Soil Water Potential Measurements Research Articles

Water Movement from Soil to Root Investigated through Simultaneous Measurement of Soil and Stem Water Potential in Potted Trees

Legge, N. J. 1985. Water movement from soil to root investigated through simultaneous measure ment of soil and stem water potential in potted trees.—J. exp. Bot. 36: 1583-1589. Osmotic tensiometers implanted in the stems of three mountain ash (Eucalyptus regnans F. Muell.) saplings growing in large plastic bins recorded stem water potential, Wsv while soil water potential, f „ was simultaneously recorded by instruments near the trees' roots and in the surrounding root-free soil. Early in a drying cycle, with the soil still wet, the diurnal variation in W, was often slight, despite diurnal variations in Vst approaching 2 0 MPa. Late in a drying cycle the diurnal fluctuations in fs and fst were very similar although changes in fs still lagged up to 1-5 h behind changes in fst. f5 values at this time occasionally reached —3-0 MPa with no apparent damage to the trees. Watering the bins in daytime led to a response in fs values within about 5 min, whereas ¥n values did not respond for a further 20 min. fst values then rose rapidly but after only 1 h began to decline again, while fs values remained at or near saturation for the rest of the day. Water uptake hypotheses which attribute an important role to a soil-root interface resistance are not supported by these data. Key words—Soil water potential, perirhizal gradients. Correspondence to: School of Agriculture, La Trobe University, Bundoora, Victoria 3083, Australia.

Response of Soybean Cultivars to Irrigation of a Clay Soil1

AbstractNumerous investigations have shown that soybeans [Glycine max (L.) Merr.] will give a significant yield response to irrigation, especially when the supplemental water is applied during reproductive development. However, the majority of these results have been obtained with soils ranging in texture from silt loam to sandy loam. Since soybeans are frequently grown on finer‐textured soils, we conducted field experiments in 1979 and 1980 on Sharkey clay (Vertic Haplaquept, very‐fine, montmorillonitic, thermic), a soil representative of that occurring in about 3 750 000 ha of the alluvial plain of the Mississippi River. Variables included initiation of furrow irrigation at the prebloom (V), beginning of bloom (BL), beginning of pod set (PS), and beginning of pod fill (PF) stages, plus a nonirrigated (NI) treatment, and three cultivars representing Maturity Groups V (‘Bedford’), VI (‘Tracy’), and VII (‘Bragg’). Data were collected for seed yield, mature plant height, lodging, xylem pressure potential, leaf area, and seasonal soil moisture status. In the ‘wet’ year of 1979, yields from irrigated Bedford and Tracy were not significantly greater than the respective NI yields of 2754 and 3368 kg ha−1. The PF treatment of Bragg produced a yield of 3630 kg ha−1, however, and this was significantly higher than the yields of all other treatments of Bragg, as well as all treatments of Bedford. In the dry year of 1980, all irrigation treatments of all cultivars increased yields above the NI treatment, although the PF treatment yields were only about one‐half of those from PB or BL treatments, which were nearly equal. Seed yield was highest from Bragg and lowest from Bedford in all cases. Lodging was not significantly affected by any irrigation treatment. Mature plant height was significantly increased only in the BL treatment of Bedford in 1980. Measurements of xylem pressure potential, leaf area, and soil water potential in 1980 indicated that the NI plots were severely stressed; thus, response to irrigation should have been expected. These results show that irrigation of soybeans growing in a clay soil should be initiated at or near bloom in a dry year, and at any period of stress during reproductive development in a “wet” year.

An Improved Thermocouple Hygrometer for In‐situ Measurements of Soil Water Potential

AbstractA new dewpoint hygrometer for in‐situ measurements of soil water potential was designed and its performance evaluated. A porous silver membrane serves as the hydraulic connection between the soil and the measuring cavity which encloses a four‐terminal thermojunction made by cross welding chromel and constantan wires 0.001 in. (0.00254 cm) in diameter. The membranes were screwed to the copper body of the hygrometer and such configuration minimized the temperature difference between the vapor source and the reference junctions. When a thermal gradient was imposed on a hygrometer placed horizontally in a soil column, the maximum voltage offset observed was 0.05µV, but the soil water potential values as indicated by the hygrometer readings remained constant at −0.56MPa.This new device has a fast response for water flow with a time constant of approximately 5 min, and the time constant for salt diffusion in and out of the membranes is about 8 min. The results of soil water potential measurements under corn (Zea mays) plant hybrid United 108 grown in a growth chamber environment have shown that this new device is capable of following the changes in soil water potential very well. The difference in water potential observed between leaf and soil after the plant had been left several hours in the dark appeared to be related to uneven moisture distribution in the pot, and the plant was responding to the average soil water potential.

