151 research outputs found
An Instrument for in situ Measurements of Soil Moisture Flow and Suction
A soil moisture flux transducer was built and tested in the
laboratory under steady-state conditions in a column of Portneuf
silt loam soil. Initial data indicate that the instrument may
be developed into a useful field research tool. Its principal
advantage is that measurements of moisture flow may be made
without any prior information concerning the unsaturated
hydraulic conductivity of the soil. Rather, one needs to know
only a soil moisture convergence factor which is dependent on
the state of the soil and the design of the transducer. While
this convergence factor is dependent upon the soil moisture
content, the dependence appears to be nearly an order of magnitude
less than the dependence of hydraulic conductivity on
soil moisture content. It may prove possible to develop the
unit for installation in the field to provide continuous measurements
of the unsaturated soil moisture flow in the tensiometer
range with errors no greater than those arising from the natural
heterogeneity of the soil
Freezing Point Method
Research in soil-plant water relations during the last two
decades has successfully addressed itself to the problems of expressing water status in soils and plant tissues in terms of free
energy. The initial impetus was provided by Spanner (1951}
in his classical work on the psychrometer. During the ensuing
years the science of water relations was provided with a new
theoretical approach based on thermodynamic principles and
terminology. This was complemented by a substantial amount
of research on basic techniques. The result has been a rather
sophisticated technology which provides a means of describing
the free energy status of water in natural systems in quantitative
terms consistent with modern thermodynamic theory. This
marks a significant turning point, because now we are capable
of expressing the flow of water in the soil-plant continuum in
meaningful terms of energy status. The recent comprehensive
review by Barrs (1968) cites over 50 references on the use of
the Spanner psychrometer and related vapor exchange methods,
and a great many other useful references on various techniques
for evaluating other aspects of plant water status. However, with
this and other reviews (Boyer, 1969; Brown, 19TH; Pack 1968,.
1969; Rawins, 1966) there still appears to be a need for a practical guide on the use of thermocouple psychrometers and other
methods of measuring water potential and its components
Punch Planting to Establish Lettuce and Carrots Under Adverse Conditions
It was observed in both the greenhouse and field that
lettuce and carrots may be established by planting single
seeds in holes punched through the soil crust. When the
holes are not back-filled, seeds planted as deep as 60 mm
will produce healthy plants in a few days. Preliminary
experiments indicate that in some climatic regions a practical
field management system could be developed which
will eliminate emergence problems arising from soil crusting
and premature drying of shallow seedbeds. The open
hole punch method may also prove useful in controlling
the temperature and salinity adjacent to the seed and in
reducing initial tillage and irrigation requirements
Double Cropping Dry Peas and Forage in Southern Idaho
Dry peas grown as a cash crop in southern Idaho
are usually harvested by the first of August, which
leaves the land bare and unproductive for the last
third of the growing season, unless the
peas
were
used as a nurse crop to establish alfalfa. This raises
the question: Could August, September and the
early part of October be utilized for forage production
from plants other than alfalfa
Relations Between CO2 Exchange Rate, CO2 Compensation, and Mesophyll Resistance from a Simple Field Method
An equation is developed that relates apparent photosynthesis
to CO? light compensation values, resistance to
CO? diffusion in air, and the apparent mesophyll resistance
to CO? transport. The equation also yields values
for total photosynthesis and light respiration from measurements
of apparent photosynthesis. A simple method
for measuring leaf CO? exchange rates in the field with
a hand-operated syringe is described. Results obtained
with this device and data published in recent literature
are used with the new equation to show that photosynthesis
may be limited more by mesophyll resistance than
by photorespiration
Vapor Pressures of an Air-Water Interface During Evaporation
Low evaporation rates from soil caused
solutes in the soil solution to accumulate at the air-water
interface and reduce the vapor pressure. Nearly tenfold
increases in concentration at the interface were
observed with ZnSO? in a cool moist soil. CaCll?, AICI?,
sucrose, and hexadecanol accumulated to lesser degrees,
depending on the evaporation rate and the temperature.
The small amount of water movement required to
develop this increase suggests the phenomena may also
occur next to semipermeable membranes, such as plant
roots
Experimental Measurements of Soil-Moisture Hysteresis and Entrapped Air
A moisture-release curve may be determined
by removing, stepwise, increments of water
and measuring the steady-state soil-moisture
suction that develops. A soil-moisture-adsorption
curve may be determined in the same
way, except that increments of water are allowed
to enter the sample. The failure of these
two curves to coincide is commonly known
as soil-moisture hysteresis (see, for example,
figs. 3 to 6)
Soil Water Hysteresis: Temperature and Pressure Effects
The effects of transient temperatures and pressures on soil water hysteresis were
studied under controlled laboratory conditions. Variations in ambient pressure and
temperature caused small changes in soil water hysteresis. These changes suggest that
at least two mechanisms may be present, neither of which is easily rationalized by the
classical hourglass-shaped pore liquid jump model.
The variation between duplicate samples of a silty clay, a silt loam, and a loamy sand
suggested that normal temperature and pressure changes in the field will not cause
changes in soil water hysteresis that are larger than those due to natural soil
heterogeneity. Consequently, soil water hysteresis curves measured in the laboratory on
representative samples may be used in computer models of field situations
Here's How Frost Damages Seedlings
HOW MANY TIMES have you stepped
out in your field or garden after a
cold night and seen a row of beans
or tomatoes with wilted, black,
and dead seedlings intermingled
with plants that show no sign of
frost damage? Do freezing temperatures
really vary that much
in the row, or are some seedlings
more frost resistant; and if so,
why don't agronomists select these
plants and develop new varieties?
Direct answers to questions like
these are not simple, but we are
beginning to understand why frost
affects tender seedlings in a variety
of different ways
Irrigating Row Crops from Sod Furrows to Reduce Erosion
Kentucky bluegrass (Poa pratensis L.) sod furrows were established
to stabilize the soil. The furrows were used to irrigate corn
(Zea mays L.), wheat (Triticum aestivum L.), barley (Hordeum vulgaris
L.), beans (Phaseolus vulgaris L.), and sugar beets (Beta vulgaris
L.). The sod strips containing the furrows were 0.3-m wide and
managed as a permanent living entity. A strip mowing machine and
a miniature rotary ditcher were developed to maintain the sod furrows.
Crops were grown in clean tilled strips 0.9-m wide between
the sod strips. Normal use of fertilizer, herbicides, and cultivation
was utilized for the crop rows in the clean tilled strips. This management
system eliminated soil erosion, at least doubled the infiltration
rate of irrigation water, and allowed the production of satisfactory
yields of wheat, barley, dry beans, and corn for silage.
Sugar beet production was unacceptable due to competition from the
sod
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