Soil Moisture Tension Research Articles

Geographic isolates of Glomus increase root growth and whole-plant transpiration of Citrus seedlings grown with high phosphorus

Four Glomus species/isolates from arid, semi-arid and mesic areas were evaluated for their effects on growth and water use characteristics of young Citrus volkameriana (′Volkamer′ lemon) under well-watered conditions, followed by three soil-drying episodes of increasing severity (soil moisture tensions of –0.02, –0.06, and –0.08 MPa) and recovery conditions. Arbuscular mycorrhizal (AM) plants were also compared to non-AM plants given extra phosphorus (P) fertilizer. AM plants and non-AM plants had similar shoot size (dry weight and canopy area), but all AM fungus treatments stimulated root growth (dry weight and length). Leaf P concentrations were 12–56% higher in AM plants than non-AM plants. Enhanced root growth was positively correlated with leaf P concentration. In general, AM plants had greater whole-plant transpiration than non-AM plants under well-watered conditions, under mild water stress and during recovery from moderate and severe soil drying. This suggests a faster recovery from moisture stress by AM plants. AM plants had lower leaf conductance than non-AM plants when exposed to severe soil drying. Although the greatest differences were between AM and non-AM plants, plants treated with Glomus isolates differed in colonization level, leaf P concentration, root length, transpiration flux and leaf conductance.

IRRIGATION OF ROSA HYBRIDA L. ‘KARDINAL’ BASED ON SOIL MOISTURE TENSION INCREASES PRODUCTIVITY AND FLOWER QUALITY

An irrigation control system based on soil moisture tension (SMT) for potted plants was able to reduce the amount of water used to produce commercially acceptable crops. This system was adapted for use on plants growing in ground beds for cut flower production. Since this type of system reduces the amount of water applied, the possibility of a detrimental effect on flower quality was tested. The irrigation control system consisted of tensiometers, modified with high flow ceramic tips and pressure transducers, an analog-digital signal converter, a computer, and custom written software. The system continuously monitored the moisture condition of the soil, initiated irrigation when the soil dried to a specific level, and turned off the water when an adequate amount was applied. When the system was installed in a commercial greenhouse of Rosa hybrida L. ‘Kardinal’ plants, water use in the test area was 26% less than the amount applied by the grower. Productivity (stems harvested@m) was 66% greater in the test area. Flowers harvested from the test plants were not lower in quality than those from the grower irrigated controls. In fact, except for the initial harvest, the flowers from the test area were significantly longer (up to 8 cm) than those from the grower’s area. An additional harvest, 71 days after the irrigation control system was removed, showed no differences in stem length. The use of this irrigation control system can reduce both water and fertilizer usage when a liquid feed program is utilized. Increases in productivity and quality can result in significant increases in profitability for commercial producers.

Relationship between soil moisture tension and the mass mortality of oak trees

Relationship between soil moisture tension and the mass mortality of oak trees

Water Use by Drip-irrigated Watermelons on Plastic Mulch

Watermelon is grown under a range of moisture regimes from rainfed to heavily irrigated, but water requirement patterns are not well documented. Drip irrigation and plastic mulch provide the opportunity to control water applications to optimize yield and quality. Water applied through subsurface drip irrigation was measured in two watermelon trials in 1998 (25 seeded and 20 seedless cultivars) and 1999 (26 seeded and 14 seedless cultivars) at Lubbock, Texas. Melons were transplanted in plastic-covered raised beds 13.6 m long spaced 2 m apart. Irrigation was applied when morning soil moisture tension measured by tensiometers exceeded 20 kPa. Watermelon yields ranged from 50 to 100 t·ha-1 with excellent quality. Weekly water use averaged 14 mm during the first 3 weeks of establishment then increased to 28 mm during the next 3 weeks as plants were running and blooming. During the 5-week fruit-enlargement period, water uptake averaged 57 mm, then decreased as full fruit size was attained. Similar uptake patterns in both years suggest that meaningful crop coefficients for scheduling watermelon irrigation could be based on phenological growth stages.

203 Influence of Irrigation Levels and Application Moments on Growth and Performance of an Early Sweetpotato Cultivar

The objective of this work was to establish the dry-matter production and yield at different irrigation levels and moments of water application. An early cultivar, INIA-Sandu, was transplanted in Nov. 1996. Treatments were a combination of: no irrigation, irrigation at 25 kPa and 75 kPa of soil moisture tension during the first 60, 30 to 90, 60 to 120 days, and during all the cycle after crop establishment. Sequential samples were done throughout the growth cycle. The parameters evaluated were fresh and dry weight of leaves, stems, and roots. At the end of the crop cycle, yield was determined. Irrigation levels and early applications affected weight of leaves, stems, and roots. Yield was greater at early water applications during the growth cycle. Lower soil moisture tensions tend to have greater yields.