Total Resistance to Water Flow in Field Soybeans: II. Limiting Soil Moisture1

AbstractExperiments were conducted to determine the total (soil plus plant) resistance to water flow through field grown soybeans [Glycine max (L.) Merr.] subjected to two drying cycles during pod addition and early pod fill (4 to 16 May) and mid‐to‐late pod fill (30 May to 7 June). Root length density distributions with depth were measured during each drying cycle and soil water potential distributions were measured daily. The experiment was.conducted in the field on Arredondo fine sand (hypothermic, coated Typic Quartzipsamments). Transpiration flux and leaf water potential measured hourly every 2 days during each drying cycle were used with daily measurements of soil water potential to calculate total resistance. Midday transpiration rates decreased relative to transpiration rates of irrigated plots starting 8 and 5 days after irrigation for the first and second drying cycles, respectively, when average soil water potential was −0.04 MPa. The increase in total resistance which corresponds to the decrease in transpiration flux was attributed to increased soil resistance. However, calculated soil resistances based on experimental results were four to six orders of magnitude higher than theoretical calculations of the bulk soil resistances based on the Gardner (1960) model. Calculations of water potential at the root surface during drying conditions indicate that unsaturated hydraulic conductivity of soil adjacent to the root may be several orders of magnitude lower than that of the bulk soil and could explain the observed reduction in the transpiration flux.

The Relationship of the Rock Outcrop Microhabitat to Germination, Water Relations, and Phenology of Erythrina flabelliformis (Fabaceae) in Southern Arizona

In southern Arizona, the western coral bean, Erythrina flabelliformis Kearey, reaches its northernmost limit of distribution in isolated mountain ranges where it apparently is restricted to stream channels and areas immediately adjacent to rock outcrops. To ascertain the relationship between the species and outcrops, the germination requirements, phenology, and water relations of E. flabelliformis were studied at Molino Basin in the Santa Catalina Mountains of southern Arizona. Measurements of soil temperature and soil water potential were made to relate the species' biology and distributional pattern to the physical environment. The seeds were found to germinate best when buried and placed under temperature regimes typical of the study site from March to October. Flowering and leaf initiation occurred just prior to the summer monsoons in early July, and leaf growth increased dramatically with the first rains. Plants of E. flabelliformis growing in stream beds flowered more and underwent leaf senescence later than did poorly watered individuals. Measurements of leaf water potential and osmotic potential indicate that this species is mesophytic in character as opposed to Haplopappus laricifolius Gray, a co-occurring xerophyte not restricted to outcrops. Data on soil water potentials suggest that areas immediately adjacent to the outcrops have greater soil moisture than areas away from the outcrops. Soil temperature gradients were discernible and may result in reduced soil water loss near outcrops. Restriction of E. flabelliformis to the outcrop habitat is partially explained by the more favorable conditions for seed burial and germination and by the improved soil water relations due to greater runoff, shading, and soil temperature stability provided by the outcrops. The predominantly tropical and subtropical genus Erythrina, comprising 108 species (Krukoff and Barnabey 1974), is represented in the Sonoran Desert by a single species, E. flabelliformis Kearney. Near the U.S./Mexican border and south into Mexico, E. flabelliformis is typically arborescent, reaching 9 m in height (Gentry 1942). At its most northern limit of distribution in the mountains of southern Arizona (Hastings et al. 1972), the plant is a shrub 1-2 m tall, but retains the large seeds ( > 1 cm) and deciduous leaves characteristic of a mesophyte. In the mountains of southern Arizona, E. flabelliformis exists singly or in scattered colonies from 900 m to 1800 m in elevation on rocky slopes and in canyons. Below about 1200 m, the species typically occurs in canyon stream beds and on north-facing slopes. Above approximately 1300 m, it usually grows on south-facing slopes. When occurring on mountain slopes, E. flabelliformis almost always grows in cracks within or immediately adjacent to rock outcrops. The relationship apparently is not caused by a nutrient deficiency since it may occur on a variety of different soils ranging from those derived from limestone to quartz monzonite. Nor is

A system for automatically measuring and recording soil water potential and rainfall

A system is described for the automatic simultaneous measurement and recording of soil water potential and rainfall. Tensiometers with fluid scanning switches and pressure transducers measure soil water potential. The fluid switch system also measures water depth in a series of bottles collecting rain from a catching trough. A versatile electronic system controls the rate and frequency of scanning by the fluid switch; a data logger collects the tensiometer and rain gauge signals and calibration values for calculating hydraulic potential and depths of collected rain. The system has been tested for accuracy, reliability and response to rapid changes of water potential and rainfall.

