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Marco Bittelli

Soil water content has an important impact on many fundamental biophysical processes. It affects the germination of seeds, plant growth and nutrition, microbial decomposition of the soil organic matter, nutrient transformations in the root zone, as

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Y. Song, J.M. Ham, M.B. Kirkham, and G.J. Kluitenberg

Measurements of soil water content near the soil surface often are required for efficient turfgrass water management. Experiments were conducted in a greenhouse to determine if the dual-probe heat-pulse (DPHP) technique can be used to monitor changes in soil volumetric water content (θv) and turfgrass water use. `Kentucky 31' Tall fescue (Festuca arundinacea Schreb.) was planted in 20-cm-diameter containers packed with Haynie sandy loam (coarse-silty, mixed, calcareous, mesic Typic Udifluvents). Water content was measured with the DPHP sensors that were placed horizontally at different depths between 1.5 and 14.4 cm from the surface in the soil column. Water content also was monitored gravimetrically from changes in container mass. Measurements started when the soil surface was covered completely by tall fescue. Hence, changes in θv could be attributed entirely to water being taken up by roots of tall fescue. Daily measurements were taken over multiple 6- or 7-day drying cycles. Each drying cycle was preceded by an irrigation, and free drainage had ceased before measurements were initiated. Soil water content dropped from ≈0.35 to 0.10 m3·m-3 during each drying cycle. Correlation was excellent between θv and changes in water content determined by the DPHP and gravimetric methods. Comparisons with the gravimetric method showed that the DPHP sensors could measure average container θv within 0.03 m3·m-3 and changes in soil water content within 0.01 m3·m-3.

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Sanjit K. Deb, Manoj K. Shukla, and John G. Mexal

gained wide acceptance because many features of the plant’s physiology respond directly to changes in water status in the plant tissues rather than to changes in the bulk soil water content (or potential) ( Jones, 2004 ). Direct physiological methods

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P. Parchomchuk, R.G. Berard, and C.S. Tan

We have found time domain reflectrometry (TDR) to be a rapid and effective method of measuring soil water content (SWC) in microirrigated orchards, particularly in applications where many sites are monitored frequently. With simple modifications to commercially available systems, it has been possible to measure up to 100 sites per hour. TDR SWC measurements have been successfully applied for scheduling irrigation and for in situ determination of SWC characteristics. The determination of plant water use from changes in SWC of microirrigated trees, however, requires that a sufficient number of probes be used to detect the spatial distribution of water within the root zone. Due to water redistribution in the soil following an irrigation, measurements made near drip emitters depend highly on the time after irrigation that the measurement is made. It is therefore important to be consistent in the timing of SWC measurements relative to irrigation events if the effects on SWC of different irrigation management practices are to be compared.

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Milton E. McGiffen Jr., John B. Masiunas, and Morris G. Huck

Field and greenhouse experiments were conducted to determine the response of eastern black nightshade (Solanum ptycanthum), black nightshade (S. nigrum), and tomato (Lycopersicon esculentum Mill. cv. Heinz 6004) to water stress and the effect of nightshade-tomato competition on soil water content. In the greenhouse, plants were exposed to three water regimes induced by watering either daily, weekly, or biweekly. Water deficit caused a similar decrease in height, weight, and leaf area in all three species. There was more than a 50% reduction in height when the plants were watered biweekly compared with daily watering. Water stress caused a shift in biomass from shoots to roots in all three species. Black nightshade and tomato produced thinner leaves in response to water deficit. Companion field experiments were conducted during the 1989 and 1990 growing seasons in Urbana, Ill. Eastern black nightshade and black nightshade were transplanted at densities of 0.8, 1.6, 3.2, and 4.8 plants/m2, 5 days after tomatoes were transplanted. These nightshade densities caused significant reductions in soil water content. In 1989, only the highest density of either nightshade species reduced topsoil water content. In 1990, all densities of nightshade, except the two lowest densities of black nightshade, reduced topsoil water content. Eastern black nightshade consistently had a greater effect on tomato yield than black nightshade. Tomato yields averaged over both years were 17,000 and 8,000 kg·ha-1 at the highest (4.8 plants/m*) density of black and eastern black nightshade, respectively. The decrease in soil moisture from high densities of nightshade could not account for the reduced yields.

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R. Nuñez-Elisea, B. Schaffer, M. Zekri, S.K. O'Hair, and J.H. Crane

Tropical fruit trees in southern Florida are grown in porous, oolitic limestone soil that has very low organic matter content and water-holding capacity. Thus, trees require frequent irrigation during dry periods. In these soils, a quantitative basis for monitoring soil water content to determine when and how much to irrigate has been lacking. Multi-sensor capacitance probes (EnviroSCAN™, Sentek, Australia) were installed in commercial carambola, lime, and avocado orchards to continuously monitor changes in soil water content at depths of 10, 20, 30, and 50 cm. Eight probes were installed per orchard. Volumetric soil water content was recorded at 15-min intervals with a solar-powered datalogger. Results were downloaded to a laptop computer twice a week. Monitoring the rate of soil water depletion (evapotranspiration) allowed irrigation before the onset of water stress. The time at which soil reached field capacity could be determined after each irrigation (or rain) event. Soil water tension was recorded periodically using low-tension (0–40 cbars) tensiometers placed adjacent to selected capacitance probes at 10- and 30-cm depths. Soil water tension was better correlated with volumetric soil water content at a 10-cm depth than at 30-cm depth. Using multi-sensor capacitance probes is a highly accurate, although relatively expensive, method of monitoring soil water content for scheduling irrigation in tropical fruit orchards. Whereas tensiometers require periodic maintenance, the multi-sensor capacitance probe system has been virtually maintenance free. The correlation between soil water content and soil water tension obtained in situ indicates that tensiometers are a less precise, but considerably cheaper, alternative for scheduling irrigation in tropical fruit orchards in southern Florida.