412 Organic Mulch Improves Yield and Economic Returns From Chive Production

Chive (Allium schoenoprasum) is one of the most popular culinary herbs in the Virgin Islands, and local demand is always high throughout the year. However, local production is not sufficient to meet increasing demands. Chive production is constrained by insect pests, weeds, and high cost of irrigation water. A study was conducted to compare the influence of organic and synthetic mulches on yield and economic returns from chive production. The study also evaluated the effect of mulch on weeds and water use. Chives were planted in plots consisting of three rows 3.6 m long. Plants were spaced 20 cm within rows 41 cm apart. The plots were mulched with grass straw, wood chips, shredded paper, and white plastic. A control plot (no mulch) was also planted for comparison. Plots were arranged in randomized complete-block design with four replications. All plots were drip-irrigated and soil moisture tension maintaned at 30 kPa. Chives grown with grass straw mulch produced taller plants and higher number of tillers (slips) than all other mulch treatments. Total fresh yield of plots with grass straw mulch was superior to all other mulch treatments including the control. On the average, plots with grass straw mulch produced 1203 g/m2 of fresh chives. All mulches resulted in reduced weed population compared to the control (no mulch). Due to high rainfall during the growing season, differences in irrigation water use were not significant. Economic comparison indicated that the net return above mulch costs was 50% higher with grass straw than with other mulch treatments. To improve production and income, herb growers should consider using grass straw and realize other benefits, including weed control and improved soil fertility.

Switchgrass Growth and Development: Water, Nitrogen, and Plant Density Effects

Switchgrass (Panicum virgatum L.), an important component of the tallgrass prairie, is a productive warm-season forage grass. Interest in growing switchgrass for alternative uses has raised questions about resource use during production. The objective of our study was to examine how resource inputs affected interspecific plant competition in switchgrass. 'Alamo' switchgrass was established from seed in outdoor lysimeters in May 1993 and grown under 22 or 112 kg N ha(-1), and under field capacity or water-deficit conditions until August 1994. Plant spacing varied systematically from 10 to 70 cm. Plants were harvested in late summer each year and individual plant dry weight, tiller number, leaf area, and morphological development stage were measured. Soil moisture tensions below -45 kPa reduced switchgrass photosynthetic rates and xylem pressure potential. As plant spacing increased, tiller number, leaf area, plant dry weight, and morphological development stage increased. Plant dry weight and tiller number in the establishment year was not affected by N input. Established plants in 1994, however, responded to high N input at low plant densities with 50 to 100% greater leaf area and up to 3-fold greater plant dry weight compared to the low-N treatment. The increased plant dry weight at high N input resulted from increased individual tiller weight and not increased tiller number. Our data indicate that competitive responses of switchgrass plants at high plant densities were controlled by competition for aboveground resources, as plant yield and morphology at high densities were not affected by water or N inputs. DOI:10.2458/azu_jrm_v53i2_sanderson

Switchgrass growth and development: water, nitrogen, and plant density effects

Switchgrass (Panicum virgatum L.), an important component of the tallgrass prairie, is a productive warm-season forage grass. Interest in growing switchgrass for alternative uses has raised questions about resource use during production. The objective of our study was to examine how resource inputs affected interspecific plant competition in switchgrass. 'Alamo' switchgrass was established from seed in outdoor lysimeters in May 1993 and grown under 22 or 112 kg N ha(-1), and under field capacity or water-deficit conditions until August 1994. Plant spacing varied systematically from 10 to 70 cm. Plants were harvested in late summer each year and individual plant dry weight, tiller number, leaf area, and morphological development stage were measured. Soil moisture tensions below -45 kPa reduced switchgrass photosynthetic rates and xylem pressure potential. As plant spacing increased, tiller number, leaf area, plant dry weight, and morphological development stage increased. Plant dry weight and tiller number in the establishment year was not affected by N input. Established plants in 1994, however, responded to high N input at low plant densities with 50 to 100% greater leaf area and up to 3-fold greater plant dry weight compared to the low-N treatment. The increased plant dry weight at high N input resulted from increased individual tiller weight and not increased tiller number. Our data indicate that competitive responses of switchgrass plants at high plant densities were controlled by competition for aboveground resources, as plant yield and morphology at high densities were not affected by water or N inputs.

