04/26/2000 Env Bd Packet - Laserfiche WebLink

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A. illu Fl• This intro grass ecosy the ment are consi turfy GRA past sive s devel is in proce Di MAN cesses thoro chapt growt emph on th turfgr and c of gr compel the a ences gatio and qualit guide A ch meth tions chapt yield of to i (contli 106 THE TURFGRASS ENVIRONMENT used for soils. Particle density (PD) is the density of dry, solid soil particles. It averages 2.65 g /cc. Bulk density (BD) is the dry weight of an undisturbed volume of soil. A well - structured, fine- or medium- textured soil will have an abundance of large and small pores, and its bulk density will be low compared to the same soil in a compacted state. If a soil could be so com- pacted that virtually all pore spaces were removed, bulk density would equal particle density. Comparisons of the bulk densities of different soils may not provide reliable indices for deter- mining their suitability for growing plants. Sands have predomi- nantly large pores; yet, their bulk densities are high because of the relatively large mass of solids making up these media. For example, a bulk density of 1.5 g /cc may indicate a rather com- pacted loam soil with insufficient large pores for drainage. How- ever, in a coarse sand of the same bulk density, aeration would not be a limiting factor in plant growth, but water retention might be. Within soil type, bulk density values are valuable for assessing the physical condition of a soil. Soil porosity can also be determined from bulk density: percent porosity = 100 - (PD X 100) A loam soil with a bulk density of 1.3 would thus have 49 percent pore spaces: 1.30 100 - ( X 100) = 49 percent 2.65* The percent pore space is only a measure of total porosity and does not directly indicate the distribution of different size pores. Within a particular soil type, however, the percent porosity can provide a reasonable assessment of pore size dis- tribution if a sufficient bank of information exists to compare plant growth response to different soil porosities. SOIL MOISTURE The importance of soil structure and density lies in their influence upon the number and size of pores and the conse- quent movement of water and air within the soil. Water drains rapidly from large (aeration) pores due to gravitational force. This is called gravitational water (Figure 4.16). The remaining water is retained as thin films on the surfaces of soil particles and as wedges where two or more particles come together. To *PD of soil particles averages 2.65 g /cc. EDAPHIC ENVIRONMENT 107 Saturated Soil Capillary (available & unavailable) Water Unavailable Water Gravitational Water Decreasing Soli Moisture Dry Soil Figure 4.16. Illustration of different types of soil water. Gravitational water drains from the large (aeration) pores, while capillary water is held as films on soil surfaces. absorb this water, plants must overcome forces of adhesion and cohesion that hold water in the soil. Adhesion is the attraction between soil surfaces and water; cohesion is the attraction be- tween water molecules. The portion of retained water that plant roots can absorb is called available water. When water films are reduced in thickness the attraction of the soil for water be- comes greater, and eventually plants can no longer secure enough water to satisfy their needs. The tightly held water that is essentially unavailable to plants is simply called unavailable water. A well- structured soil will release enough water in response to gravity so that aeration porosity is adequate to sustain healthy plants. The water content of the soil following drainage of the aeration pores is called field capacity and the air content is called aeration capacity. These are not precise values but are useful as indices of soil structure for a given soil. In a poorly structured soil, removed water may not be replaced by air because of soil shrinkage. Such soils form massive clods with large cracks instead of friable granules. The thickness of a water film at which plants can no longer absorb sufficient water to sustain growth is about the same for all soil types. However, the water content, measured in grams of water per gram of dry soil, at which this occurs varies among soils because of large differences in total surface area. When plants growing in a particular soil wilt irreversibly, the soil is said to be at its permanent wilting point, and the amount of water remaining in the soil is called the permanent wilting percentage. This amount ranges from 1 to 2 percent for sandy