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Hui-lian Xu, Laurent Gauthier, and André Gosselin

`Capello' tomato plants (Lycopersicon esculentum Mill.) were grown in a greenhouse in peat-based substrate (70% sphagnum peat and 309'. perlite, by volume) and supplied with nutrient solutions of high (4.5 mS·cm-1) or low (2.3 mS·cm-1) electrical conductivity (EC) under high (95% ± 5%) or low (55% ± 8% of capillary capacity) soil water conditions. Three weeks after treatments started, stomatal transpiration (TRst) and cuticular transpiration (TRcu) rates were measured by three methods: 1) analyzing TRst and TRcu from a water retention curve obtained by drying excised leaves in air under a photosynthetic photon flux (PPF) of 400 μmol·m-1·s-1, 2) analyzing TRst and TRcu from a transpiration decline curve obtained by measuring transpiration rates after cutting the leaf from the stem of the dehydrated plant in the gas-exchange system, and 3) measuring transpiration rates under light and in dark respectively using the gas-exchange method. TRst and TRcu were decreased by high EC and/or low soil water content. For method 1, the transpiration decline curve shows two distinct phases: the initial steep slope that indicates TRst and the gently sloped section that indicates TRcu. Both slopes were lower for high EC and/or water-stressed plants compared to the control (low EC and high soil water content). The tangent lines of these two phases of the curve intersect at one point (t, w). The value oft that indicates the time for stomatal closure was longer and the value of w that indicates the critical tissue water level for stomatal closure was lower for high EC and/or water-stressed plants. In method 2, the initial rate of total transpiration was higher in high EC and/or water-stressed plants. Leaf wax content increased, especially under high EC stress. This suggests that increased deposition of wax prevents water loss from the cuticle. A delay in complete stomatal closure, complete closure at lower RWC, and reduced TRcu or an increase in wax deposit were adaptations to water and salinity stresses in tomato plants under our controlled environmental conditions.

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David R. Bryla and Robert G. Linderman

total amount of water applied to each treatment during the first two growing season is shown in Table 1 . Table 1. Total irrigation water applied to ‘Duke’ blueberry in 2004 and 2005. Soil water content was measured monthly (June to

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Dariusz Swietlik

Several soil moisture measuring sensors are used to schedule irrigation of horticultural crops. However, without precise knowledge on root distribution, these devices may not provide accurate data on soil moisture conditions around the majority of roots. Also, some devices are expensive and/or require a tedious calibration and careful maintenance, limiting their use by growers. ET models have been successfully used for many crops grown in arid climates; however some models are not very precise and do not account for seasonal differences in plant water needs. Moreover, in humid regions it is difficult to assess the magnitude of rainfall contribution to the plant water needs particularly when only a part of the root system is irrigated. Research is needed to characterize plant variables which are useful for scheduling irrigation.

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Mongi Zekri and Lawrence R. Parsons

The development of improved equipment for measuring soil water content has created the need for a better understanding of soil water drainage and movement. Without this understanding, it is impossible to know if an observed decrease in soil water content at a particular depth is due to evapotranspiration and/or continual drainage. This study was designed to determine the length of time for different soil depths of a Florida Candler fine sand to reach field capacity. A field site with no vegetation on it was saturated with water and covered with a plastic tarp to prevent evaporation. At 6- to 24-hour intervals, soil water content was measured gravimetrically in the top 15 cm (6 inches) and with the neutron probe from 30 to 150 cm (12 to 59 inches). The 15-cm depth reached field capacity after one day, but it took 4 days for the 30- to 150-cm depths to reach field capacity because of rewetting by water draining form higher horizons. The time required for drainage to stop must be considered when evaluating changes in soil water status at a particular depth. This is important for distinguishing between plant water uptake and drainage for different soil layers.Soil water characteristic curves of undisturbed soil samples, bulkdensity, porosity, and field capacity in situ were also determined for this soil. Field capacity values found in situ were compared to those found using the pressure plate technique. Laboratory values were higher than field values because the laboratory data were closer to hydrostatic conditions than the field data and the degree of saturation provided during wetting of the cores was higher in the laboratory. Water was not readily retained in Candler fine sand because the soil was very porous, infiltration rates were high, drainage was rapid, and water storage capacity was limited. Using field measurements, field capacity values of soil at different depths ranged from 4.8% to 6.2% volume for Candler fine sand. These are considered to be low values when compared to other types of soil.