Predicting nitrogen supply in plantation eucalypt forests

Nitrogen mineralization was measured experimentally and predicted by simulation modelling in three plantations of Eucalyptus globulus in south-western Australia where annual rainfall ranged from 600 to 1500 mm. Nitrogen mineralization was measured in the field each 4 weeks for 3 yr (2 sites) and 2.8 yr (1 site) using the in situ coring technique. Potentially available N was estimated at each sampling time using anaerobic incubations in the laboratory. In the field, soil temperature was logged each hour and soil moisture was estimated gravimetrically each month at the start and end of each field exposure period. The relationship of N mineralization to environmental variables was determined through aerobic incubations of soil samples in the laboratory at 6 temperatures (4 to 42°C) and 10 soil moistures (air dry to 10 kPa soil moisture tension). A generalized model using an exponential function to describe temperature response and a logistic function to describe moisture response was fitted to the laboratory N mineralization incubation data. This function was used as a basis for predicting N mineralization in the field from measured soil moisture and temperature. Model output was scaled by tuning the simulated and observed N mineralization rates over the experimental period at each site. Finally, independent scale factors were derived by regressing the tuned scale factors against soil chemical (anaerobic N) and physical characteristics (soil moisture content at 25 kPa moisture tension) and these were used in model simulations to predict N mineralization. There was a marked seasonal pattern in monthly rates of mineralization related to the seasonally dry climate in south-western Australia. Rates were highest during late winter and spring when microbial activity was not limited by soil moisture deficits and soil temperatures were rising, and lowest in summer when the surface soils dried to below wilting point. Model simulations generally reflected the observed pattern of mineralization and mean annual rates of N mineralization at each sites were predicted within about plus or minus 20%

Scheduling strawberry irrigation based upon tensiometer measurement and a climatic water balance model

A field experiment was carried out over three cropping seasons with 1 year old strawberry plants ( Fragaria x ananassa Duch. `Elsanta' and `Honeoye'), where irrigation was scheduled based upon tensiometer measurements and a climatic water balance model. Both irrigation schedules were compared to the non-irrigated one. The influence on fruiting response, the impact on soil moisture tension, mineral nitrogen content in the soil and nitrogen content in the leaves were investigated. In all the years, irrigated plants had significantly higher yields than the non-irrigated ones. The mean fruit weight was also increased by irrigation. Optimisation of irrigation was best achieved in both varieties when the climatic water balance model was used to schedule irrigation. During dry periods, soil moisture tension under non-irrigated strawberries increased to values above 300 hPa at 20 cm soil depth. In a year, with dry conditions and high evapotranspiration, maximum values above 700 hPa were reached. Differences in soil moisture were observed between the two irrigation schedules within and over the years of the experiment. The hydraulic gradients calculated from tensiometer measurements showed that in the periods with irrigation, percolation did not occur below 40 cm soil depth. Therefore, leaching of mineral nitrogen out of the rooted area of strawberries could be excluded for both irrigation schedules. The nitrogen level in dry leaf matter was not influenced by irrigation, except in one year when irrigation followed the climatic water balance model. The present study shows the potential to optimise the irrigation of strawberries using the climatic water balance model, which is less labour intensive and time consuming than tensiometer measurements. Besides the economical aspects, the results from the analysis of different soil parameters indicate that scheduling irrigation based upon this model was also environmentally safe.

Nearly Isogenic Cucumber Genotypes Differing in Leaf Size and Plant Habit Exhibit Differential Response to Water Stress

Four nearly isogenic cucumber lines (Cucumis sativus L.) differing in leaf size [standard leaf (LL) vs. little leaf (ll)] and plant habit [indeterminate (DeDe) vs. determinate (dede)] were compared for their response to high soil moisture tensions in 1990 and 1996. Comparisons were made between lines for aboveground vegetative and fruit response, between two irrigation regimes, and among three postharvest treatments. Differences in vegetative plant response between lines were documented by wilting ratings, plant dry weight, fruit number and fresh weight, and fruit quality [i.e., fruit shape, seed size, seed cavity size, and pillowy fruit disorder (PFD)] ratings depending upon the stress environment. Postharvest treatment affected the quality of fruit recovered from plants subjected to water stress. Exposure of fruit at 15 °C and 85% relative humidity (RH) for 4 days after hydrocooling, resulted in lower PFD than storage of fruit at 26 °C and 60% RH for 2 days without hydrocooling. Cucumber genotypes showed differential response to water stress indicating that plant habit and leaf size can be important genetic determinants of plant response to water stress. Although plant productivity was not affected by water stress, PFD, shape, seed size, and seed cavity size of fruit from lldede plants were more severely affected by water stress than its llDeDe counterpart. Plants homozygous ll, in either a determinate or indeterminate background, were less susceptible to wilting under water stress conditions than their normal leaf (LL) counterparts. However, plant dry weight and fruit number and weight were higher in LLDeDe plants when compared to their llDeDe counterparts. Fruit recovered from LLDeDe plants were of higher quality than those recovered from llDeDe plants. Thus, wilting response to water stress is not necessarily indicative of a cucumber plant's tolerance to water stress in the reproductive stage.

