YOUR BEST PRACTICE


is assigned zero, so the soil matric potential is always negative. This is because any water attached to the solid particles is not free and therefore considered to be at a lower energy state than free water.


Ideally, the compilation of the soil water retention curve for a given soil can be tedious and time-consuming and demands the use of specialized instruments such as a pressure plate apparatus. However, for practical pur- poses of managing water flow for irrigation and drainage, it is noteworthy that soil water retention curves for various soil types are strongly influenced by soil texture and soil structure (see fig. 2).


Soil texture & structure


The degree of fineness or coarseness of a soil is referred to as the soil texture and is defined as the relative amounts or percentages of sand, silt and clay. Although texture is a measure of the solid components of the soil, it does not include the soil organic matter content. Furthermore, since the proportion of sand, silt and clay is strongly influenced by the breakdown of the soil’s parent material during the soil formation process, then soil texture is generally not affected by irrigation and agronomic management practices.


The natural arrangement of soil particles (sand, silt, clay) into larger units (aggregates) of varying degrees of coherence is termed soil structure. These structural units are named peds and are characterized on the basis of size, shape and grade.


Unlike texture, soil structure includes organic matter and other binding agents such as calcium and other salts. Soil structure has an important influence on plant growth, primarily as it affects moisture relationships, aeration, heat transfer and mechanical impedance of root growth, which in turn can affect water flow and supply within the soil profile.


Identifying soil types


A sound knowledge of how to identify the soil textural and structural classes of various soil types is critical for managing the water supply. Soil textures can be determined in accordance with the U.S. Department of Agriculture Textural


irrigationtoday.org


Triangle (see fig. 3) by utilizing laboratory techniques to accurately quantify the relative percentages of sand, silt and clay, or by adopting the “feel” method.


Since soil structure can also be correlated to the predominating abiotic (e.g., climatic and environmental conditions) and biotic (e.g., soil organisms) factors, it is also important to conduct an in-field assessment of the influence of these factors.


During the field assessments, it is vital to at least note the following properties in both the surface and subsoil: the amount of soil organic matter, coarse gravel and rocks; soil color; infiltration and percolation rates; root channels and worm holes; and the presence of restricting clay layers or hardpans.


Based on this information, it is obvious that in order to efficiently manage water supply within the soil profile, it is important to have a comprehensive knowledge of soil texture and soil structure of the soil types. It is also necessary to understand the interrelatedness between the soil water retention characteristics and other distinguishing features that can potentially influence soil water flow and solute transport.


While this discussion focused on the identifica- tion of soil types and soil water retention, there are also other direct or indirect techniques for monitoring soil water status in fields in order to assess the plant water availability and to schedule irrigation events. For more information, go to www.nrcs.usda.gov and choose “Soils” in the Topics menu.


Figure 3: U.S. Department of Agriculture soil textural triangle


for characterizing soil textures. Source: USDA


A so


ound knowledge


of how to identify the so nd


s il textural and structural


classes of various soil types is critical for managing the water supply.


Dave Goorahoo, PhD, is a professor in vegetable crops production – plant science department and a soil scientist at the Center for Irrigation Technology at California State University, Fresno.


Winter 2020 | Irrigation TODAY 27


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40