This is because water molecules are extracted from the large soil pores first and water is held more tightly in the smaller pores and the bond between water molecules and soil pores becomes even stronger as the soil gets drier. As the soil gets drier, the plants must exert increasingly more energy to extract water molecules. In more practical terms, SMP is a direct indication of how much energy must be exerted (application of pressure) by plants to extract the water molecules from soil particles. The quantity of soil-water from SMP is determined through soil-water characteristics curves developed for specific soil types. Soil matric potential (SMP) represents the relative availability of the amount of water held in the soil profile for plant uptake/use. Volumetric soil water content (SWC) indicates the quantity of water in the soil but does not directly indicate the availability of this water to plants. Soil-water status in the soil profile can be expressed in two very different ways: It is a field-specific variable that must be measured for each field. Furthermore, unlike some weather variables, soil moisture is not a transferrable variable between locations or fields.
Given its vital importance to numerous processes, plant physiological functions, and soil-water-atmosphere relationships, soil moisture determinations and irrigation management decisions must be made based on technology rather than non-technological approaches (i.e., hand-feel method, calendar day approach, neighbor’s schedule, visual observations of soil and/or crop status) to optimize crop production efficiency. As a result, stomata closure reduces the transpiration rate and yield because transpiration and yield are strongly and linearly correlated (Irmak, 2016). Some researchers have reported that soil-oxygen deficiency can cause stomata closure as well, even when plants do not experience water deficit stress. Reduced oxygen concentrations in soil due to wet soil or flooding conditions can cause stomatal closure of plants, which causes plant stress because plants cannot transpire water vapor at an optimal/potential rate although water is available. For example, a multi-year study at the UNL South Central Agricultural Laboratory near Clay Center (Irmak, 2014) reported that having 25% more water than was needed in the maize root-zone reduced grain yields as much as 15 bu/ac, which is a substantial portion of attainable maize yield in south-central Nebraska. Too much water or frequent irrigations may cause anaerobic soil conditions and promote undesirable chemical and biological reactions in the soil, which can reduce yields and water and energy resources. SMP indicates how much energy plants will have to exert to extract the water molecules from soil particles. This can lead to reductions in crop yield and yield quality. Too light or infrequent irrigation applications can impose water stress on crops, which can also cause irreversible damage. Irrigation management requires the knowledge of “when” and “how much” water to apply to optimize crop production and increase and maintain a high level of water use efficiency. Accurate determination of soil-water status (either matric potential or water content) is not only important for irrigation and water resources management, it is also a fundamental element of soil-water movement, chemical (fate) transport, crop water stress, evapotranspiration, hydrologic and crop modeling, climate change, and other important disciplines. Soil-water in the plant root-zone must be maintained in a balance so that plants can optimize their transpiration (biomass/yield production process) as well as water, nutrient, and micronutrient uptake. Soil-water status (soil moisture) plays a critical role in determining the yield potential of crops.
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