597 Tree Establishment and Soil Surface Covers

The purpose of this study is to determine the effects of a variety of commonly used soil surface covers on the establishment of Cercis canadensis L. var. texensis (Wats.) Rose. `Alba'. Containerized 23.3-L trees were planted into 1.1 × 1.5-m plots separated by metal landscape edging in 20-cm-deep raised beds constructed of exterior treated landscape timbers. Eight soil surface treatments were imposed with five replicates each. Controls included bare soil with no cover. The remaining seven treatments consisted of 8-cm depth of pine bark mulch, Trachelospermum asiaticum (Siebold & Zucc.) Nakai on 30-cm centers mulched with 8 cm of pine bark, solid sodded Stenotaphrum secundatum (Walt.) Kuntze, 8 cm of decorative gravel, 3 cm of recycled paper mulch, brick pavers underline with 5 cm of coarse builders sand (brick-on-sand), and seasonal color mulched with 8 cm of pine bark. The color rotation for summer, fall / winter, and spring is Catharanthus roseus (L.) G. Don, Viola × wittrockiana Gams., and Petunia × hybrida Hort. Vilm.-Andr., respectively. Preliminary data indicates that mulching with pine bark resulted in similar or slightly increased trunk diameter growth over that of bare soil. Brick-on-sand and the recycled paper mulch had smaller increases in trunk diameter than the other treatments. Mid-day leaf water potentials of most treatments were similar to bare soil, but the mid-day water potentials of trees mulched with recycled paper were among the most negative. All treatments except brick-on-sand trees recovered as well or better than trees in the bare soil by the next morning. Poor predawn water potential recovery of brick-on-sand trees may be due to the elevated temperatures observed in their root zone. Bare soil, recycled paper mulch, and the annual treatments had the most negative soil moisture tensions on average. The bare soil may be due to increased evaporative loose over mulched soils. The large biomass of vinca as the season progressed may account for the more negative readings in the annual seasonal color plots.

High temperature rather than drought stress is responsible for the occurrence of double pistils in `Satohnishiki' sweet cherry

Effects of temperature and soil moisture level on the occurrence of double pistils in `Satohnishiki' sweet cherry were studied. Trees were grown in sunlit growth chambers controlled at 25°C/25°C, 30°C/25°C and 35°C/25°C, day/night from late July to early September. The higher the temperature, the slower the progression of flower differentiation. At 25°C, all the flower buds had a single pistil, whereas above 30°C, the percentage of flowers with double pistils increased remarkably. More than 80% of the flowers differentiated double pistils at 35°C and some had triple pistils. In another experiment, trees were grown under moist [soil moisture tension (SMT) <6 kPa] and dry (SMT < 32 kPa) conditions in chambers controlled at 25°C/15°C and 35°C/25°C. Though the trees did not show symptoms of water stress under the low temperature/moist soil regime, the leaf water potential under the high temperature/moist soil regime decreased to almost the same level as that under both dry soil regimes. At high temperatures, the occurrence of double pistils was severe. However, the soil moisture did not affect appreciably the frequency. In contrast, pistil doubling was not observed at low temperatures at all. Under field conditions, by the restriction of watering, the leaf water potential obviously decreased. However, the frequency of occurrence of double pistils under the dry soil regime (SMT < 51 kPa) was not higher than that under the moist soil regime (SMT < 13 kPa). These results suggested that high temperature above 30°C is a critical factor in the formation of double pistils in `Satohnishiki' sweet cherry.