A Screen‐Caged Thermocouple Psychrometer and Calibration Chamber for Measurements of Plant and Soil Water Potential

AbstractEstimates of water potential with thermocouple psychrometers are subject to errors due to temperature gradients. The objective here is to describe the design and construction procedures of a new screen‐cage single‐junction psychrometer that minimize these errors. The screen‐cage design restricts entry of contaminants into the psychrometer cavity; allows rapid vapor equilibration between the sample and the psychrometer; and provides a 360° concentric evaporation surface that reduces the influence of vapor condensation due to temperature gradients. In addition, a calibration chamber that can also be used for plant and soil samples is described.

Managed Soil Moisture System for Studying Plant Water Relations under Field Conditions1

AbstractA Managed Soil Moisture System (MSMS) was installed at the Purdue Univ. Agronomy Farm for plant water relations research. Sixteen plots, each 6.1 m ✕ 9.8 m, can be controlled individually for water application and removal. The MSMS was designed on the principle of the Prescription Athletic Turf (PAT) system developed at Purdue. The area was excavated and lined with a moisture barrier. After the drainage/irrigation system was installed, the plots were filled to a depth of 40 cm with dune sand. During normal. “non‐moisture stress” operation a water table is maintained at a depth of 35 cm. Each plot has its own supply tank for monitoring the water use. An overflow allows drainage of excess rainfall, and a pump can be used to hasten drainage. Given a period of dry weather, this facility allows rapid adjustment of soil moisture for studying the effect of drought stress on plant growth. The plot size permits plant measurements and periodic destructive sampling without significantly affecting the crop canopy environment. In preliminary experiments in 1974, corn (Zea mays L.) grew normally to maturity with a water table at 35 cm. During a hot, dry period, severe leaf curling began on the stress plots 4 days after the drainage system was opened. Coupled with meteorological and soil water data, plant growth, and water potential measurements, this facility provides a relatively low cost means for studying environmental‐plant response data under near‐natural environments.

Improved thermocouple psychrometers for measurement of soil water potential in a temperature gradient

The improved psychrometer is constructed from high-thermal-conductivity materials to minimise temperature gradients within it. To minimise errors from condensation on the psychrometer, it is symmetrically constructed with respect to the porous windows where moisture exchange with the soil occurs. Zero offset of the new units is reduced by more than an order of magnitude compared with previous versions and measurement errors due to temperature gradients are less than one-third the value for the best designs previously available.

An in‐situ Dewpoint Hygrometer for Soil Water Potential Measurement

AbstractAn in situ device for determining the soil water potential by measuring the dewpoint temperature depression is described. A saturated ceramic plug is used to provide hydraulic contact between the soil and an air‐tight measuring chamber housing a four‐terminal thermojunction. The ceramic and the thermojunction references are thermally clamped to the copper body of the instrument in order that temperature differences between the vapor source (the internal evaporating surface of the ceramic) and the reference junctions are minimal. Thus, the device reduces the major source of error that plague conventional soil psychrometers in the presence of temperature gradients. A hygrometer placed perpendicular to an imposed thermal gradient in a soil column recorded a maximum zero offset voltage of 0.22 µV as compared with 130 µV from a Wescor psychrometer under the same conditions.Provided the ceramic remains saturated, the response of the hygrometer to changes in soil water matric potential is extremely rapid. Two bean plants (Phaseolus vulgaris L., c.v. “Tender‐green”) growing in a 2‐liter container of silt loam soil were subjected to wetting and drying cycles in the laboratory. Simultaneous measurements of leaf and soil water potential illustrate the capacity of the soil hygrometers to track changes in soil water matric potential.In the presence of solutes, the instrument is diffusion limited. The time constant for salt equilibration, with our geometry and using KCl as the solute was 23 hours. This is consistent with a diffusion coefficient of 2 to 3 × 10−6 cm3 sec−1. For this reason the application of this design should be restricted to circumstances where the osmotic contribution to the total water potential is either small or not changing rapidly. These criteria are generally satisfied in the humid regions.

