as the water content of a given soil
<br />increases, the amount of energy from solar radiation or at-
<br />mospheric air required to raise its temperature increases
<br />air-
<br />prop ortionately.
<br />Changes in
<br />soil a in the soil. eSandy soils warm and cool at
<br />moisture -solid bala
<br />a faster rate than clayey soils, due to generally higher aeration
<br />
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