Comportamento físico-hídrico de um podzólico vermelho-amarelo câmbico fase terraço sob diferentes sistemas de manejo

O estudo foi desenvolvido no município de Viçosa, MG, na fazenda Fundão, pertencente ao câmpus da Universidade Federal de Viçosa (UFV), com o fim de verificar o comportamento físico- hídrico de um Podzólico Vermelho-Amarelo câmbico fase terraço, sob diferentes sistemas de manejo. Foi utilizado delineamento experimental em blocos ao acaso, com cinco tratamentos e quatro repetições. Os tratamentos foram: 1 - arado de aiveca; 2 - arado de discos; 3 - grade pesada; 4 - plantio direto; 5 - testemunha, com as parcelas mantidas em pousio. Os parâmetros avaliados foram: teores de matéria orgânica; granulometria; densidade do solo; densidade de partículas; porosidade total, macroporosidade, microporosidade, agregados e curva de retenção da água no solo. Pode-se concluir que: o uso da grade pesada reduz os teores de matéria orgânica na camada de 0-15 cm; a utilização da grade pesada acarreta adensamento da camada superficial; o revolvimento do solo ocasiona maior macroporosidade; o sistema plantio direto proporciona menor desagregação do solo; o sistema plantio direto garante maior retenção de água.

Influence of slope and stone cover on tillage operations in the Ethiopian highlands

Human population pressure has forced smallholder farmers in some areas of the central plateau region of the Ethiopian highlands to cultivate plots located on slopes or hillsides having varying amounts of stone cover. The objectives of this pilot study were to determine if slope and(or) stone cover influenced tillage operations on smallholder farms in the Debre Birhan area of the Ethiopian highlands, and to examine the relationship between different soil/tillage-related factors and draft of the indigenous single-tined ard ( maresha). This paper also briefly describes the traditional systems of cropping and soil management, and provides data on selected physical and chemical properties of soils found in the study area. A land classification (LC) system was developed to categorize plots according to the slope and stone cover. Surface stone cover of plots was calculated and a method of estimating subsurface stones was tested. Tillage measurements included plowing frequency or number (PN) per area per annum, soil moisture tension, plowing depth, furrow “wedge” of soil moved, area plowed, contour angle, and the speed of travel and pulling force exerted by oxen. Plots were located on slopes ranging from 0% to 23%. Degree of slope was a key factor in determining the acuteness of angle of the contour at which plowing is done prior to seed covering, but had no significant effect on maresha draft. Stone cover of plots ranged up to 27%, with individual stones moved by the ard weighing up to 2.5 kg. Plowing depth ranged from 10.1–15.3 cm for all PN across LC. Oxen travelled at speeds of 0.35–0.58 m s −1. Mean values for draft of the maresha across LC ranged from 0.83–1.04 kN. Farmers plowed from 0.10–0.15 ha during a 5–6 h work period, irrespective of LC or PN. Soil moisture tension (range 16–33 kPa) was found not to have a significant effect on draft, due to tillage being carried out prior to the major and minor rains. Percent stone cover explained very little of the variation in implement draft ( R 2=0.10). Adding stone weight to the model increased R 2 slightly (0.22), but significance was low ( P<0.16). Furrow wedge of soil displaced by the ard head accounted for most of the variation in draft of the maresha across all LC and PN ( R 2=0.75, P<0.008). The design and construction of the maresha allows it to be used equally well for tillage operations on plots having minimal or high stone cover which may be located on flat or gentle sloping land, or on steep hillsides.

Evaluation of Soil Moisture Tension for Starting Irrigation Applying Decrease of Consumptive Use with Soil Dry on Paddy Based Land in Tropical Area

Evaluation of Soil Moisture Tension for Starting Irrigation Applying Decrease of Consumptive Use with Soil Dry on Paddy Based Land in Tropical Area

Iron oxide mineralogy of Terre Rosse and Rendzinas in relation to their moisture and temperature regimes

SummaryTerre Rosse and Rendzinas occur on limestones close together under the same climatic conditions. Their differences in iron oxide mineralogy are believed to be due to differences in the pedoclimate. We monitored the soil moisture tension and the soil temperature over nearly 3 years in Terra Rossa‐Rendzina pairs on hard limestone and soft chalk in Israel. The Terre Rosse dried out more rapidly than the Rendzinas mainly because of their smaller water‐holding capacity. In summer, average soil temperatures in the Rendzinas were significantly lower than in the Terre Rosse.Haematite dominates the red Terre Rosse and goethite the yellower Rendzinas. This difference appears to be related to the soil climate, specifically the moisture regime of the two soils, and supports the hypothesis that release of Fe and formation of ferrihydrite in Terre Rosse during the wet winter is followed by transformation to haematite during marked desiccation in the dry summer. By contrast, wetter soil conditions in the Rendzinas direct the formation of Fe oxides more towards goethite. either directly or by transformation of ferrihydrite via solution.