Resistance to water movement through wheat root systems

A three-layer electrical analogue model was used to calculate resistance to water movement through the roots of wheat plants growing in small weighing lysimeters. In one experiment the wheat was grown in two soil types; in a second experiment one soil type was used but different root systems were induced by controlling the water table before the start of the experimental period. Resistance calculations were based on hourly measurements of transpiration rate, leaf water potential and water uptake from three soil layers (qi), calculated from measurements of soil water potential at three depths. The number of main roots per stem (required for the model) and root surface area in each layer (Ai) were obtained from measurements of root lengths and diameters in soil cores taken at the end of each experiment. Estimates of the resistance to flow through stems led to estimates of ψ0), the water potential at the stem base, at any stem flow rate. Axial (main) root resistances (Rxi) were calculated from the Poiseuille equation. Values of the resistance to water movement through the roots in layer i were calculated from the set of equations describing uptake from each layer in terms of flow rates, potential gradients and resistances; these values, inserted in the solution for 1/10 from the set of three equations, yielded total root resistances (RT) and estimates of the effective soil moisture potential (^ψs) for the whole profile. (RT) ranged from 63.9 to 627.3 bar sec mm-3 (cf. stem resistance between 24 and 70 bar sec mm-3) and was inversely related to flow rate through the main roots, which indicated a constant potential drop (^ψs – ψ0) of about 10 bars, irrespective of soil type or root system. Radial root resistances, estimated as At(<ψsi – ψ0)/qi, ranged from 4.6 x 104 to 4.2 x 106 bar sec mm-l and were inversely related to qi. Inaccuracies in estimates of Rxi do not affect the results much and the model used is potentially valuable as a framework for field research.

Transpiration and leaf water potentials of wheat in relation to changing soil water potential

Changes in the transpiration rate of wheat in drying soils were followed in experiments in which plants were grown in two small weighable lysimeters in a glasshouse. Hourly measurements of soil water potential (Ψs) were made at three depths in each lysimeter. The water potential of flag leaves was measured with a pressure chamber, and stomatal resistance with a pressure drop porometer. Data on root densities and distribution were also obtained. Transpiration rates fell below estimated potential levels when the average value of Ψs in the root zone was reduced to –1 to –5 bars, depending on soil storage, root distribution and potential transpiration rate. From this point Ψs fell rapidly in the surface layers, more slowly at depth. It was found that accurate calculations of daily water uptake could be made from changes in soil water content. The minimum value of leaf water potential (�1 )attained each day declined progressively through the drying cycle, but there was evidence that stomatal resistance (rs) is not uniquely related to Ψ1; initial stomatal closure occurred at Ψ1, values which decreased from –11 to –25 bars as drying progressed. This adaptive mechanism is related to changes in osmotic potential of the leaves. Whole plant resistances (Rp), derived from leaf water potentials and fluxes through individual stems, increased as stem populations increased. In the high population lysimeter Rp decreased from 300 to 100 bar sec mm-3 as canopy transpiration rates increased from 1.5 to 4.5 x 10-4 mm sec-1. In the low population lysimeter Rp decreased from 70 to 30 bar sec mm-3 as transpiration increased from about 2.2 to 4.5 x 10-4 mm sec-1. The higher resistances appear to confer significant advantages in terms of water conservation and adaptation to drought.

Extended Field Use of Screen‐Covered Thermocouple Psychrometers1

AbstractThe use of screen‐covered thermocouple psychrometers for in situ measurements of soil water potential has been criticized because of possible soil and micro‐organism contamination through the screen covering. Double‐junction Peltier psychrometers with a screen covering were installed in the soil under field conditions for periods ranging from 2 to 40 months of continuous exposure, and were then removed and examined for evidence of loss of calibration sensitivity and contamination by soil particles and micro‐organisms. The uncleaned psychrometers were recalibrated in the laboratory and then disassembled and examined under a microscope (250 ✕) for contamination. Although slightly more than half of the units lost some sensitivity after field exposure, the average decrease was less than 5%. The psychrometer cavities were all nearly free of soil particles, the thermocouple junctions were shiny, and no evidence of microbial attack was evident. It appears that long‐term field exposure of screen‐covered psychrometers is no more detrimental to their performance than that of ceramic or other psychrometers.