The effect of water vapour adsorption on soil moisture content under Mediterranean climatic conditions

The effect of water vapour adsorption on the soil moisture content of different soils under semi-arid climatic conditions (Greece) was evaluated. Time domain reflectometers were used for continuously monitoring soil moisture content at various depths. Water vapour adsorption by the soil was measured using weighing lysimeters. The obtained data indicated that nighttime water vapour adsorption is very important for areas characterised by high oscillation of air humidity. In a period of 8 months, viz. from February to August 1996, a total amount of 226 mm of water vapour was adsorbed by the soil, while the total rainfall was only 179 mm in the same period. Water vapour adsorption was found to be responsible for the diurnal fluctuations of the volumetric soil moisture content. This fluctuation was related to the clay content, the depth and the initial moisture content of the soil. Water vapour adsorption could be related to the soil moisture tension of the uppermost soil layer, the daily minimum value, and the amplitude of air humidity. Rock fragments restricted water vapour adsorption, but their contribution to the reduction in evaporation waS greater under certain climatic conditions.

Pot soil air composition in conditions of high soil moisture and its influence on chrysanthemum growth

Chrysanthemums were grown in 15.2 cm standard pots in a heavy mix of clay loam soil+sphagnum peat moss (2:1). A fine texture mix was used to accentuate undesirable gas profiles in the soil. Soil air was analyzed at five depths in the soil profile. In one set of tests, water was applied to the top of the pot at a matrix potential in the center of the soil profile of −5 kPa. The average gas concentrations in soil air in the top and bottom fifths of soil were for O 2—20.0 and 14.5%, for CO 2—0.8 and 2.4%, and for C 2H 4—0 and 0.08 μl dm −3. Smooth concentration gradients of each gas occurred from top to bottom of the soil profile. The composition of soil air changed greatly during the drying cycle. At soil moisture tensions of −0.7, −2.5, and −5 kPa in the center of the soil profile, the gas concentrations in the lowest fifth of soil were for O 2—9.6, 15.3, and 20.3%, and for CO 2—4.5, 3.5, and 0.6%, respectively. Thus, soil atmospheric conditions for plant growth were poorest immediately after watering and continuously improved up to the time of watering. When pots of chrysanthemum were watered by capillary action from mats, the average concentration of gases in soil air in the lowest fifth of soil were 5.8% O 2, 3.6% CO 2, and 0.38 μl dm −3 C 2H 4. This gas profile was less desirable for growth than the profile found in top-watered pots. Unlike the situation in top-watered pots, the gas concentrations in mat-watered pots were stable. Roots in top-watered pots were restricted to the upper two thirds of the soil ball, and were distributed evenly in the inner part of the soil and at the periphery. Roots of mat-watered plants developed further down the vertical periphery of the pot than roots of top-watered plants, but they did not grow inside the ball. Chrysanthemum plants were grown through a hole in the side wall of each of five 3.9 cm tall by 15.2 cm diameter plastic rings stacked vertically and separated by stainless steel screens that allowed for passage of water but not roots. Water was applied to the top of these cylinder stacks. The largest plants developed in the top ring with progressively smaller plants at lower depths. Plants in the lower two rings developed interveinal chlorosis and did not reach commercial size.

ERODIBILITY AND CRITICAL SHEAR OF A PREVIOUSLY FROZEN SOIL

Many areas of the northern United States and southern Canada, and particularly the four million ha of nonirrigatedcropland of the Northwestern Wheat and Range Region in the United States, suffer severe erosion under thawingsoil conditions. Ability to predict soil erosion in these areas is hampered by lack of knowledge of soil properties andhydraulic resistance of thawing soils. This study was conducted to determine erodibility parameters of soil frozen andthawed under controlled moisture tension. A tilting flume was designed to allow near-natural freezing and thawing of asoil mass and apply shear stress from flowing water. Tests were conducted under soil moisture tensions of 50, 150, and450 mm. A linear relationship was found between detachment and shear stress. Critical shear stress values showed littlechange with time. Rill erodibility was related to soil moisture tension and changed rapidly during the course of a 90-mintest flow. At a tension of 450 mm, the erodibility was similar to that found in field tests under summer conditions. Thisstudy indicates the transient nature of rill erodibility during soil freezing and thawing that will influence the accuracy ofcontinuous simulation erosion models for winter conditions.