Improved Thermocouple Psychrometer for the Measurement of Plant and Soil Water Potential

Improved Thermocouple Psychrometer for the Measurement of Plant and Soil Water Potential

Dynamic Measurement of Soil and Leaf Water Potential With a Double Loop Peltier Type Thermocouple Psychrometer

AbstractDetails on the construction, calibration, and performance of a three‐terminal double loop thermocouple psychrometer are given. (This psychrometer design was originally conceived and constructed by Dr. W. H. Gardner, Dep. of Agronomy, Washington State University, Pullman, Wash., USA). The thermal stability of this psychrometer is about 40 times better than that of the two‐terminal psychrometer (Spanner type) for ambient temperature fluctuations with a time rate of change greater than 0.2C/min. The response behavior of a fritted glass bulb and a ceramic bulb psychrometer was tested for vapor and for liquid phase water movement. For vapor phase flow the fritted glass bulb exhibited a shorter response time than the ceramic bulb psychrometer, whereas the reverse was true when water movement was predominantly in the liquid phase. Water potential measurements carried out on silty clay and silt loam soil samples were within ±0.4 bar of the porous plate extractor equilibrium values. A system that facilitates automatic and continuous in situ measurement of soil water potential using the three‐terminal psychrometer is described.

Psychrometric Measurement of Soil Water Potential: Temperature and Bulk Density Effects

CHANGES IN SOIL WATER POTENTIAL WITH TEMPERATURE WERE DETERMINED USING THERMOCUPLE PSYCHROMETERS ON SAMPLES OF FOUR SOULS. RESULTS INDICATE THAT CHANGE IN SOIL WATER POTENTIAL WITH TEMPERATURE BECOMES GREATER AS SOILS BECOME DRIER. THE FINER TEXTURED SOILS SHOWED A GREATER CHANGE IN WATER POTENTIAL WITH TEMPERATURE THAN DID COARSER SOILS AT SIMILAR WATER POTENTIALS. WATER POTENTIAL CHANGE WITH BULK DENSITY ALSO WAS DETERMINED FOR SAMPLES OF THREE SOILS. LITTLE CHANGE WAS NOTED EXCEPT ON A CLAY SUBSOIL SAMPLE. /AUTHOR/

Improved Thermocouple Psychrometer for the Measurement of Plant and Soil Water Potential

The operation characteristics (the Peltier cooling current i and the cooling period tc), the range of measurement of water potential, the calibration sensitivity, and the accuracy are evaluated for an improved psychrometer and compared with data in the literature, and with a recent theory. The range increases with increasing Peltier cooling current up to the optimal value of i « 9 m A and has been found to agree fairly well with the predictions of theory. A maximum Tange of 0 to about —13 000 J/kg is obtainable with the improved psychrometer. This is nearly twice that previously reported for a Spanner-type psychrometer. The experimental calibration sensitivities C's increase with increasing water potential, •cooling period, and temperature and, with tc — 60 s, are in general larger than the theoretical values at high water potentials but less than the theoretical values at low water potentials. They are mostly larger than those reported by Monteith and Owen but about the same as those of Lang and Trickett. The accuracy of the improved psychrometer is significantly greater than that previously reported for a Spanner-type psychrometer. While the theory predicts a linear relationship between water potential and peak output, the •observed calibration curves are non-linear and vary between psychrometers. Because of these •discrepancies between observation and theory, calibration of each psychrometer is necessary.

Improved Thermocouple Psychrometer for the Measurement of Plant and Soil Water Potential

Improved Thermocouple Psychrometer for the Measurement of Plant and Soil Water Potential

In Situ Measurement of Soil and Plant Leaf Water Potential

AbstractFor a pepper plant (Capsicum frutescens L.) growing in a greenhouse, soil water potential was measured with tensiometers and a thermocouple psychrometer, plant water potential was inferred from a calibrated beta gauge, and transpiration was measured by weighing the soil‐plant system. Water moved through the transpiration stream in the direction of the gradient of the water potential. Regardless of the distribution of water in the soil, transpiration was unaffected by soil water content until it dropped to a value corresponding to −6 to −8 bars water potential. Below this water content, transpiration decreased linearly with soil water content to a soil water potential of −37 bars. At this time the plant water potential was below −50 bars and the transpiration rate was near zero. After irrigation the plant regained full turgor.

Psychrometric Measurement of Soil Water Potential without Precise Temperature Control

AbstractA theoretical analysis shows that by imposing certain boundary conditions it should be possible to eliminate the major effects of temperature fluctuation on psychrometric measurements of soil water potential. Tests on a psychrometer constructed to meet these boundary conditions were carried out on a soil column in a greenhouse. Although diurnal fluctuation of soil temperature was as great as 5C, water potential was measured with an apparent accuracy of ±0.5 bars throughout two irrigation cycles.